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Dynamic and equilibrium aspects of micellar and microemulsion systems

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Dynamic and equilibrium aspects of micellar and microemulsion systems
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Leung, Roger Yi-Ming, 1955-
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ix, 262 leaves : ill. ; 28 cm.

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Alcohols ( jstor )
Colloids ( jstor )
Curvature ( jstor )
Interfacial tension ( jstor )
Kinetics ( jstor )
Micelles ( jstor )
Molecules ( jstor )
Salinity ( jstor )
Solubilization ( jstor )
Surfactants ( jstor )
Chemical Engineering thesis Ph. D
Dissertations, Academic -- Chemical Engineering -- UF
Emulsions ( lcsh )
Liquid-liquis equilibrium ( lcsh )
Micelles ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 235-261.
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Typescript.
General Note:
Vita.
Statement of Responsibility:
by Roger Yi-Ming Leung.

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DYNAMIC AND EQUILIBRIUM ASPECTS
OF MICELLAR AND MICROEMULSION SYSTEMS








BY



ROGER YI-MING LEUNG


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA

1987




DYNAMIC AND EQUILIBRIUM ASPECTS
OF MICELLAR AND MICROEMULSION SYSTEMS
BY
ROGER YI-MING LEUNG
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1987
c


To
My parents and my wife


ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Professor D.O.
Shah for his guidance and encouragement during the course of this
research. I would also like to thank Professor John P. O'Connell,
Professor Gerald B. Westermann-Clark, Professor Dale W. Kirmse, Pro
fessor Brij M. Moudgil and Professor Paul W. Chun as members of the
supervisory committee for their time and advice.
I wish to thank Nancy, Ron and Tracy for their help and coopera
tion. I would also like to gratefully acknowledge the financial sup
port from the National Science Foundation and the ALCOA foundation
for this research.
Finally, I owe my gratitude to my parents, my brother and sis
ter, my wife and all my friends and colleagues in the Chemical
Engineering Department for their encouragement, help and friendship
throughout all these years.
iii


TABLE OF CONTENTS
ACKNOWLEDGEMENTS iii
ABSTRACT viii
CHAPTERS
1 INTRODUCTION 1
1.1 Surfactant Molecules 1
1.2 Surfactant-Containing Aggregates 1
1.3 Equilibrium Aspects of Surfactant-Containing 3
Aggregates
1.4 Dynamic Aspects of Surfactant-Containing 6
Aggregates
1.4.1 Application of Fast Relaxation 8
Techniques to the Dynamic Study of
Surfactant Solutions
1.4.2 Kinetics of Micellization 8
1.4.3 Dynamic Aspects of Microemulsions 17
1.5 Scope 19
2 REVIEW OF THE LITERATURE: FORMATION, STRUCTURE 22
AND PROPERTIES OF MICROEMULSIONS
2.1 Introduction 22
2.2 Spontaneous Emulsification and Thermodynamic 24
Stability of Microemulsions
2.2.1 Ultralow Interfacial Tension 25
2.2.2 Interfacial Curvature, Elasticity 27
and Entropy
2.3 Geometric Aspects and Structure of 36
Microemulsions
2.3.1 Geometric Packing Considerations in 36
Amphiphile Aggregation
2.3.2 Non-Glcbular Domain and Microemulsion 41
Structure in Phase Inversion Region
2.3.3 Design Characteristics of Microemulsions ... 46
2.3.4 Shape Fluctuations and Structural 47
Dynamics of Microemulsions
2.4 Solubilization and Phase Equilibria of 48
Microemulsions
2.4.1 Solubilization and Structure of 48
Microemulsions
2.4.2 Phase Equilibria of Microemulsions 54
iv


2.4.3 Phase Behavior of Winsor-Type 56
Microemulsions
2.4.4 Pseudophase Hypothesis and 59
Dilution Method
2.5 Experimental Studies and Properties 63
of Microemulsions
2.5.1 Experimental Techniques for 63
Characterization of Microemulsions
2.5.2 Middle-Phase Microemulsions and 65
Ultralow Interfacial Tension
2.6 Novel Applications 68
3 EFFECTS OF ALCOHOLS ON THE DYNAMIC MONOMER-MICELLE .... 70
EQUILIBRIUM AND CONDUCTANCE OF MICELLAR SOLUTIONS
3.1 Introduction 70
3.2 Experimental 71
3.3 Results and Discussions 71
3.3.1 Effect of Mixed Solvents on CMC 77
3.3.2 Effect of Alcohols on the Conductance 79
Micellar Solutions
3.3.3 Formation of Swollen Micelles 86
by Alcohols
3.4 Conclusions 94
4 DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS: 96
EFFECTS OF SHORT-CHAIN ALCOHOLS AND POLYMERS
ON MICELLE STABILITY
4.1 Introduction 96
4.2 Experimental 97
4.3 Results and Discussions 98
4.3.1 Labilizing Effect of Short-Chain 98
Alcohols on Micelles
4.3.2 Influence of Alcohol Chain Length 104
and Surfactant Concentration on
Labilizing Effect of Alcohols
4.3.3 The Concept of Micelle Stability Ill
4.3.4 Micelle Nucleus Formation as the 113
Rate-Limiting Step: Evidence
from Polymer Additives
4.4 Conclusion 121
5 DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS: 123
EFFECTS OF MEDIUM- AMD LONG-CHAIN ALCOHOLS
AND OILS
5.1 Introduction 123
5.2 Results and Discussions 123
5.2.1 Slow-Down of Step-Wise Association 123
Kinetics by Alcohols
5.2.2 Transition in Micellization Kinetics 128
with Addition of Alcohols
v


5.2.3 Effect of Alcohol Chain Length on the 129
Transition of Micellization Kinetics
5.2.4 Formation of Swollen Micelles by 136
Alcohols and Oils with Resultant
Slow-Down in Micellization Kinetics
5.3 Conclusions 141
6 SOLUBILIZATION AND PHASE EQUILIBRIA OF WATER-IN-OIL .... 144
MICROEMULSIONS: EFFECTS OF SPONTANEOUS CURVATURE
AND ELASTICITY OF INTERFACIAL FILM
6.1 Introduction 144
6.2 Basic Concepts and Theory 145
6.2.1 Interfacial Free Energy of Microemulsions .. 145
6.2.2 Free Energy of Interdroplet Interactions ... 147
6.2.3 Phase Equilibria of W/0 Microemulsions 150
6.2.4 Equilibrium Droplet Size and Solubilization 151
of W/O Microemulsions
6.3 Discussions 157
7 SOLUBILIZATION AND PHASE EQUILIBRIA OF WATER-IN-OIL .... 161
MICROEMULSIONS: EFFECTS OF OILS, ALCOHOLS AND SALINITY
7.1 Introduction 161
7.2 Materials and Methods 161
7.3 Effect of Oil Chain Length 163
7.3.1 Influence of Oil Penetration into 163
Interfacial Films
7.3.2 Formation of Birefringent Phases 166
7.3.3 Chain Length Compatibility in 171
W/O Microemulsions
7.4 Effect of Alcohols 176
7.4.1 Effect of Alcohol Chain Length 176
7.4.2 Effect of Alcohol Concentration 179
7.5 Effect of Salinity 184
7.5.1 Optimal Salinity in Single-Phase W/O 184
Microemulsions
7.5.2 Effects of Alcohol and Oil 188
on Optimal Salinity
7.6 Conclusions 191
8 REACTION KINETICS AS A PROBE FOR THE DYNAMIC 196
STRUCTURE OF MICROEMULSIONS
8.1 Introduction 196
8.2 Experimental 197
8.2.1 Materials and Methods 197
8.2.2 Preparation of AgCl Sols 198
8.2.3 Coagulation Rate Measurement 198
vi


8.3 Results and Discussions 200
8.3.1 Coagulation of Hydrophobic AgCl Sols 200
8.3.2 Physico-Chemical Properties of the 204
Microemulsions
8.3.3 Interrelationship between the Reaction 221
Kinetics and the Dynamic Structure
of Microemulsions
8.4 Conclusions 225
9 CONCLUSIONS AND RECOMMENDATIONS 227
9.1 Effects of Alcohols on the Dynamic Monomer- 227
Micelle Equilibrium and Conductance of
Micellar Solutions
9.2 Effects of Alcohols, Oils and Polymers on the 228
Dynamic Property of Micellar Solutions
9.3 Effects of Spontaneous Curvature and Interfacial .. 230
Elasticity on the Solubilization and Phase
Equilibria of W/0 Microemulsions
9.4 Effects of Oils, Alcohols and Salinity on the 231
Solubilization and Phase Equilibria of W/0
Microemulsions
9.5 Reaction Kinetics as a Probe for the Dynamic 232
Structure of Microemulsions
9.6 Recommendations for Further Investigations 233
REFERENCES 235
BIOGRAPHICAL SKETCH 262
vii


Abstract of Dissertation Presented to
the Graduate School of the University of Florida
in Partial Fulfillment of the Requirement for the
Degree of Doctor of Philosophy
DYNAMIC AND EQUILIBRIUM ASPECTS OF
MICELLAR AND MICROEMULSION SYSTEMS
By
Roger Yi-Ming Leung
May, 1987
Chairman: Professor Dinesh 0. Shah
Major Department: Chemical Engineering
The dynamic and equilibrium aspects of micellar and microemul
sion systems have been investigated with focus on the influence of
alcohols, oils and salinity on the systems. The pressure-jump method
was used to study the micellization kinetics. The obtained slow
relaxation time is related to the average life-time of micelles.
Microemulsions are studied by centering on the effect of interfacial
elasticity and curvature on the systems.
The addition of alcohols to micellar solutions increases the
thermodynamic stability of micelles at low alcohol concentrations. A
maximum thermodynamic stability of micelles has been observed at


about 2-3 alcohol/surfactant ratios in the micellar phase. However,
the addition of alcohols may increase or decrease the kinetic stabil
ity (life-time) of micelles depending on the micellization kinetics
and alcohol chain length. The addition of alcohols can induce a
transition of micellization kinetics from a step-wise association to
a reversible coagulation-fragmentation process. The formation of
swollen micelles due to addition of alcohols and oils increases the
slow relaxation time and hence the life-time of the micelles, with a
concomitant decrease in the specific conductivity of the micellar
solution.
The spontaneous curvature and elasticity of interfacial films
have been shown to influence the solubilization and phase equilibria
of water-in-oil (w/o) microemulsions, when the interfacial tension is
very low. Maximum water solubilization in a w/o microemulsion can be
obtained by minimizing both the interfacial bending stress of rigid
interfaces and the attractive interdroplet interaction of fluid
interfaces at an optimal interfacial curvature and elasticity. The
study of phase equilibria of microemulsions serves as a simple method
to evaluate the property of the interface and provides phenomenologi
cal guidance for the formulation of microemulsions with maximum solu
bilization.
Being sensitive to the dynamic structure of surfactant solu
tions, the reaction kinetics and dynamic measurements have been used
as a probe for the dynamic structure of micellar and microemulsion
systems.


CHAPTER 1
INTRODUCTION
1 .1 Surfactant Molecules
Surfactant molecules have a unique feature of possessing both a
polar (hydrophilic) and a nonpolar (hydrophobic) part within the same
molecule. It is this unique feature that causes them to spontaneously
aggregate in a solution, or to adsorb at an air/water or oil/water
interface, which is known as the surface activity of surfactants.
This was first recognized by Hartley (1) in 1936 who originated the
concept of "micelles" as aggregates of surfactant molecules in aque
ous solutions. The molecules were organized in such a way that the
polar heads of the molecules were in contact with the surrounding
water, while the nonpolar tails were shielded from the surrounding
water by aggregating in the interior of the micelle.
1.2 Surfactant-Containing Aggregates
It is well known that two immiscible liquids, e.g., oil and
water, can form a macroscopically clear, homogeneous mixture upon
addition of a third liquid (dispersing agent) which is miscible with
both the liquids (2). This has been conventionally represented by a
ternary phase diagram as shown in Figure 1-1. The single phase
region in the diagram is usually considered as a simple molecular
solution. However, when surface-active molecules, e.g., surfactants
1


2
C
Figure 1-1. A schematic ternary phase diagram representing the
formation of a homogeneous mixture of two immiscible
liquids (A & B) upon addition of a third liquid (C)
which is miscible with both the liquids. Shaded area
represents the two-phase region. P is the plait point.


3
or detergents, are used as the dispersing agent, the single phase
region may consist of raicrodomains of the dispersed phase and complex
association structures of surfactant molecules. Such a single phase
region is often referred as a MmicroemulsionM (3). Figure 1-2
represents a schematic ternary microemulsion phase diagram composed
of microdomains with various possible association structures of sur
factants. At low surfactant concentrations, normal and inverted
micelles are usually expected. At high surfactant concentrations,
water-in-oil (w/o) and oil-in-water (o/w) microemulsions may exist
when the system contains a considerable amount of oil and water. At
even higher surfactant concentrations, a liquid crystalline phase may
be observed. One special character of such a system is that dif
ferent structures may exist in a single-phase region without a phase
separation during structural transition.
1.3 Equilibrium Aspects of Surfactant-Containing Aggregates
The simplest type of micelles is a spherical one. Micelles are
observed only at surfactant concentration just above the "critical
micellization concentration" (CMC) and usually have an aggregation
number of the order of 100. The "CMC" can be defined as a relatively
small range of concentrations separating the limit below which virtu
ally no micelles are detected and above which virtually all addi
tional surfactants form micelles Increasing surfactant concentration
will alter the size and shape of micelles. Thus, with increasing sur
factant concentration one may observe various structures such as ran
domly oriented cylindrical micelles, hexagonally packed cylindrical
micelles or lamellar micelles (4-16).


4
SURFACTANT
Figure 1-2. A schematic ternary phase diagram of an oil-water-
surfactant microemulsion system consisting of
various association microstructures: A, normal
micelles or o/w microemulsions; B, reverse (or
inverted) micelles or w/o microemulsions; C,
concentrated microemulsion domain; D, liquid
crystal or gel phase. Shaded areas represent the
multi-phase regions and the clear area is the
single-phase region.


5
It should be stressed that the aggregation of surfactants in an
aqueous solution arises not from the attractive forces between the
nonpolar tails, but is due to the cohesiveness of water molecules.
The surrounding water, while accepting the polar surfactant heads,
squeezes out the nonpolar surfactant tails. This "hydrophobic
effect, together with the repulsion between the surfactant head
groups, appears to determine the formation, structure and stability
of micelles (6,9,17-29). There have been many theories proposed for
micellization process, including phase separation models (30-32),
statistical mechanical models (33-34), thermodynamic models
(14,28,34-45), a Monte Carlo simulation (31), shell models (46-47),
geometric models (48-49) and a variation theory (50). All these
theories were concerned with the prediction of micellar equilibrium
properties, most notably the CMC, mean aggregation number, structure
and size distribution. Surfactants also form reverse micelles in non
polar media, but the process of micelle formation in nonpolar media
is significantly different from that in aqueous solutions (51-64). It
is known that the presence of water promotes the formation of reverse
micelles.
Microemulsions are isotropically clear, thermodynamically stable
dispersions of oil and water consisting of microdomains stabilized by
surfactant films. The most basic types of microemulsion structure are
oil-in-water (o/w) and water-in-oil (w/o) droplets. More details
about the formation, structure, and properties of microemulsions as
well as their structural relationship to micelles will be reviewed in
chapter 2.


6
1.4 Dynamic Aspects of Surfactant-Containing Aggregates
In addition to the equilibrium picture of surfactant solutions
described above, it is noted that surfactant aggregates are in
dynamic equilibrium with surfactant monomers in the solution. In
fact, micelles and microemulsions should be viewed as dynamic struc
tures. They are thermodynamically stable, but there are constant
formations, dissolution and (shape and size) fluctuations of these
aggregates--in the words of Winsor, "micelles are of statistical
character, and it is important to guard against a general picture of
micelles as persistent entities having well-defined geometrical
shapes." (9, page 3). Figure 1-3 schematically depicts a dynamic
equilibrium between surfactant monomers and micelles in an aqueous
solution. A micelle may constantly pick up a surfactant monomer at
rate constant k from the bulk and may also lose a monomer at rate
constant k The critical micellization concentration actually
represents the equilibrium point of this dynamic process following
the relation CMC = k /k+ (65). The rate of micelle formation and
dissolution as well as the average life-time of micelles depend on
the reaction rate of this dynamic process.
An examination of two review articles on amphiphile aggregation
in aqueous solvents (66) and in nonpolar solvents (67) reveals that
the equilibrium and thermodynamic aspects of surfactant solutions
have received extensive attention ovr the past fifty years, but the
study on the dynamic aspect of these systems is still in its early
stage. However, some initial explorations of kinetics and dynamic
aspects of micelles and microemulsions have greatly advanced our
basic understanding of these systems during the past decade.


7
Figure 1-3. A schematic diagram depicting the dynamic monomer-
micelle equilibrium in a micellar solution. Surfactant
monomers keep exchanging between the micelles and+
the bulk solvent with association rate constant k
and dissociation rate constant k The symbol "+"
represents the counterions of the surfactants.


8
1.4.1 Application of Fast Relaxation Techniques to
the Dynamic Study of Surfactant Solutions
The main objective of the kinetic study is to understand the
rate and mechanism of a chemical reaction when the reaction at
equilibrium relaxes back to its original or new equilibrium state
after a small perturbation on the thermodynamic parameters of the
system. Experimental techniques, such as stopped-flow, pressure-
jump, temperature-jump and ultrasonic relaxation, have existed for a
long time in conventional studies of fast chemical kinetics (68).
Research groups in Germany, Sweden and France as well as in Japan
have applied these techniques to study surfactant systems. These
studies have led to important conclusions about the aggregation
kinetics and dynamic structure of surfactant solutions. They will be
discussed in two categories as the kinetics of micellization and
dynamics of microemulsions respectively.
1.4.2 Kinetics of Micellization
The first documented attempt to measure the rate of micelle
breakdown was by Jaycock and Ottewill (69) in 1964. Using a
stopped-flow technique, they found that the breakdown of anionic
(SDS) and cationic (alkylpyridinium salts) micelles was rapid with
half-life on the order of 10 milliseconds. They also found qualita
tively that the rate of micelle breakdown could be expedited by
increasing the temperature, decreasing the surfactant chain length or
by decreasing the size of the counterion. However, the foundation of
relaxation kinetics of micellar solutions was established by a paper
(65) jointly published in 1976 by three research groups in France,
Germany and Sweden. A number of studies (70-117) on micellization


9
kinetics have been reported during the past ten years. It appears
that, through a combination of research efforts in both theoretical
and experimental aspects, the kinetic study has culminated into one
of the most fruitful approaches for the fundamental understanding of
micellar systems.
It is now well established that the relaxation of pure micellar
solutions is composed of two processes. One fast process is associ
ated with the exchanging of surfactant monomers between the micelles
and the bulk solution with a fast relaxation time x ^ in the order of
microseconds. The slow process is related to the micelle formation
and dissolution with a slow relaxation time I ^ in the order of mil
liseconds. Figure 1-4 schematically illustrates these two relaxation
processes. At low surfactant concentrations, micelles are formed
as described by equation
\ [1.1]
A
n
through a step-wise association process
[1.1]:
A1 + A1
A1 + A2
A1 + A3
A. + A T-
1 n-1


10
T.j (microseconds)
Slow
^(milliseconds)
Figure 1-4. A schematic diagram describing the mechanisms of two
relaxation processes observed in a pure dilute
surfactant solution. The fast relaxation process
is associated with the exchange of surfactant
monomers between the micelles and the bulk solution.
The slow relaxation process is related to micelle
formation and dissolution process .


11
while the overall reaction is
k+
n A. A [1.2]
I n
k"
where A^. denotes a surfactant monomer and A^ is an aggregate contain
ing n surfactants. Figure 1-5 represents a schematic size distribu
tion curve of aggregates in a micellar solution. The relaxation times
T j and T ^ were formulated by Aniansson et al. (65) through an anal
ogy to a heat conduction process as follows:
1/ Tj k"/ a2 + (k'/n)a [1.3]
1/ t2 = nV^R^tl + a 2a/n)_1 [1.4]
a = (C A1)/A1 [1.5]
where k is the dissociation rate constant of one surfactant monomer
from a micelle; o is the half-width of the Gaussian distribution of
micelles; n is the mean aggregation number of micelles; a is the
ratio of surfactants in micellar state to that in monomer state; A^
is the free monomer concentration in the bulk phase; C is the total
surfactant concentration; R is the rate-determining resistance of
micelle nucleus formation.


NUMBER DENSITY
12
Figure 1-5. A schematic size distribution curve of aggregates
in a micellar solution


13
Many experimental results obtained on micellar systems are to a
large part well described by the above theory. By applying this
theory to the experimental results, it is possible to determine two
statistical parameters of micellar solutions, namely the average
residence time of a monomer in micelles and the average life-time of
micelles. By plotting 1/ vs. a in equation [1.3], one can deter-
- 2
mine k /n from the slope and k / a from the intercept. Using the
value of n obtained from other techniques (e.g., light scattering,
fluorescence or osmotic pressure measurements), one can obtain both
the kinetic (k ) and statistical (n, a ) parameters of the micelles.
The mean residence time of a surfactant monomer in micelles is equal
to n/k and the life-time of micelles T.. is obtained from the fol-
M
lowing equation (113):
Tm = T 2na o + 02a/n) 1 [1.6]
When surfactant concentration is much greater than the CMC, T._ is
M
approximately equal to n Thus, the study of the fast relaxation
process provides information about the residence time of a surfactant
monomer in micelles, while the measurement of slow relaxation time
allows an estimation of average life-time of micelles. The experimen
tally determined values of various kinetic and statistical parameters
of sodium alkyl sulfate micelles are listed in Table 1-1.
According to Aniansson and Wall theory, the controlling step in
the micellar aggregation process is the formation of micelle nuclei,
which has been conceptualized in analogy to a resistance offered to a
pseudo-stationary flow of heat from one block to the other through a


Table 1-1 Values3 of Kinetic and Statistical Parameters
of Sodium Alkylsulfate Micelles at 25 C
Surfactant
CMC (mM)
n
a
k (see
-1)
r./k
(sec )b
T (sec)C
M
C.H..SO.Na
6 1J 4
420
17
6
1.32
X
109
13
X
Hf9
C8Hi7S4Na
130
27
1.0
X
108
0.27
X
io'6
:iOH21SVad
33
41
9
X
io7
0.45
X
io'6
0.35 x 10'3
!llH23SVa
16
52
4
X
io7
1.3
X
io'6
:i2H25S04Na
8.2
64
13
1
X
io7
6.4
X
io'6
ro
X
o
LO
:14H29SVa
2.05
80
16.5
9.6
X
io6
8.3
X
io'6
3.4
:.,H,,S0. Nae
16 33 4
0.45
100
11
6
X
104
1.7
X
io'3
37
a. All values are taken from Aniansson et al., J. Phys. Chem., 80, 905 (1976)
b. Average residence-time of surfactant monomers in micelles
c. Average life-time ( n T^) of micelles at critical micellization concentration
d. Measured at 40 C
e. Measured at 30 C


15
wire during the equilibration process of micelles (65). Any additive
which alters the micellar nucleus state will concomitantly influence
the micellar aggregation process. Kinetic measurements can provide
information about the micellar nuclei even though these species are
present in such small concentrations (10 ^ to 10 ^ M as reported
in reference 75) that they can usually be neglected in the mass bal
ance equation, and can not be detected by other techniques. The
experimental results have provided a new insight into the thermo
dynamics and kinetics of aggregation processes. In fact, it is due to
our better understanding of micellization kinetics that we are in the
position to vary the life-time of micelles over several orders of
magnitude by an appropriate choice of the chain length of surfac
tants, additives and counterions.
In addition to the above-mentioned step-wise association pro
cess, micelles can also form through a reversible coagulation-
fragmentation process of submicellar aggregates for ionic surfactants
at sufficiently high concentrations or for nonionic surfactants. This
process can be written as
A. + A.
1 J
' i+j
[1.7]
where A^, A^ are submicellar aggregates. A review article by
Kahlweit states clearly that "ionic micelles should be considered as
charged colloidal particles. At low counterion concentrations the
electrostatic repulsion prevents the coagulation of submicellar
aggregates so that micelles grow by incorporation of monomers only.
At high counterion concentrations, however, this reaction path is


16
bypassed by a reversible coagulation of submicellar aggregates. With
nonionic systems, both reaction paths compete right from the CMC
onwards." (83, page 2069). The slow relaxation time of this reversi
ble coagulation process is formulated as follows (83):
1/ T00 = 3 nail + a2a/n) *(A /A )^ [1.8]
22 K o g go
where is the average mean dissociation rate constant of equation
[1.7] in the absence of potential barrier; A^ is the counterion con
centration; A is the counterion concentration at the onset of
go
coagulation of submicellar aggregates; and q is a complex function of
the charge of counterions and of the surface potential of submicellar
aggregates.
Besides the above theories for pure micellar solutions, a theory
for the kinetics of mixed micellar systems has also been proposed
(73,116). This theory predicts two fast and one slow relaxation
processes for two-component mixed micellar systems. Two fast relaxa
tion processes are associated with the monomer exchange between the
micelles and the bulk solution for each of the two components, while
the slow relaxation process is the same as in the case of pure micel
lar solutions. The application of this theory to mixed systems of
alcohols and surfactants has received considerable attention due to
its relevance to microemulsions. A series of reports by Zana et al.
(94-98) on the effect of alcohols on micellar solutions demonstrates
the strength of combining both static and dynamic studies in probing
a complex surfactant system. Their results have experimentally veri
fied the kinetic theory for mixed micellar systems.


17
Although most of the dynamic studies have been focused on relax
ation time measurements, the analysis of relaxation amplitude (88-
90,110-111) has also been shown to provide information about the
dependence of CMC and mean aggregation number of micelles on tempera
ture, pressure or surfactant concentration, depending upon the per
turbation method applied to the system.
1.4.3 Dynamic Aspects of Microemulsions
The dynamic aspects of microemulsions have been investigated in
recent years by NMR, electron-spin probe, fluorescence and chemical
relaxation techniques (112). The results of these studies suggest
that microemulsions are highly dynamic in nature. Many dynamic
processes are reported in microemulsions (112), some of which will be
discussed in chapter 2. Similar to a micellar solution, there exists
a fast exchange of surfactants and cosurfactants between the interfa
cial film and the continuous or the dispersed phase. However, the
dynamic process of most concern in our study is probably the
emulsification-coalescence equilibrium through which microemulsions
are formed. This is schematically shown in Figure 1-6. The decrease
of interfacial tension with the adsorption of surfactants and cosur
factants onto the interface induces the formation of emulsion dro
plets by self-emulsification or spontaneous emulsification. The big
emulsion droplets may break further into small microemulsion droplets
under proper thermodynamic conditions. On the other hand, small
microemulsion droplets can also coalesce to form bigger droplets,
leading to an ultimate phase separation of oil and water. By adjust
ing the thermodynamic conditions, this dynamic equilibrium can be
shifted toward the end favoring microemulsion formation.


18
Figure 1-6. A schematic diagram depicting the dynamic
emulsification-coalescence equilibrium for
the formation of microemulsions.


19
One of the long-recognized thermodynamic conditions for the for
mation of microemulsions is the ultralow interfacial tension. When
the interfacial tension is low, the system favors the expanding of
interfacial area to form small microemulsion droplets. However, two
additional parameters of the interfacial film, namely interfacial
curvature and elasticity, are also important which have not yet
received equal attention before. These two parameters are related to
the organization and fluidity of the interfacial film, which would in
turn influence the dynamic structures and interdroplet interactions
of microemulsions. In fact, much of the dynamic character of
microemulsions originates from the thermal fluctuations of interfa
cial films. Highly fluid interfacial films can result in strong
interdroplet interactions which may shift the emulsification-
coalescence equilibrium toward phase separation.
1.5 Scope
The major thrust of this dissertation is to explore the dynamic
aspects of micellar and microemulsion systems, with emphasis on the
influence of alcohols, oils and salinity on the systems. The dynamic
properties of these systems are studied mainly in the following three
aspects: 1. kinetics of aggregation and dissolution of surfactant
aggregates in aqueous solution; 2. fluidity and curvature of the sur
factant interfacial film; 3. aggregate-aggregate interactions. The
correlation between dynamic and equilibrium properties of the systems
has also been particularly noted. The study starts with simple aque
ous micellar solutions. With additives such as alcohols and oils


20
added to the solutions, the study extends to mixed micelles and
microemulsions.
Following a brief introduction to the general equilibrium and
dynamic aspects of surfactant solutions and a review on the develop
ment of micellization kinetics of simple aqueous micellar solutions,
chapter 2 reviews the formation, structure and properties of
microemulsions.
The influence of alcohols on the equilibrium parameters of
sodium dodecyl sulfate (SDS) aqueous micellar solutions, i.e., the
CMC and the degree of counterion dissociation of micelles, is
reported in chapter 3. The factors which affect the CMC and the ther
modynamic stability of micelles are delineated. The solubilization
site of alcohols in micelles and its influence on the properties and
structure of the micelles are also discussed. Chapters 4 and 5
present experimental results on the effects of alcohols and oils on
the dynamic parameter, namely the slow relaxation time T^, of SDS
micelles. The results are basically explained by the change of
micelle nucleus concentration and the alteration of micellization
kinetics by the additives. A concept which distinguishes the thermo
dynamic stability from the kinetic stability of micelles is proposed.
Chapters 6 and 7 focus on the microemulsion system. Theoretical
aspects of the effects of the spontaneous curvature and elasticity of
interfacial films on the solubilization and phase equilibria of oil-
external microemulsions are presented in chapter 6. The effect of
both interfacial parameters on aggregate-aggregate interactions and
its consequences on solubilization in microemulsions are discussed.
Chapter 7 reports the experimental verification of the proposed


21
theory and further delineates the influence of the molecular struc
ture of various components of microemulsions on the interfacial cur
vature and elasticity, and its consequences on the solubilization and
phase equilibria of oil-external microemulsions. Some phenomenologi
cal guidelines for the formulation of microemulsions with maximum
solubilization capacity are suggested. Chapter 8 demonstrates the
use of reaction kinetics of AgCl precipitation as a probe for the
dynamic structure of microemulsions. Finally, Chapter 9 concludes
this dissertation with conclusions and recommendation for future stu
dies .


CHAPTER 2
REVIEW OF THE LITERATURE: FORMATION, STRUCTURE
AND PROPERTIES OF MICROEMULSIONS
2.1 Introduc tion
In 1943, Hoar and Schulman (118) first described a microemulsion
as a transparent or translucent system formed spontaneously upon mix
ing oil and water with a relatively large amount of ionic surfactant
together with a cosurfactant, e.g., an alcohol of medium chain length
(C^ to C-y). The system contained dispersion of very small oil-in
water (o/w) or water-in-oil (w/o) droplets with radii in the order of
o
100-1000 A. Figure 1-2 shows schematically these two basic microemul
sion structures. Since Hoar and Schulman's report, considerable
interest and attention have been focused on microemulsions. This can
be attributed to the fact that microemulsions possess special charac
teristics of relatively large interfacial area, ultralow interfacial
tension and large solubilization capacity as compared to many other
colloidal systems. These special features offer great potential for a
wide range of industrial and technological applications, e.g., terti
ary oil recovery, detergency, catalysis, drug delivery, etc.
In general, the formation of microemulsions involves a combina
tion of three to five components, namely, oil, water, surfactant,
cosurfactant and salt. The chemical structure of surfactant, cosur
factant and oil strongly influences a microemulsion phase diagram
22


23
(119-121). In fact, the complexity and diversity in properties,
structures and phase behavior of microemulsions have always posed a
persistent challenge for many theoretical and experimental research
ers. During the past decade, scientific literature on microemulsions
has grown at a fast pace. Several books, symposium proceedings and
review articles have been published (122-136). An exhaustive cover
age of all aspects of microemulsions is virtually impossible in this
chapter. Hence, the review will focus only on some fundamental ques
tions and some recent developments of microemulsions which are of
particular scientific interest or technological relevance.
At present, there exists no precise, or commonly agreed-upon,
definition of microemulsions. As a matter of fact, there has been
much debate about the terminology of "microemulsions," and as a
consequence, many other terms such as "swollen micelles" or "solubil
ized micelles" have been suggested (135). The debate centers on dis
tinguishing microemulsions from a true micellar solution (137-139).
Historically, microemulsions were defined from a phenomenological
viewpoint, i.e., the observation of a homogeneous, transparent and
low viscosity system containing a considerable amount of dispersed
phase with the presence of suitable surfactant and cosurfactant. At
very low volume fraction of dispersed phase, however, the system
actually resembles a true micellar solution. The transition between
these two structures generally shows no apparent break in many of the
physical properties of pure surfactant systems (140), but may exhi
bit a discontinuity for commercial mixed surfactant systems (141).
Based on a temperature dependence study of photon correlation spec
troscopy, Zulauf and Eicke (142) have established a clear transition


24
from Aerosol-OT reverse micelles in iso-octane to w/o microemulsions
at a water to Aerosol-OT molecular ratio about 10. But the relation
ship between normal micelles and o/w microemulsions is not as
straightforward. It has been shown that the kinetics of solubiliza
tion of oil is much slower for o/w microemulsions than that for nor
mal micelles (143). Hence, the key to this long-arguing problem prob
ably lies more in kinetic or dynamic measurements of the system
rather than in static measurements.
In spite of the controversy mentioned above, the designation of
a clear isotropic single-phase region in a phase diagram as
microemulsions does offer practical convenience in terminology. In
our opinion, a microemulsion can be defined phenomenologically as a
thermodynamically stable, isotropically clear dispersion of two
immiscible liquids, consisting of microdomains of one or both liquids
stabilized by an interfacial film of surface-active molecules.
2.2 Spontaneous Emulsification and Thermodynamic
Stability of Microemulsions
Two most fundamental questions in dealing with a microemulsion
are probably the mechanism of microemulsion formation and its thermo
dynamic stability as compared to a conventional emulsion, i.e., a
macroemulsion. A macroemulsion, upon standing, has been known to
coalesce and eventually to separate into an oil and water phase due
to a lack of thermodynamic stability (144). However, it has been
pointed out that some emulsion systems may be thermodynamically
unstable but could exhibit long term stability for practical purposes
(124). This has been referred to as "kinetic stability" of the sys
tem due to a high energy barrier for coalescence between droplets


25
(124). The distinction between thermodynamic stability and kinetic
stability of a system is probably only a matter of concern from a
thermodynamic rather than an operational point of view.
2.2.1 Ultralow Interfacial Tension
One of the early, important contributions from Schulman and
coworkers was the realization that the stabilization of microemul
sions required a low solubility of surfactant (or surfactant mixture)
in both oil and water phases (145), resulting in the adsorption of
the surfactant at the water-oil interface to lower the interfacial
tension. This can be described by the well-known Gibbs adsorption
isotherm for multiple-component systems (146):
d/ = Z T. dy. = zr.RTdUna.) [2.1]
i i i i i 1
where y is the interfacial tension, p^ is the surface excess of com
ponent i (amount of component i adsorbed per unit area), y ^ is the
chemical potential of component i, and a^ is the activity of the
solute i. Equation [2.1] basically dictates that the increase of
surfactant activity a. in the solution would result in a decrease of
interfacial tension if the surface excess of the surfactant is posi
tive. Moreover, the addition of a second positively adsorbed surfac
tant to the system would always cause a further decrease in interfa
cial tension. Hence, it has been proposed that the role of cosurfac
tant, together with the surfactant, is to lower the interfacial ten
sion down to a very small--even a transient negative--value at which
the interface would expand to form fine dispersed droplets and


26
subsequently adsorb more surfactants and cosurfactants until their
bulk concentration is depleted enough to make interfacial tension
positive again. This process, known as "spontaneous emulsification,"
forms the microemulsion.
The concept of transient negative interfacial tension and its
relation to spontaneous emulsification have been proposed and experi
mentally examined for some time (147-150). The value of this concept
is to emphasize the importance of ultralow interfacial tension for
the formation and thermodynamic stability of microemulsions. In fact,
the mechanism of microemulsion formation has been analyzed by Ostrov
sky and Good (151) based on a dynamic equilibrium process in which
the rate of self-emulsification is equal to the rate of coalescence
of microemulsion droplets. The analysis established a boundary of
interfacial tension between a thermodynamically stable microemulsion
and an unstable macroemulsion. For interfacial tensions lower than
-2
10 dyne/cm, stable microemulsions can be obtained. In other ther
modynamic models (152-159), lower inter facial tensions in the order
-4 -5
of 10 to 10 dyne/cm have been employed to satisfy the stable con
dition of microemulsions.
It is known that some surfactants, e.g., many double-chain sur
factants and nonionic surfactants, can form microemulsions without
the addition of a cosurfactant (160-161). Although this has been
attributed to different abilities of surfactants in lowering the
interfacial tension (133), it seems that additional factors besides
the ultralow interfacial tension may have to be considered for a com
plete explanation. In fact, the interfacial bending instability
resulting from the thermal fluctuations of interface and the


27
dispersion entropy of droplets in the solution may also contribute
significantly to the formation of microemulsions when the interfacial
tension is low.
2.2.2 Interfacial Curvature, Fluidity and Entropy
The formation of small microemulsion droplets requires a bending
of the interface. It has been shown by Murphy (162) that the bending
of an interface requires work to be done against both interfacial
tension and bending stress of the interface. Although always
present, the bending stress is important only for very low interfa
cial tension or highly curved interfaces. This can be described
schematically by Figure 2-1. At an equilibrium condition with very
low interfacial tension, an interface would assume an optimal confi
guration and curvature, known as the spontaneous curvature 1/R^, at
which the bending energy of the interface is minimized. Further
bending the interface away from this spontaneous curvature will cause
an increase in bending energy, which can be represented by a constant
K, known as the curvature elasticity (or bending elasticity) of the
interface. The constant K with the unit of energy actually dictates
the ease of interfacial deformation. A large K value corresponds to
a "rigid" interface for which large energy is required to bend the
interface. A small K value represents a "fluid" interface for which
little energy is necessary for bending. Hence, K is also called the
"rigidity constant" of the interface. When K is close to k^T, where
k^ is the Boltzmann constant, the interface is subject to a bending
instability resulting from thermal fluctuations.


28
*
Curvature Elasticity
(Rigidity Constant)
Figure 2-1. A schematic diagram for spontaneous curvature and
' curvature elasticity of an interfacial film.
The filled circles represent oil molecules penetrating
into the interfacial film.


29
Safran and Turkevich (163) have expressed the interfacial free
energy F^. of microemulsion droplets in terms of both interfacial ten
sion and bending energy for an uncharged interface:
FT = n [ 4 ttXR2 + 16 1TK R/R )2] [2.2]
1 o
where n is the number density of droplets, ) is the interfacial ten
sion, R is the droplet radius and Rq the radius of spontaneous curva
ture (or the natural radius). Equation [2.2] only contains the ener
getic term, and the entropic term of interface will be discussed
later. Equation [2.2] is applicable to ionic w/o or nonionic
microemulsions where the electrostatic energy can be neglected. The
-14
value of K has been found to be in the order of 10 erg for
microemulsions (164); hence the bending energy term is important only
when y is close to zero. Accordingly, Murphy (162) has concluded
that a planar interface having a low but positive interfacial tension
could nevertheless be unstable with respect to thermal fluctuations
if the reduction in interfacial free energy due to bending exceeds
the increase in free energy due to expansion of the interface.
Therefore, he suggested that the bending instability at low interfa
cial tension might be responsible for spontaneous emulsification.
The preceding discussion focused on the effect of thermal fluc
tuations on a "fluid" interface (small K). Although the role of mem
brane "fluidity" for the formation of microemulsions has been noted
earlier (148), its significance is better elucidated by recent
theories and experimental results (164-166). It has been shown
(164-165) that when K is larger than kfeT, oil, water and surfactant


30
may form a birefringent lamellar phase, and only when K is small,
isotropic disordered microemulsions are obtained. A lamellar
birefringent phase is often observed in the vicinity of a microemul
sion phase in the phase diagram (167). The addition of cosurfactant
is found to increase the fluidity of the interface, leading to a
structural transition from birefringent lamellar phase to isotropic
microemulsions (164,166). In practice, the fluidity of an interface
can be increased by choosing a surfactant and co-surfactant with
widely different sizes of the hydrocarbon moiety (148), or by setting
a temperature so that there is a balance between hydrophilic and
lipophilic properties of the surfactant (168).
The thermal fluctuations of a fluid interface lead to an
increase in the entropy of interfacial film. The entropy of such a
fluctuating interface has been approximated by the mixing entropy of
oil and water (124). The decrease in free energy of the system due
to this dispersion entropy may exceed the increase of free energy
caused by newly created interfacial area due to emulsification, thus
resulting in spontaneous emulsification and stabilization of a
microemulsion. This has been quantitatively accounted for on the
basis of phenomenological thermodynamics by many researchers (152
159). Be cause excellent reviews on various thermodynamic models of
microemulsions have been published (124,133), only certain important
concepts and results will be described here.
Ruckenstein and Chi (152) have expressed the Gibbs free energy
change of microemulsion formation by three terms:
AGm(R) = Agl +ag2 +ag3
[2.3]


31
where A is the interfacial free energy including a positive term
due to creation of uncharged interface and a negative term due to the
formation of electric double layer; A G^ is the free energy of inter
droplet interactions composed of a negative term due to van der Waals
attraction and a positive term due to repulsive double layer interac
tion; A G^ is the entropy term accounting for dispersion of
microemulsion droplets in the continuous medium. From equation
[2.3], the condition for spontaneous formation of microemulsions with
the most stable droplet size (R ) at a given volume fraction of
dispersed phase may be obtained:
( 3Agm/3r)r=r* = 0 [2-41
(32AG /3R2) > 0 [2.5]
m kK"
Equations [2.4] and [2.5] indicate that a negative, minimum
k
A GAR ) is required to obtain a stable microemulsion as shown in
curve A of Figure 2-2. Curve B in Figure 2-2 represents a kineti-
cally stable macroemulsion providing that the height of energy max
imum is significant, and curve C corresponds to an unstable emul
sion. Figure 2-3 shows the influence of interfacial tension on the
formation of microemulsions. When interfacial tension is less than 2
-2
x 10 dyne/cm, a stable microemulsion can be formed. Figure 2-4
shows the individual contribution of the three terras in equation
[2.3] to the stability of microemulsions. The dispersion entropy
predominantly contributes to the thermodynamic stability of
microemulsions. Rosano and Lyons (169) using a titration method have


32
Figure 2-2. A schematic illustration of the Gibbs free energy
change of microemulsion formation AG as a function
of droplet radii R. Curve A shows a stable micro
emulsion with droplet radius R* at the minimum AG .
Curve B shows a kinetically stable emulsion and M
curve C an unstable emulsion


33
a Gm (cal-cm3)
Figure 2-3. The influence of interfacial tension on the formation
of microemulsions. Small but positive values of
interfacial tension can result in a stable microemulsion.


34
0,001
o
-0.001
Figure 2-4.
The contribution of AG^, AG^ and AG^ to the free
energy of microemulsion formation


35
shown that the formation of microemulsions is indeed entropically
driven. Ruckenstein's model further predicts a phase inversion from
one type of microemulsions to another, i.e., w/o to o/w, as well as a
phase separation (152-157).
As a criterion for the formation of a thermodynamically stable
dispersion system with low interfacial tension, an inequality has
been proposed (151):
- (d lny)/(d InR) 2 [2.6]
Though the form of this inequality may differ depending on different
thermodynamic treatments (151), many analyses do agree upon a similar
trend that for microemulsions the average equilibrium radius of dro
plets increases with decreasing interfacial tension (159,170), but
the reverse is true for macroemulsions (151).
To recapitulate the discussion so far, it is concluded that the
spontaneous formation of microemulsions with decrease of total free
energy of the system can only be expected if the interfacial tension
is so low that the free energy of the newly created interface can be
overcompensated by the dispersion entropy of droplets in the medium.
The bending instability resulting from the thermal fluctuations of
interface with low tension and high fluidity could be responsible for
spontaneous emulsification. Two necessary conditions for the forma
tion of microemulsions are as follows:
(1) Large adsorption of surfactant or surfactant mixture at the
water-oil interface. This can be achieved by choosing a surfactant
mixture with proper hydrophilic-lipophilie balance (HLB) for the


36
system. One can also employ various methods to adjust the HLB of a
given surfactant mixture, such as adding a cosurfactant, changing
salinity or temperature etc.
(2) High fluidity of the interface. The interfacial fluidity
can be enhanced by using a proper cosurfactant or an optimum tempera
ture.
The role of cosurfactant in microemulsion formation is to (a)
decrease the interfacial tension; (b) increase the fluidity of the
interface; and (c) adjust the HLB value and spontaneous curvature of
the interface leading to the spontaneous formation of microemulsions.
2.3 Geometric Aspects and Structure of Microemulsions
Two types of most commonly encountered microemulsions are o/w
and w/o globular droplets as shown in Figure 1-2. Some theories such
as mixed (or duplex) film theory (147-148,171-172), "R" theory (173)
and the concept of hydrophilic-lipophilic balance of surfactant
(174-175), have long been proposed in attempt to delineate the fac
tors which determine the formation of a specific structure, i.e., w/o
or o/w, for a given water-oil-surfactant system. Recently, a
geometric model concerning the surfactant packing at the interface
has also been proposed (170,176). All these theories define certain
parameters which can dictate the curvature of a given interfacial
film and hence predict the corresponding structure. Since reviews of
these theories are in the literature (124,141,177), only the
geometric model will be discussed.


37
2.3.1 Geometric Packing Considerations in Atnphiphile Aggregation
Basically, the geometric model emphasizes the importance of
geometric constraints in the packing of amphiphiles at the interface
for determining the structure and shape of amphiphilic aggregates.
Following the concept of duplex film, which was first proposed by
Bancroft (178) and Clowes (179), and later applied to microemulsions
by Schulman (147), the model essentially considers the interfacial
film as duplex in nature, i.e., the polar heads and hydrocarbon tails
of amphiphiles are acting as separate uniform liquid interfaces, with
water hydration in the head layer and oil penetration in the tail
layer. The key element of describing the geometric packing of sur
factants at the interface is a packing ratio defined as the ratio of
cross-sectional area of hydrocarbon chain to that of polar head of a
surfactant molecule at the interface, v/a 1 where v is the volume
o c1
of hydrocarbon chain of the surfactant, a^ is the optimal cross-
sectional area per polar head in a planar interface, and 1^ is
approximately 80-90% of the fully extended length of the surfactant
chain (176).
The direction and the degree of interfacial curvature are basi
cally a result of this packing ratio and are further influenced by
differential tendency of water to swell the head area and oil to
swell the tail area. It is intuitively clear that a greater cross-
sectional area of tail than that of head (v/a 1 >1) will favor the
o c
formation of w/o droplets, while a smaller cross area of tail than
that of head (v/a 1 < 1) would favor the o/w droplets. A planar
o c r
interface requires v/a 1 =1 which leads to the formation of
o c


38
lamellar structure. Figure 2-5 schematically depicts the above
descript ion.
Assuming that the optimal head area a^ will not change with
interfacial curvature, Mitchell and Ninham (176) suggested a neces
sary geometric condition for the existence of o/w droplets,
1/3 < v/aolc < 1 [2.7]
Equation [2.7] predicts the formation of (1) normal micelles for
v/aQlc < 1/3; (2) o/w droplets for 1/3 < v/aQlc < 1; and (3) w/o
(inverted) droplets for >1. It should be pointed out that
the increase in packing ratio v/a^^ corresponds to an increase in
o/w droplet size, but to a decrease in w/o droplet size. The boun
dary at v/aQlc = 1 indicates a structural transition from o/w to w/o
droplets. The structure and molecular mechanism of this phase inver
sion domain remain poorly understood and will be discussed next.
Similar geometric criteria have also been proposed to describe the
structure of biological lipid aggregates (180). These results seem
to suggest that the geometric packing of amphiphiles plays an impor
tant role in determining the structure and shape of aggregates.
One of the advantages of this geometric model is that the pack
ing ratio can quantitatively account for the HLB of a surfactant. A
low HLB value in the range of 4-7 favoring w/o emulsions corresponds
to v/aQlc > 1, while a high HLB value in the range of 9-20 favoring
o/w emulsions corresponds to v/aQlc < 1. Further, taking the
geometric packing term into account in a thermodynamic model can
serve as a simple approach to establish a unified thermodynamic


Increasing Chain Area and
Oil Solubility of Surfactant
< 1
Oil-In-Water
Chain
m-
t
Head
v/a0lc = 1
Water Solubility of Surfactant
v/a0lc > 1
Figure 2-5. A schematic diagram representing the interfacial curvature of o/w and
w/o microemulsions and phase inversion based on the geometric model


40
framework for amphiphilic aggregation (176). In addition, the con
cept of the geometric model can easily account for the influence of
salt, cosurfactant and oil on interfacial curvature. For a simple
water-oil-surfactant system, a surfactant with bulky head group and
relatively small tail area, like some single-chain surfactants, tends
to form o/w droplets. To obtain w/o droplets in this case, one has
to employ cosurfactant (e.g., medium-chain alcohols), high salinity,
or oil with a smaller molecular volume or a higher aromaticity in the
system. The incorporation of cosurfactant in the interface is
expected to increase the mean hydrocarbon volume per surfactant
molecule without affecting appreciably either aQ or 1^ (176). The
addition of salt is expected to decrease head area a^ due to the
suppression of electric double layer. Oil with smaller molecular
volume or higher aromaticity can enhance the penetration of oil into
the surfactant layer thus increasing the surfactant hydrocarbon
volume (176,181-183). All these effects tend to increase the packing
ratio, hence favoring the formation of w/o droplets. On the other
hand, a surfactant with a laterally bulky hydrocarbon part and a
relatively small head group, like some double-chain surfactants,
favors the formation of w/o droplets. This can explain why Aerosol-
OT (sodium bis-2-ethyl hexyl sulphosuccinate) forms w/o microemul
sions spontaneously without the addition of a cosurfactant.
The effect of temperature on packing ratio is difficult to
predict due to a lack of understanding of all forces in the system.
However, experimental data for biological lipid and non-ionic surfac
tant systems seem to suggest an increase in v/a 1 with increasing
o c
temperature (176). This can be explained partly by the decrease in


41
water hydration of the head group (decreasing aQ) at elevated tem
perature (184). One thus expects a growth of normal micelles formed
by nonionic surfactants with increasing temperature due to increasing
v/aolc until the cloud point (185), beyond which phase separation
occurs. On the other hand, flocculation of micelles due to attrac
tive interaction between micelles at elevated temperature has also
been observed (186-188). At even higher temperatures, known as the
phase inversion temperature (PIT), phase inversion from normal to
inverted micelles occurs (161,189-190). At this PIT, one expects
v/a 1 =1 and thus a zero curvature,
o c
2.3.2 Non-Glcbular Domain and Microemulsion Structure
in Phase Inversion Region
Apart from the consideration of geometric packing presented
above, two additional geometric constraints have to be observed for
the existence of globular structure in a ternary phase diagram (191).
First, there exists an upper limit of 0.64 as the maximum volume
fraction of dispersed droplets in the solution according to a simple
random close-packing model of hardspheres (192). Second, there must
also exist a lower limit of the polar head area a below which elec-
o
trostatic repulsion between polar heads increases. Such a limit
imposes a lower bound on the size of w/o droplets because decreasing
size requires a decreasing polar head area aq and/or an increasing
hydrocarbon volume v according to the geometric model. But this con
straint does not apply to o/w droplets because aQ increases as the
droplet size diminishes.


42
Taking these two constraints into account, Biais et al. (191)
have identified some domains which cannot have any globule in the
ternary phase diagram shown in Figure 2-6(a). Figure 2-6(b) shows
the experimentally determined region where no globules are observed
(193). Instead, a lemellar structure has been found in region 2. In
region 1, the solution probably consists of small hydrated soap
aggregates solvated by alcohol molecules, and dispersed in the oil
medium (193).
It has also been proposed, as shown in Figure 2-6(a), that at
least two different mechanisms of phase inversion are possible (191).
Path 1 indicates a continuous transition from w/o to o/w with an
intermediate region in which o/w and w/o droplets may coexist
(190,194-195), or a bicontinuous structure has been suggested (196-
197). A discontinuous transition is also possible along path 2
through a structure that cannot be spheres. Usually, a birefringent
lamellar structure has been observed in this case (193).
According to the prediction of the geometric model, zero curva
ture is expected at phase inversion, thus justifying the existence of
lamellar structure along the path 2. However, the rationale for the
continuous transition along path 1 is not so obvious. In fact, the
mechanism and structure of this continuous phase inversion remain
poorly understood. Talmon and Prager (198) have proposed a statisti
cal mechanical model of bicontinuous structure to account for this
continuous phase inversion without a priori concerning about the
geometric features of aggregates. It was based on a Voronoi tessela-
tion followed by random segregation of oil and water domains. Subse
quently, de Gennes et al. (165,199) proposed a modification by


43
SURFACTANT
Figure 2-6. Nonglobular microemulsion domains (a) Two nonglobular
domains due to the close-packing constraint (hatched
area) and the limitation of minimum head area (dotted
area). Two possible mechanisms of phase inversion
are shown.


44
Butanol/SDS = 2
Water Toluene
Figure 2-6. Continued, (b) An experimentally determined nonglobular
domain (dotted area) in water/toluene/butanol/SDS
microeraulsion system.


45
taking a cubic lattice model instead of the Voronoi tesselation. It
was shown (165) that a persistence length £ ^ can be defined as the
characteristic length of the water-oil interface. The value of £ ^
increases exponentially with increasing curvature elasticity K of the
interface. An isotropic microemulsion phase can exist when K and
consequently are small; otherwise, periodic ordered structures
such as lamellae are expected. This result elucidates the importance
of fluidity and thermal fluctuations of interface for the formation
of microemulsions. It further delineates the correlation between
isotropic random microemulsion phase and periodic ordered phase of
lyotropic nematics. In addition, the physical meaning of elementary
size of a system without well-defined geometry such as the bicontinu-
ous structure has been clarified. The bicontinuous structure (196-
197) is envisioned as containing continuous interpenetrating domains
of both oil and water with neither one surrounding the other. Though
the equilibrium mean curvature of the interface is zero, complying
with the prediction of geometric model, the interface is constantly
subject to thermal fluctuations resulting in continuous sinusoidal
bending with no specific preference toward either water or oil
phases. It should be mentioned that in addition to the spherical and
bicontinuous structures discussed above, other microemulsion struc
tures, such as cylinders and lamellae, have also been proposed (200).
Phase inversion phenomena bear important technological
relevance. In general, one can obtain phase inversion by changing a
large number of variables in a systematic manner. Of greatest impor
tance among these changes are to increase the volume fraction of
dispersed phase, to vary the salinity of the system, and to adjust


46
the temperature. When salinity is varied, a middle-phase microemul
sion with equal solubilization of brine and oil can be obtained at
phase inversion salinity--the so-called "optimal salinity." This has
important implications for tertiary oil recovery because maximum
solubilization and ultralow interfacial tension can be obtained at
this optimal salinity. More details of this will be discussed later.
Shinoda and Kunieda (161) have also established that maximum solubil
ization can be obtained for nonionic surfactant systems at phase-
inversion temperature (PIT). Maximum or optimal detergency is often
obtained at the vicinity of PIT, i.e., the cloud point (201-202).
2.3.3 Design Characteristics of Microemulsions
Based on all the preceding discussions, it can be concluded that
the geometric features (or HLB) of a surfactant play an important
role in determining the formation and structure of microemulsions.
To design a microemulsion, the use of surfactant or surfactant mix
ture is required to lower the interfacial tension according to equa
tion [2.1]. But the addition of alcohol is not a theoretical require
ment, although alcohol is often used to fluidize the interfacial film
(decrease K). Actually, one can also obtain a fluid interfacial film
by using a double- or branched-chain surfactant at temperature above
the thermotropic phase transition temperature (203). However, when a
cosurfactant is not used in microemulsions, it is a necessary but not
a sufficient condition that surfactant hydrocarbon volume v, effec
tive chain length lc and head group aQ should satisfy the relation,
v/aQlc = 1, as the elementary design characteristic for a simple
three-component, namely oil, water and surfactant, microemulsion


47
system (160). Several other variables such as the chemical nature of
cosurfactant and oil, salinity and temperature can alter the packing
ratio. Thus many parameters are available for manipulation in design
and formulation of microemulsions.
2.3.4 Shape Fluctuations and Structural Dynamics of Microemulsions
Thus far, our discussion has focused mainly on the equilibrium
structure and character of microemulsions. It should be mentioned
that, on one hand, the thermal fluctuations of interface result in a
thermodynamic stability of microemulsions, but on the other hand a
highly dynamic character of microemulsions also results due to ther
mally induced size and shape fluctuations (polydispersity) of spheri
cal microemulsions (204-205). In fact, a microemulsion should be
viewed as a dynamic structure (204). They are thermodynamically
stable, but there is a constant coalescence, break-down and deforma
tion of microemulsion droplets. The picture of microemulsions as
persistent entities having definite geometric shape is not accurate.
The structural dynamics of microemulsions have been investigated
by a variety of techniques and methods, such as nuclear magnetic
resonance (NMR), electron spin resonance (ESR), chemical relaxation
techniques, chemical reaction or fluorescence quenching kinetics in
microemulsions (112), and quasi-elastic light scattering (206-207).
The results of NMR and ESR studies confirm that there exists a con-
- 8
stant and fast exchange (characteristic time on the order of 10
-9
10 second) of microemulsion components (e.g., surfactant and cosur
factant) between the interfacial film and the continuous phase (112).
This corroborates the view that the interfacial film of


48
microemulsions is highly fluid. Further, the content of microemul
sion droplets, specially w/o droplets, is found to be rapidly
exchanged between the droplets through collisions and formation of
"transient dimers." This is evidenced by studying the kinetics of
chemical reactions and fluorescence quenching in microemulsions
(112,208-211). The formation of dimers has been attributed to "sticky
collisions" between droplets resulting from attractive interdroplet
interactions as suggested by neutron and light scattering studies
(212-215). Such an exchange process and formation of dimers have
important relevance to the chemical reactions occurring in microemul
sions. This will be demonstrated in chapter 8. The study of dynamic
aspects of microemulsions has actually advanced the fundamental
understanding on the stability, fluidity of interface, interaction
forces and collision rate of microemulsion droplets.
2.4 Solubilization and Phase Equilibria of Microemulsions
2.4.1 Solubilization and Structure of Microemulsions
Solubilization is one of the most salient features of the
microemulsion system from which most applications stem. Many early
studies of solubilization reported in a classic book by Laing et al.
(216). are based on simple soap (micellar) solutions, i.e. the abil
ity of surfactants to increase the solubility of hydrophobic com
pounds in water. Since Marsden and McBain (217) published one of
the very first phase diagrams illustrating solubilization phenomena
in a solution, the field of solubilization has expanded considerably.


49
Figure 2-7 presents a series of schematic ternary or pseudoter
nary (in which two components are grouped at the same vertex)
microemulsion phase diagrams. These diagrams show the changes of
general features of microemulsions when varying the alcohol chain
length, and varying the surfactant from single-chain to double-chain,
or from ionic to nonionic surfactant. Each clear microemulsion phase
region represents a solubilization area with a specific structure.
The two mechanisms of phase inversion from o/w (L^) to w/o (L^)
microemulsions described earlier can be seen in Figure 2-7. The con
tinuous phase inversion is often observed when a short-chain cosur
factant is used (K is very small), resulting in a large connecting
homogeneous solubilization area (Figure 2-7(a) and 2-7(c)). A
discontinuous phase inversion is seen in most cases with and
regions separated by some intermediate liquid crystal regions.
The factors which determine solubilization have not been com
pletely delineated. However, based on current theories and under
standing of solubilization (156-157,163,198-199,216,218-223), some
important parameters can be identified on a qualitative basis. It
has been shown that three solubilization sites are possible in a sur
factant aggregate (224-227). Taking a normal micelle as example,
hydrocarbons and other nonpolar compounds are thought to be incor
porated in the micelle interior (swollen micelles, Figure 2-8(a)).
Some solubilizate molecules may distribute themselves among the sur
factant molecules at the interface (Figure 2-8(b)). Polar solubil
izate molecules may adsorb at the micellar surface (Figure 2-8(c)).
Here the discussion of solubilization is limited only to the first
case, which is relevant to the formation of swollen micelles or
microemulsions.


50
Butanol/SDS = 2 b Pentanol/SDS = 2
Sodium Caprylate d Sodium Caprylate
Figure 2-7. Schematic ternary (or pseudoternary) phase diagrams of
various microemulsion systems. represents a normal
micelle or o/w microemulsion region. shows a reverse
micelle or w/o microemulsion region. M represents a
middle-phase microemulsion. B refers to anisotropic
phases, a & b are based on reference 119; c & d on
reference 244; f is shown at phase inversion temper
ature. Note that detailed liquid crystalline regions
are not shown


51
AOT f Nonlonlc Surfactant
Figure 2-7. Continued.


52
Figure 2-8. A schematic view of three possible solubilization
sites in surfactant aggregates, namely (a) the
micelle interior; (b) the palisade layer; and
(c) the micellar surface.


53
From a simple geometric calculation, it can be shown that the
total solubilization volume V in a microemulsion is equal to:
V = A R/3 [2.8]
where At is the total interfacial area and R is the radius of dro
plets. A^ is related to the total emulsifier concentration in a sys
tem. At constant total emulsifier concentration, the solubilization
is directly related to the droplet radius and hence the curvature of
the interface (163,199,219-222). Therefore, solubilization depends
on the structure of microemulsions. Equation [2.8] predicts that
solubilization is large when R approaches infinity (zero curvature).
This explains the maximum solubilization observed at phase inversion
region as discussed earlier. Further, one often observes a smaller
solubilization area in o/w (L^) than in w/o (l^) microemulsions as
shown in Figure 2-7. This can be attributed partly to the highly
curved interface (large aQ) associated with o/w droplets resulting
from strong electric repulsion between polar heads and from strong
water hydration of polar heads (for nonionic surfactants). Increas
ing salinity or temperature (for nonionic surfactants) can decrease
aQ and consequently decrease the curvature, thus increasing the solu
bilization. It is also generally observed that that w/o microemul
sions form more readily than o/w microemulsions (161,228).
The above analysis focuses only on the influence of interfacial
curvature (or bending energy) on solubilization. The interaction
between microemulsion droplets can also influence the stability,
structure and hence the solubilization of microemulsions


54
(165,223,229). The long-range electrostatic repulsive force in aque
ous micellar solutions at higher surfactant concentrations can lead
to a structural transition from isotropic micelles to an anisotropic
ordered structure such as hexagonal or lamellar phase (230). On the
other hand, attractive force between droplets can cause coagulation
or coalescence between droplets, and consequently a phase separation
of microemulsions (165,223,229). Coagulation of o/w droplets can usu
ally be obtained by increasing the salinity of the system (231-232),
while increasing the fluidity of interface leads to coalescence of
w/o droplets (233-234). In any of the above events, a corresponding
change in solubilization is usually observed.
Apart from the above viewpoint that solubilization depends on
structure and properties of microemulsions, solubilization itself
also induces changes in shape, size and structure of microemulsions
(235-238). Hence, solubilization, structure and properties of
microemulsions are all interrelated.
2.4.2 Phase Equilibria of Microemulsions
When the limit of solubilization of a microemulsion is reached,
phase separation occurs and the microemulsion phase can coexist in
equilibrium with other phases. The phase equilibria of microemul
sions are conventionally described by a phase diagram with tie lines
as shown schematically in Figure 2-7(d).
According to the Gibbs phase rule, the degrees of freedom of a
given system at constant temperature and pressure are equal to:
C P
[2.9]


55
where C is the number of components, and P is the number of phases in
the system. Thus, a general four-component microemulsion system,
namely oil-water-surfactant-cosurfactant system, can be constituted
by one, two, three or four phases in equilibrium. Consequently, the
approach of studying microemulsions becomes a matter of choice
depending on the problem of concern. The most popular approach is to
study the one-phase microemulsion region. However, the study of two-
and three-phase equilibria is important for understanding the stabil
ity and interaction forces in microemulsions. This will become more
clear as our discussion proceeds. Such a phase-equilibrium approach
is also useful for determining the composition of phase boundary.
At this time, there is very little known about four-phase
equilibria of microemulsions (239-240), hence only two- and three-
phase equilibria will be discussed. At least three types of
two-phase equilibria in microemulsions have been elucidated: (i)
Microemulsions in equilibrium with excess internal phase (i.e., w/o
microemulsions with water or o/w microemulsions with oil). This type
of phase equilibria is driven by the bending stress (or curvature) of
the interfacial film (163,199,219-222), and the phase separation
occurs due to the resistance of interfacial film to bending for
growth of microemulsion droplets. (ii) Two isotropic microemulsions
phases (containing high and low density of droplets respectively)
coexist. This phase separation is driven by attractive interdroplet
interactions (165,223,229). Critical-like behavior and sometimes a
critical point may be observed in this case (2,233-234,241-243).
(iii) Both w/o and o/w microemulsion phase coexist (221). This phase
equilibrium is driven by the balance of hydrophilic and lipophilic


56
property of a surfactant (i.e., equal solubility of surfactant in
both oil and water). Experimentally, one often observes this type of
phase equilibria at very low surfactant concentrations (244). At
sufficiently high surfactant concentrations, birefringent mesophases
are often present between w/o (L^) and o/w (Lt) microemulsions
(discontinuous phase inversion); hence no direct phase equilibrium
between w/o and o/w is observed. It has further been shown that the
first two types of phase equilibria together can give rise to three-
phase equilibria of microemulsions (i.e., microemulsions in equili
brium with both excess oil and water) when both bending stress and
attractive force act in parallel upon the system (163,165,199,245).
Some theoretical treatments of the above-mentioned phase equilibria
of microemulsions can be found in the literature (163,165,199,219
223,229,245). It can be concluded that the study of phase equilibria
leads to a better understanding of the stability of microemulsions
and serves as a simple measure to assess the driving force for phase
separation of microemulsions. This is important for the design and
formulation of microemulsions.
2.4.3 Phase Behavior of Winsor-Type Microemulsion Systems
The most studied phase equilibria in microemulsions are probably
the Winsor type microemulsions (246-248) using a salinity scan as
shown in Figure 2-9(a). One can prepare such a system by mixing
equal volumes of brine and oil with a proper surfactant and cosurfac
tant. By increasing the salinity, one observes a progressive change
in phase diagram and behavior as described by Figure 2-9(a) and 2-
9(b).


57
s s s
Increasing Salinity *
Figure 2-9. A schematic presentation of a typical Winsor-type
microemulsion showing the progression of phase
diagrams, phase volumes and interfacial tensions by
salinity scan. M, W, 0 represent microemulsion,
excess water and excess oil phases respectively.
represents the interfacial tension between the
microemulsion and excess oil phases, and y^ is
the interfacial tension between the microemulsion
and excess water phases .


58
In the low salinity region, the Winsor I system represents a
lower-phase o/w microemulsions in equilibrium with excess oil. In
the high salinity region, the Winsor II system consists of an upper-
phase w/o microemulsion in equilibrium with excess brine. It is
clear that both Winsor I and Winsor II phase equilibria are driven by
the bending stress of interfacial films.
In the intermediate salinity region, the Winsor III system is
composed of a middle-phase microemulsion in equilibrium with both
excess oil and brine. The optimal salinity is defined as the salin
ity at which equal volumes of brine and oil are solubilized in the
middle-phase microemulsion. The structure of this middle-phase
microemulsion has not been determined conclusively. Based on the
data of ultracentrifugation, Hwan et al. (195) proposed that the
middle phase is a o/w microemulsion near the boundary close to low
salinity region, and a w/o microemulsion near the boundary close to
high salinity region. Thus, a middle-phase microemulsion at the
optimal salinity would represent a continuous phase inversion from
o/w to w/o structure. A bicontinuous structure (196) has been pro
posed for the middle-phase microemulsion at optimal salinity and has
been widely examined both experimentally and theoretically
(194,223,249-253).
It is the attractive force between microemulsion droplets that
leads to a transition of both Winsor I and Winsor II to Winsor III
microemulsions (223). The transition from Winsor I to Winsor III
microemulsions has been attributed to the coacervation of normal
micelles (232), while the transition from Winsor II to Winsor III
microemulsions is associated with the percolation phenomena of w/o


59
droplets (254-259). Both these transitions have also been associated
with critical phenomena (241). Thus the phase equilibria of Winsor
III systems are governed by both attractive forces between droplets
and interfacial bending stress.
Apart from the conventional salinity scan, the transition of
Winsor type systems from o/w to w/o structure can also be produced by
changing any of the following variables in a systematic way
(232,245,253): (1) increasing the alkyl chain length or molecular
weight of surfactant; (2)increasing the surfactant concentration; (3)
increasing the aromaticity of oil; (4) decreasing the oil chain
length; (5)increasing the alcohol chain length (more oil soluble) or
concentration; (6) increasing the temperature for nonionic surfactant
system or decreasing the temperature for ionic surfactant system; (7)
decreasing the number of hydrophilic groups (e.g., ethylene oxide) of
nonionic surfactant. All these changes may be accounted for by a
corresponding change of packing ratio v/aQlc according to the
geometric model. Some important properties of middle-phase
microemulsions and their relation to tertiary oil recovery remain to
be discussed later.
2.4.4 Pseudophase Hypothesis and Dilution Method
All the phase equilibria discussed so far refer to the equili
bria between "macroscopic" phases. However, it is a well accepted
concept today that a bulk homogeneous microemulsion phase consists of
three microscopic domains, namely a dispersed domain separated from a
surrounding continuous domain by a domain of interfacial film. The
three-compartment model (147) of microemulsions treats each


60
microdoma in as a "microscopic" phase in equilibrium with the other
two. Components of microemulsions, such as surfactant and cosurfac
tant molecules, will partition in all three domains under an equili
brium condition. Since it is assumed that equilibria between the
microdomains obey the same thermodynamic laws as the equilibria
between macroscopic phases (191), each domain has been referred as a
thermodynamic "pseudophase." This is the essence of the pseudophase
model (191,260) upon which many thermodynamic frameworks (124,261-
262) for micellar and microemulsion systems have been proposed.
Based on the pseudophase model and some simple geometric con
siderations, Biais et al. (191) have justified the existence of
dilution lines and the use of a dilution method for w/o microemul
sions. A dilution line in a pseudoternary phase diagram represents a
locus along which the volume of continuous phase of a raicroemulsion
can be increased without significantly altering the size, shape and
composition of the droplets. The existence of dilution lines is
important for the structural study of elementary microemulsion dro
plets by scattering techniques or centrifugation. Since the data
obtained from these experiments are themselves a function of droplet
concentration, a dilution of droplets and extrapolation to zero dro
plet concentration are often employed in experiments to exclude the
concentration dependence. Further, a dilution procedure is also used
to obtain information about interactions between droplets (124,263).
By diluting a w/o microemulsion, one can also determine the composi
tion of each pseudophase. Most importantly, the distribution of
alcohol between continuous and interfacial domains can be determined,
which by no means can be obtained from other methods.


61
One of the great difficulties in diluting a microemulsion is to
ensure the constancy of structure and composition of the droplets
during dilution. In the course of dilution, water in the dispersed
phase to surfactant ratio has been kept unchanged to ensure a con
stant droplet size. A dilution procedure, first proposed by Bowcott
and Schulman (145) and modified by Graciaa (264), can be implemented
as follows: first, oil is added to a transparent microemulsion until
turbidity occurs, then the transparency is reinstated by adding
alcohol together with a certain amount of water. By repeating this
titration many times and plotting the volume of added alcohol versus
that of added oil, one can obtain a titration curve as shown in Fig
ure 2-10. Only at a correct alcohol/water ratio corresponding to that
in the continuous phase, a linear dilution line can be obtained
(curve b in Figure 2-10). The dilution line can be described by the
following equation:
V = kV + rV [2.10]
aso
where V V and V are the volume of surfactant, added alcohol, and
sao 7 7
added oil respectively. Assuming that the dispersed phase contains
only water and that the surfactant molecules only partition at the
interface, r gives the volumetric ratio of alcohol to oil in the con
tinuous phase and k provides the volumetric ratio of alcohol to sur
factant at the interface. The composition of all pseudophases can
thus be deduced (124,191,264).
The validity of this dilution procedure has been examined exper
imentally using neutron scattering (265). It was concluded that the


62
Figure 2-10. Dilution curves of a water/SDS/butanol/toluene
microemulsion. The curvature in curves a & c
indicates a change of continuous phase and
droplet composition of microemulsions during
dilution. Only the linear curve b corresponds
to the dilution line.


63
dilution method only applies to a microemulsion with well-defined
droplet structure exhibiting weak interdroplet interactions. As a
result, the use of dilution method is limited to systems with small
volume fraction of droplets (266). At higher volume fraction of
dispersed phase, such as a middle-phase microemulsion which can not
be described by a droplet structure, the dilution method fails. Many
o/w microemulsion systems cannot be diluted unless enough salt is
added to screen the electric repulsive force between droplets (124).
It may also be pointed out that a dilution line in a pseudoter
nary phase diagram always corresponds to a demixing line where a
phase separation from one-phase to two-phase occurs (124). No dilu
tion line can be observed in a one-phase region. The dilution line
should also be a straight line due to the constancy of composition
and structure of droplets during dilution. Further, dilution can only
be applied to the demixtion (or phase separation) of microemulsions
resulting from the interfacial bending stress, not from the interac
tions between droplets.
2.5 Experimental Studies and Properties of Microemulsions
2.5.1 Experimental Techniques for Characterizing Microemulsions
Microemulsions have been studied using a great variety of tech
niques. The shape, size, structure and many other physico-chemical
properties of microemulsions have been determined for various systems
(267-275). A widely used method for structural study of microemul
sions is the scattering method, including static and dynamic light
scattering (263,276), small angle neutron and X-ray scattering


64
(277-281). These scattering techniques not only provide detailed
structural information about microemulsions, but also measure the
interactions between droplets which can influence the structure, pro
perties and phase behavior of microemulsions (282). Some other tech
niques used in structural studies of microemulsions include sedimen
tation and ultracentrifugation (195,283), electron microscopy
(232,284-286), positron annihilation (287-288), static and dynamic
fluorescence methods (210,235,289), and NMR (290-291). The tech
niques probing the dynamics of microemulsions are NMR (181,272), ESR
(164,194), ultrasonic absorption (292-293), electric birefringence
(294-295) etc. The measurements for various properties of microemul
sions include conductivity and dielectric measurements (258,296-297),
viscometry (241,249,283), interfacial tension and ellipticity meas
urements (298-299), density and heat capacity measurements (268-269),
and vapor pressure measurements (275) etc. It is not intended here
to describe elaborately these techniques or measurements and the
information thereby obtained because reviews on some of these tech
niques are available in the literature (112,123-124,131,133-134).
Many of these techniques are complementary. A result obtained from
one technique often requires a comparison with other techniques to
avoid possible artifacts associated with each technique. The remain
ing discussion will be devoted to describing some important proper
ties of microemulsions which are of technological relevance, and some
applications of microemulsions.


65
2.5.2 Middle-Phase Microemulsions and Ultralow Interfacial Tension
The middle-phase microemulsion has been widely studied due to
its relevance to tertiary oil recovery processes (300). After the
primary and secondary oil recovery, a large amount of oil remains
trapped as oil ganglia in the porous rocks of the oil reservoir due
to capillary forces (301). A surfactant solution is then injected
into the reservoir to mobilize the oil ganglia by lowering the inter
facial tension between the oil and water phases. In tertiary oil
recovery, a lowering of oil-water interfacial tension from about 20-
-2 -3 .
30 dynes/cm to at least 10 -10 dyne/cm is required under practical
reservoir conditions (232). The formation of in-situ middle-phase
microemulsions with sufficient solubilization of oil and brine in the
reservoir by the injected surfactant solution can fulfill this
requirement.
It has been shown that as salt concentration approaches the
optimal salinity, the solubilization parameter of microemulsions
(defined as the ratio of volumes of solubilized phase to that of the
surfactant, V /V and V /V ) increases in both lower- and upper-phase
O S W S rr r
microemulsions as shown in Figure 2-9(c). At the same time, interfa
cial tension between the microemulsion phase and the excess phases
decreases as shown in Figure 2-9(d). Apparently, interfacial tension
is related to the solubilization parameter of microemulsions. A
higher solubilization parameter corresponds to a lower interfacial
tension. At the optimal salinity, equal solubilization of brine and
oil in microemulsions as well as equal interfacial tension of the
microemulsion phase toward both excess oil and water phases are


66
observed. These are the most important properties of middle-phase
microemulsions as related to tertiary oil recovery. Other properties
of middle-phase microemulsions such as conductivity and viscosity can
be found in the literature (254).
Some empirical rules have been proposed to predict the optimal
salinity for a given oil and surfactant system (302-305), but the
precise mechanism responsible for the ultralow interfacial tension is
not well established. The study of low interfacial tension systems
can be divided into two regimes (124,232): (i) Two-phase system with
low surfactant concentrations (0.1%-2% by weight). This is basically
a micellar system. (ii) Three-phase (Winsor type) system with high
surfactant concentrations (2%-10%), containing a middle-phase
microemulsion. In both cases, the low interfacial tension has been
attributed to the presence of a thin adsorbed surfactant and/or
cosurfactant layer (Langmuir film) with high surface pressure at the
interface (306-307). This can be described by the Gibbs adsorption
isotherm of equation [2.1]. It has also been proposed that a
surfactant-rich phase at the interfacial region containing liquid
crystalline structures may be responsible for the low interfacial
tension observed in some systems (308). However, in high surfactant
concentration regime near the optimal salinity (S*), extremely low
interfacial tensions of y,_. below S* and y_w above S* (see Figure 2-
WM OM
9(d)) have been attributed to a thick diffuse interfacial region
associated with critical phenomena (124,234,241), and the ultralow
interfacial tension has been described satisfactorily by the critical
scaling laws (234).


67
Several theoretical models (165,170,229,252,309-310) have been
proposed to predict the low interfacial tension between two bulk
phases in which micelles or microemulsion droplets are present and a
surfactant monolayer layer is adsorbed at the interface between the
two bulk phases. For most two-phase systems, the result seems to
confirm that low interfacial tension can be accounted for by the
presence of a surfactant layer at the interface. The value of inter
facial tension is mainly influenced by the curvature (or size) of
micelle or microemulsion droplets. However, for the critical diffuse
interface of a middle phase microemulsion near the optimal salinity,
the dispersion entropy and interactions of droplets may become dom
inant in determining the interfacial tension. Theoretical prediction
of interfacial tension becomes less satisfactory in this case.
Although many microscopic properties such as the interfacial
curvature, dispersion entropy and interactions of microemulsion dro
plets can influence the interfacial tension as predicted by many
theoretical models, the presence of microemulsion droplets in two-
phase systems is not required in maintaining the low interfacial ten
sion once the equilibrium between two bulk phases has been reached.
It has been shown (234) that the interfacial tension of a two-phase
system, say a Winsor I microemulsion system, remains unchanged after
diluting continuously the o/w microemulsion phase by brine (but sur
factant concentration has to remain above the critical micelle con
centration in the aqueous phase). This conclusion is also valid for
Winsor II system (234). These striking results seem to further con
firm the role of a surfactant layer at the interface in obtaining a
low interfacial tension. It is not clear at this time, however,


68
whether the presence of middle-phase microemulsion structure is
important for maintaining the ultralow interfacial tension of Winsor
III system because the dilution method can not be applied.
2.6 Novel Applications
Microemulsions also offer a great variety of technological,
industrial and biomedical applications. Some advantages of the
microemulsion technology are its spontaneous formation (easy to
prepare), thermodynamic stability (long shelf-life time), isotropi
cally clear appearance (easy to monitor spectroscopically), low
viscosity (easy to transport and mix), molecularly ordered interface
(easy to control the diffusivity as membrane), large interfacial area
(accelerate surface reactions), low interfacial tension (flexible and
high penetrating power), and large mutual solubilization of water and
oil (thus possess both hydrophilic and lipophilic characteristics).
It is these special characteristics that many applications of
microemulsions are based on. Some of potential engineering applica
tions of microemulsions are (1) enhanced oil recovery; (2) lubrica
tion, metal cutting fluid; (3) detergency; (4) improved combustion
efficiency of fuels; (5) novel heat transfer fluid; (6) corrosion
inhibition; (7) media for chemical reactions. Some potential biomed
ical applications of microemulsions include (1) agricultural spray;
(2) improved radiation detection fluid; (3) cosmetic and health-care
products; (4) drug-delivery systems; (5) blood substitutes and organ
preservation fluid. Surveys on some of these applications can be
found in the literature (3,131,133,311-317).


69
In conclusion, it is clear that there has been a rapid develop
ment and better understanding of microemulsions and their applica
tions since their introduction decades ago. Today, microemulsions
still offer worthwhile scientific challenges for researchers. Many
novel applications of microemulsions will probably emerge in the com
ing years.


CHAPTER 3
EFFECTS OF ALCOHOLS ON THE DYNAMIC MONOMER-MICELLE
EQUILIBRIUM AND CONDUCTANCE OF MICELLAR SOLUTIONS
3.1 Introduction
Alcohols are the most commonly used additives in micellar solu
tions to form various solubilized systems or microemulsions for a
wide range of industrial applications. As mentioned in chapter 1,
there exists a dynamic equilibrium between monomers and micelles in
surfactant solutions. The addition of alcohols influences the surfac
tant monomer concentration and the rates of the dynamic equilibrium
process. Many studies (318-342) concerning the effects of alcohols on
the equilibrium and thermodynamic properties of micellar solutions
such as CMC, aggregation number, structure and counterion binding,
etc. have been reported. Singh and Swarup (318) have found that the
CMC goes through a minimum upon addition of alcohols from propanol to
hexanol in sodium dodecyl sulfate (SDS) and cetyItrimethylammonium
bromide solutions. Methanol and ethanol also show a similar change
(343-344). Since the Gibbs free energy change for the addition of one
surfactant monomer to a micelle is proportional to RT In (Xq), where
is the CMC in mole fraction units, the lower the CMC, the more
negative will be the Gibbs free energy change and hence the more
stable the micelle. Thus, it can be asserted that the addition of
alcohols thermodynamically stabilizes the micelles at low alcohol
70


71
concentrations, but destabilizes them at high alcohol concentrations.
The objective of this chapter is to examine systematically the effect
of alcohol chain length on the equilibrium aspect of micellar solu
tions, specifically the critical micellization concentration and the
counterion dissociation of micelles. The information obtained will be
used for comparison with the effect of alcohols on the dynamic
aspects of micelles presented in chapters 4 and 5.
3.2 Experimental
Sodium dodecyl sulfate (SDS) was used as supplied by B.D.H.
(purity 99%). All normal alkanols with purity above 99% were used
directly without further purification. Absolute ethanol was of USP
200 proof (reagent grade) from Florida Distillers Corporation. The
critical micellization concentration was determined at 20C using
electrical conductance method by diluting the micellar solution with
a mixed solvent of water and alcohol. The electrical conductance was
measured at 1000 Hz using Beckman conductivity bridge.
3.3 Results and Discussions
Figure 3-1 shows the change of electrical conductance of micel
lar solutions as a function of SDS concentration in the presence of
pentanol. The CMC can be determined from the break point of two
linear conductance curves. The CMC of SDS at 20C was found to be 8.5
mM, in agreement with the literature (345). Figure 3-2 reports the
change of CMC of SDS with addition of various alcohols from ethanol
to hexanol. The change of CMC exhibits a minimum for alcohols from
butanol to hexanol.


SPECIFIC CONDUCTIVITY ( 10 S/cm
72
Figure 3-1.
Specific conductivity of SDS solutions as a
function of SDS concentration with addition
of pentanol


73
Figure 3-2.
The change of critical micellization concentration of
SDS with alcohol concentration


74
In addition to the information about CMC, the ratio between the
slopes of the two linear conductance curves above and below the CMC
(Figure 3-1) can also provide information about the degree of coun
terion dissociation from the micelles (346-347). This can be under
stood from the following equations. The total surfactant concentra
tion in a solution can be written as:
C = C + nC [3.1]
o m
where C is total surfactant concentration, C is the monomer concen-
o
tration, C^ is the micelle concentration and n is the mean aggrega
tion number of micelles. Assuming a complete counterion dissociation
of surfactant monomers, the total free counterion concentration in
the solution is:
C = C + anC [3.2]
go m
where C^ is the total free counterion concentration, a is the degree
of counterion dissociation from micelles. Substituting equation [3.1]
into [3.2], one obtains:
C = C + a(C C )
go o
[3.3]
The specific conductivity of a micellar solution is equal to:


75
K = X+C + X'C + X C
sp o g 0 0 mm
where K is the specific conductivity (S/cm) of the solution,
sp r
X and X are the equivalent conductivity of the counterion
0 m
factant monomer anion and micelle respectively. Substituting
tions [3.1] and [3.3] into [3.4], one obtains:
[3.4]
, sur-
equa-
K = ( X
sp o
X")c + ( a X + X /n)(C C )
0 0 0 m o
[3.5]
Equation [3.5] can be simply written as:
K = X c +
sp o o
X'
m
(C C )
o
[3.6]
where X^ is the equivalent conductivity of a surfactant monomer, and
X^ is the apparent equivalent conductivity of a surfactant in
micelles. It is clear that Xq is equal to the slope of the conduc
tance curve below the CMC and corresponds to the slope above the
CMC shown in Figure 3-1. The ratio of these two slopes gives:
W Xn = ( aK + + O
mo 0 m q o
[3.7]
Since the mobility of a counterion is expected to be greater than a
surfactant monomer anion or a micelle, by assuming a X >> X /n and
0 m
X0 XQ, equation [3.7] reduces to:
A / X
[3.8]
Figure 3-3 reports the counterion dissociation of SDS micelles as a
function of pentanol concentration. The addition of pentanol to the


76
PENTANOL CONCENTRATION ( mM )
Figure 3-3. The change of degree of counterion disso
ciation of SDS micelles with pentanol
concentration


77
micellar solution increases the counterion dissociation of micelles.
The a value obtained for pure SDS micelles has been found to be
around 0.41. The reported literature values range from 0.14 to 0.54
(348).
3.3.1 Effect of Mixed Solvents on CMC
Short-chain alcohols are usually known as cosolvents which are
highly miscible with water. An extensive review of solvent effect on
amphiphilic aggregation by Magid (349) has indicated that the con
tinuous addition of cosolvents into aqueous micellar solutions usu
ally leads to higher CMC values (but may be proceeded by an initial
CMC depression for many penetrating cosolvents at low concentra
tions), smaller aggregation number and eventually a break-down of
micelles. In the literature, two factors have generally been proposed
to account for the influence of cosolvents on micelle formation. The
first one is related to the comicellization of cosolvents with sur
factants. Despite the high miscibility of cosolvents with water, many
cosolvents are known to penetrate into micelles (349). The effect of
such penetrating solvents has been analyzed by Zana et al. (94)
mainly in two aspects: an increase in distance between surfactant
head groups (steric effect) and a decrease in the dielectric constant
of micellar palisade layer. The CMC decreases as a result of dilution
of micellar surface charges (94). A quantitative analysis (326) has
shown that the factor governing the CMC depression is the mole frac
tion of the alcohol in the micellar phase, independent of the chain
length of alcohols. The chemical potential of micelles decreases
because the electrical potential of micelles decreases upon alcohol


78
penetration (326). However, a recent paper by Manabe et al. (350)
argued that the solubilized alcohols in micelles cause an increase in
the degree of ionization of the micelles but have little influence on
electrical potential at the micellar surface due to a compensation
effect from the dissociated counterions. Hence, the depression of CMC
has been attributed to an increase in mixing entropy of mixed
micelles due to comicellization (330,351).
The second factor deals with the change of structure and proper
ties of water upon the addition of cosolvents. The hydrophobic asso
ciation and micelle formation have been interpreted in term of the
structure of water. When the cosolvent is a structure-breaker, CMC
usually increases (hydrophobic interaction decreases), whereas the
CMC decreases when the cosolvent is a structure-maker (349). Frank
and Ives (352-353) have
reviewed
the structural
properties
of
alcohol-water mixtures.
Various
physico-chemical
properties
of
short-chain alcohols and water mixtures often show maxima or minima
at low alcohol fractions, suggesting a maximum structure promotion of
water by alcohols, followed by a structure disruption at higher
alcohol fractions. Hence, the decrease of CMC at low alcohol concen
trations shown in Figure 3-2 can be partly ascribed to the structure
promotion of water (333,349). The increase of CMC and break-down of
micelles at very high alcohol concentrations is connected with the
disruptive effect of alcohols on the structure of water, and the
decrease of dielectric constant of water.
However, the alcohol concentrations reported in Figure 3-2 is
probably not high enough to produce a disruptive effect on the struc
ture of water. Hence, the increase of CMC observed in Figure 3-2 is
probably due to a third factor which will be discussed later.


79
3.3.2 Effects of Alcohols on the Conductance
of Micellar Solutions
According to equation [3.5], the specific conductivity of a
micellar solution mainly depends on total surfactant concentration,
surfactant monomer concentration and the degree of counterion disso
ciation of micelles. At low surfactant concentrations near the CMC,
the first term in equation [3.6] is important and hence the conduc
tance is mainly attributed to the surfactant monomers. At higher sur
factant concentrations, micelles may contribute predominantly to the
conductance. Figure 3-4 shows the change of electrical conductance of
10 mM SDS (slightly above the CMC, 8.5 mM) with addition of alcohols
from methanol to heptanol. The change of conductance is similar to
the change of CMC shown in Figure 3-2, and the alcohol compositions
at the conductance minima (160 mM butanol, 62 mM pentanol and 24 mM
hexanol respectively) roughly coincide with that of CMC minima (160
mM butanol, 73 mM pentanol and 30 mM hexanol respectively). This sug
gests that the change of conductance upon addition of alcohols actu
ally reflects the change of surfactant monomer concentration (or CMC)
in 10 mM SDS solution.
Figure 3-4 can be replotted in Figure 3-5 as a function of
alcohol concentration partitioning in the micelles. The fraction of
various alcohols partitioning in SDS micelles can be obtained from
the literature (354). Figure 3-5 shows that all the conductance data,
except that of heptanol, roughly fall in a v-shape curve, and the
conductance minima of butanol to hexanol occur at about the same
alcohol concentration present in the micellar phase. This result
supports the assertion (326) that the mole fraction of alcohol in the


80
Figure 3-4. The change of specific conductivity of 10 mM SDS
with alcohol concentration


Specific Conductivity (10 JS/cm)
81
Alkanol Concentration (mM)
Present in the Micellar Phase
Figure 3-5. The change of specific conductivity of 10 mM SDS with
alcohol concentration present in the micellar phase.
The symbols used are the same as that in Figure 3-4 for
various alcohols. The filled symbols represent the
conductivity minima shown in Figure 3-4.


82
micellar phase is the governing factor for determining the surfactant
monomer concentration or the CMC. The surfactant monomer concentra
tion of 10 mM SDS can further be plotted as a function of
alcohol/surfactant ratio in the micellar phase as shown in Figure 3-
6. It shows that the minimum surfactant monomer concentration (or
CMC) occurs at about 2-3 alcohol/surfactant ratios in the micellar
phase, independent of alcohol chain length from butanol to hexanol.
It is noted that the 2-3 alcohol/surfactant ratios have also been
reported for various alcohol-surfactant systems, at which maximum
stability or performance of the system have been observed (355). The
result has been interpreted by the closest geometrical packing
between the alcohols and the surfactants at the optimal ratios (355).
The ratios have also been found to be the optimal compositions of
inter facial films in many microemulsion systems. The minimum CMC at
the optimal alcohol/surfactant ratios shown in Figure 3-2 is probably
due to the closest geometrical packing of the surfactants and
alcohols in the mixed micelles.
As surfactant concentration increases, the contribution from
micelles to the conductance of a micellar solution will increase as
compared to the contribution from surfactant monomers. Since the
addition of alcohol increases the counterion dissociation of micelles
as reported in Figure 3-3, the conductance will consequently increase
if the micelles contribute predominantly to the conductance. Figure
3-7 indeed supports the above statements. Upon addition of pentanol,
the decrease in surfactant monomer concentration tends to decrease
the conductance at low SDS concentrations. However, such a decrease
will be offset by an increase in the conductance due to the


83
Figure 3-6. The change of surfactant monomer concentration
in 10 mM SDS as a function of alcohol/surfactant
molar ratio in the micellar phase. The filled
symbols correspond to the conductivity minima
shown in Figure 3-4.


84
increasing counterion dissociation of micelles with the addition of
pentanol. This counteracting effect increases with increasing SDS
concentration. At about 22 mM SDS, these two opposite effects com
pensate each other, resulting in no change in the conductance with
addition of small amount of pentanol as shown in Figure 3-7. This
explanation can further be confirmed by a simple calculation. Based
on equation [3.6], the change of specific conductivity of a micellar
solution can be written as:
dK = ( X x' )dC + C d( X X' ) + Cd X'
sp omooom m
[3.9]
To observe no change in the conductance, dK
to have:
sp
= 0 and it is necessary
(X x )dC + C d( x X' ) + Cd x' = 0 [3.10]
omooom m
The total surfactant concentration at which no conductance change is
observed upon addition of pentanol can be calculated from equation
[3.10]. For pure SDS solution, C = 8.5 mM, X = 53.8 and =
o o m
22.3. Upon addition of 10 mM pentanol, Cq = 7.7 mM, Xq = 54.8 and
^ = 23.5. Then C = 22.4 mM is obtained from equation [3.10] at
m
which no change in the conductance is expected with addition of small
amount of pentanol. This is in agreement with Figure 3-7. At even
higher SDS concentrations, a direct increase in the conductance is
observed upon addition of pentanol since the micelles contribute
predominantly to the conductance.


SPECIFIC CONDUCTIVITY ( TO"* S/cm )
85
Figure 3-7. The change of specific conductivity of various SDS
solutions as a function of pentanol concentration.
The points of conductivity minima are indicated by
arrows. The inset figure shows the correlation of
SDS concentrations with pentanol concentrations at
the conductivity minima.


86
3.3.3 Formation of Swollen Micelles by Alcohols
As shown in Figure 3-4, heptanol exhibits distinctly different
effect from that of butanol to hexanol on the conductance. Since the
change of conductance reflects the change of surfactant monomer con
centration, the surfactant monomer concentration decreases with addi
tion of heptanol, instead of exhibiting a minimum as in the case of
butanol to hexanol. This can be attributed to the formation of swol
len micelles by heptanol. Figure 3-8 shows the schematic models of
swollen micelles. The added alcohols in micellar solutions partition
initially in the palisade layer of the micelles as shown in Figure
3-8(b). The penetrating alcohols may replace some surfactants to
form mixed micelles and provide steric shielding as well as dilution
effects on the micellar surface charges (94), thus resulting in an
increase in counterion dissociation from micelles. However, at
higher alcohol concentrations, alcohols start penetrating into the
hydrophobic core of micelles (98) to form swollen micelles as dep
icted in Figure 3-8(c), causing an increase in the micellar volume
and the aggregation number of surfactant in micelles. Increasing
alcohol chain length enhances the swelling of the micelles to
decrease the hydrocarbon chain density in the hydrophobic core (Fig
ure 3-8(d)). The formation of swollen SDS micelles by heptanol has
been evidenced by fluorescence method (341). Russell et al. (341)
have found that the aggregation number of SDS micelles first
decreases and then increases as the heptanol/SDS ratio increases.
The increase in hydrophobic volume of micelles can result in a
decrease in the curvature of micellar surfaces and a concomitant


87
Figure 3-8. Schematic models of swollen micelles
(a) a pure micelle; (b) alcohols are solubilized
in the palisade layer close to the micellar
surface. Some surfactants may be replaced by the
alcohols, providing steric shielding and
dilution effects on the surfactant head groups;
(c) alcohols are solubilized in the hydrophobic
core of micelles at sufficiently high alcohol
concentrations, resulting in an increase in the
volume of the hydrophobic core and a decrease
in interfacial curvature (swollen micelles);
(d) long-chain alcohols can swell the micelles
immediately; (e) nonpolar oils are solubilized
in the hydrophobic core of the micelles


88
closer packing of surfactant head groups. This will lead to a recom
bination of counterions to the micelles and a consequent decrease in
the conductance. The result shown in Figure 3-9 supports this asser
tion. Since micelles contribute primarily to the conductance of 100
mM SDS solution, the electrical conductance is expected to increase
due to increasing counterion dissociation from micelles with addition
of alcohols from propanol to octanol. The decrease of conductance
with addition of methanol and ethanol can be explained by the
decreasing dielectric constant of water. At sufficiently high alcohol
concentrations, swollen micelles are formed and consequently the con
ductance decreases. Table 3-1 lists the alcohol concentrations and
the alcohol/surfactant ratios in the micellar phase at the conduc
tance maxima observed in Figure 3-9. The alcohol/surfactant ratio
required to swell the micelles decreases with increasing alcohol
chain length. Note in Figure 3-9 that swollen micelles form right
upon the addition of decanol.
Table 3-2 tabulates the initial slopes of conductance increase
for various alcohols shown in Figure 3-9. The ratios between the
slopes (first column in Table 3-2) can provide at first approximation
the relative partitioning of different alcohols in 100 mM SDS
micelles. By taking the partitioning of hexanol in the micelles as 1,
the relative partitioning of butanol and pentanol in the micelles
obtained from the conductance measurements is close to the literature
values as indicated in the parenthesis of the first and tne second
columns in Table 3-2. The discrepancy in propanol is due to the high
solubility of propanol in water, which may significantly alter the
dielectric constant of water, hence the conductance measurement does


SPECIFIC CONDUCTIVITY ( 10' S/cm
89
Figure 3-9. The change of specific conductivity of 100 mM SDS
as a function of alcohol concentration. The inset
figure shows the enlarged details of low alcohol
concentration region. The filled points represent
turbid solutions.


Table 3-1 Alcohol/Surfactant Ratios in the Micellar Phase at the Onset
of Formation
of Swollen Micelles in 100
mM SDS Solution
Concentration3
of Alcohols (M)
Fraction'3 of Alcohol
Partioning in the
Micellar Phase
Alcohol/Surfactant
Ratio in Micelles
butanol
0.68
0.44
3.0
pentanol
0.47
0.77
3.6
hexanol
0.234
0.92
2.2
heptanol
0.099
0.96
1.0
octanol
0.051
0.985
0.5
a. Total alcohol concentration at the conductance maxima reported in Figure 3-9.
b. The fraction of total added alcohol present in the micellar phase.
All values taken from Stilbs, P. J. Colloid Interface Sci. 87, 385 (1982).
c. Assuming the surfactant monomer concentration in the aqueous phase
is negligible; hence all surfactants are present in the micellar phase.


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81,9(56,7< 2) )/25,'$


DYNAMIC AND EQUILIBRIUM ASPECTS
OF MICELLAR AND MICROEMULSION SYSTEMS
BY
ROGER YI-MING LEUNG
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1987
c

To
My parents and my wife

ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Professor D.O.
Shah for his guidance and encouragement during the course of this
research. I would also like to thank Professor John P. O'Connell,
Professor Gerald B. Westermann-Clark, Professor Dale W. Kirmse, Pro¬
fessor Brij M. Moudgil and Professor Paul W. Chun as members of the
supervisory committee for their time and advice.
I wish to thank Nancy, Ron and Tracy for their help and coopera¬
tion. I would also like to gratefully acknowledge the financial sup¬
port from the National Science Foundation and the ALCOA foundation
for this research.
Finally, I owe my gratitude to my parents, my brother and sis¬
ter, my wife and all my friends and colleagues in the Chemical
Engineering Department for their encouragement, help and friendship
throughout all these years.
iii

TABLE OF CONTENTS
ACKNOWLEDGEMENTS iii
ABSTRACT viii
CHAPTERS
1 INTRODUCTION 1
1.1 Surfactant Molecules 1
1.2 Surfactant-Containing Aggregates 1
1.3 Equilibrium Aspects of Surfactant-Containing 3
Aggregates
1.4 Dynamic Aspects of Surfactant-Containing 6
Aggregates
1.4.1 Application of Fast Relaxation 8
Techniques to the Dynamic Study of
Surfactant Solutions
1.4.2 Kinetics of Micellization 8
1.4.3 Dynamic Aspects of Microemulsions 17
1.5 Scope 19
2 REVIEW OF THE LITERATURE: FORMATION, STRUCTURE 22
AND PROPERTIES OF MICROEMULSIONS
2.1 Introduction 22
2.2 Spontaneous Emulsification and Thermodynamic 24
Stability of Microemulsions
2.2.1 Ultralow Interfacial Tension 25
2.2.2 Interfacial Curvature, Elasticity 27
and Entropy
2.3 Geometric Aspects and Structure of 36
Microemulsions
2.3.1 Geometric Packing Considerations in 36
Amphiphile Aggregation
2.3.2 Non-Glcbular Domain and Microemulsion 41
Structure in Phase Inversion Region
2.3.3 Design Characteristics of Microemulsions ... 46
2.3.4 Shape Fluctuations and Structural 47
Dynamics of Microemulsions
2.4 Solubilization and Phase Equilibria of 48
Microemulsions
2.4.1 Solubilization and Structure of 48
Microemulsions
2.4.2 Phase Equilibria of Microemulsions 54
iv

2.4.3 Phase Behavior of Winsor-Type 56
Microemulsions
2.4.4 Pseudophase Hypothesis and 59
Dilution Method
2.5 Experimental Studies and Properties 63
of Microemulsions
2.5.1 Experimental Techniques for 63
Characterization of Microemulsions
2.5.2 Middle-Phase Microemulsions and 65
Ultralow Interfacial Tension
2.6 Novel Applications 68
3 EFFECTS OF ALCOHOLS ON THE DYNAMIC MONOMER-MICELLE .... 70
EQUILIBRIUM AND CONDUCTANCE OF MICELLAR SOLUTIONS
3.1 Introduction 70
3.2 Experimental 71
3.3 Results and Discussions 71
3.3.1 Effect of Mixed Solvents on CMC 77
3.3.2 Effect of Alcohols on the Conductance 79
Micellar Solutions
3.3.3 Formation of Swollen Micelles 86
by Alcohols
3.4 Conclusions 94
4 DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS: 96
EFFECTS OF SHORT-CHAIN ALCOHOLS AND POLYMERS
ON MICELLE STABILITY
4.1 Introduction 96
4.2 Experimental 97
4.3 Results and Discussions 98
4.3.1 Labilizing Effect of Short-Chain 98
Alcohols on Micelles
4.3.2 Influence of Alcohol Chain Length 104
and Surfactant Concentration on
Labilizing Effect of Alcohols
4.3.3 The Concept of Micelle Stability Ill
4.3.4 Micelle Nucleus Formation as the 113
Rate-Limiting Step: Evidence
from Polymer Additives
4.4 Conclusion 121
5 DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS: 123
EFFECTS OF MEDIUM- AMD LONG-CHAIN ALCOHOLS
AND OILS
5.1 Introduction 123
5.2 Results and Discussions 123
5.2.1 Slow-Down of Step-Wise Association 123
Kinetics by Alcohols
5.2.2 Transition in Micellization Kinetics 128
with Addition of Alcohols
v

5.2.3 Effect of Alcohol Chain Length on the 129
Transition of Micellization Kinetics
5.2.4 Formation of Swollen Micelles by 136
Alcohols and Oils with Resultant
Slow-Down in Micellization Kinetics
5.3 Conclusions 141
6 SOLUBILIZATION AND PHASE EQUILIBRIA OF WATER-IN-OIL .... 144
MICROEMULSIONS: EFFECTS OF SPONTANEOUS CURVATURE
AND ELASTICITY OF INTERFACIAL FILM
6.1 Introduction 144
6.2 Basic Concepts and Theory 145
6.2.1 Interfacial Free Energy of Microemulsions .. 145
6.2.2 Free Energy of Interdroplet Interactions ... 147
6.2.3 Phase Equilibria of W/0 Microemulsions 150
6.2.4 Equilibrium Droplet Size and Solubilization 151
of W/O Microemulsions
6.3 Discussions 157
7 SOLUBILIZATION AND PHASE EQUILIBRIA OF WATER-IN-OIL .... 161
MICROEMULSIONS: EFFECTS OF OILS, ALCOHOLS AND SALINITY
7.1 Introduction 161
7.2 Materials and Methods 161
7.3 Effect of Oil Chain Length 163
7.3.1 Influence of Oil Penetration into 163
Interfacial Films
7.3.2 Formation of Birefringent Phases 166
7.3.3 Chain Length Compatibility in 171
W/O Microemulsions
7.4 Effect of Alcohols 176
7.4.1 Effect of Alcohol Chain Length 176
7.4.2 Effect of Alcohol Concentration 179
7.5 Effect of Salinity 184
7.5.1 Optimal Salinity in Single-Phase W/O 184
Microemulsions
7.5.2 Effects of Alcohol and Oil 188
on Optimal Salinity
7.6 Conclusions 191
8 REACTION KINETICS AS A PROBE FOR THE DYNAMIC 196
STRUCTURE OF MICROEMULSIONS
8.1 Introduction 196
8.2 Experimental 197
8.2.1 Materials and Methods 197
8.2.2 Preparation of AgCl Sols 198
8.2.3 Coagulation Rate Measurement 198
vi

8.3 Results and Discussions 200
8.3.1 Coagulation of Hydrophobic AgCl Sols 200
8.3.2 Physico-Chemical Properties of the 204
Microemulsions
8.3.3 Interrelationship between the Reaction 221
Kinetics and the Dynamic Structure
of Microemulsions
8.4 Conclusions 225
9 CONCLUSIONS AND RECOMMENDATIONS 227
9.1 Effects of Alcohols on the Dynamic Monomer- 227
Micelle Equilibrium and Conductance of
Micellar Solutions
9.2 Effects of Alcohols, Oils and Polymers on the 228
Dynamic Property of Micellar Solutions
9.3 Effects of Spontaneous Curvature and Interfacial .. 230
Elasticity on the Solubilization and Phase
Equilibria of W/0 Microemulsions
9.4 Effects of Oils, Alcohols and Salinity on the 231
Solubilization and Phase Equilibria of W/0
Microemulsions
9.5 Reaction Kinetics as a Probe for the Dynamic 232
Structure of Microemulsions
9.6 Recommendations for Further Investigations 233
REFERENCES 235
BIOGRAPHICAL SKETCH 262
vii

Abstract of Dissertation Presented to
the Graduate School of the University of Florida
in Partial Fulfillment of the Requirement for the
Degree of Doctor of Philosophy
DYNAMIC AND EQUILIBRIUM ASPECTS OF
MICELLAR AND MICROEMULSION SYSTEMS
By
Roger Yi-Ming Leung
May, 1987
Chairman: Professor Dinesh 0. Shah
Major Department: Chemical Engineering
The dynamic and equilibrium aspects of micellar and microemul¬
sion systems have been investigated with focus on the influence of
alcohols, oils and salinity on the systems. The pressure-jump method
was used to study the micellization kinetics. The obtained slow
relaxation time is related to the average life-time of micelles.
Microemulsions are studied by centering on the effect of interfacial
elasticity and curvature on the systems.
The addition of alcohols to micellar solutions increases the
thermodynamic stability of micelles at low alcohol concentrations. A
maximum thermodynamic stability of micelles has been observed at

about 2-3 alcohol/surfactant ratios in the micellar phase. However,
the addition of alcohols may increase or decrease the kinetic stabil¬
ity (life-time) of micelles depending on the micellization kinetics
and alcohol chain length. The addition of alcohols can induce a
transition of micellization kinetics from a step-wise association to
a reversible coagulation-fragmentation process. The formation of
swollen micelles due to addition of alcohols and oils increases the
slow relaxation time and hence the life-time of the micelles, with a
concomitant decrease in the specific conductivity of the micellar
solution.
The spontaneous curvature and elasticity of interfacial films
have been shown to influence the solubilization and phase equilibria
of water-in-oil (w/o) microemulsions, when the interfacial tension is
very low. Maximum water solubilization in a w/o microemulsion can be
obtained by minimizing both the interfacial bending stress of rigid
interfaces and the attractive interdroplet interaction of fluid
interfaces at an optimal interfacial curvature and elasticity. The
study of phase equilibria of microemulsions serves as a simple method
to evaluate the property of the interface and provides phenomenologi¬
cal guidance for the formulation of microemulsions with maximum solu¬
bilization.
Being sensitive to the dynamic structure of surfactant solu¬
tions, the reaction kinetics and dynamic measurements have been used
as a probe for the dynamic structure of micellar and microemulsion
systems.

CHAPTER 1
INTRODUCTION
1 .1 Surfactant Molecules
Surfactant molecules have a unique feature of possessing both a
polar (hydrophilic) and a nonpolar (hydrophobic) part within the same
molecule. It is this unique feature that causes them to spontaneously
aggregate in a solution, or to adsorb at an air/water or oil/water
interface, which is known as the surface activity of surfactants.
This was first recognized by Hartley (1) in 1936 who originated the
concept of "micelles" as aggregates of surfactant molecules in aque¬
ous solutions. The molecules were organized in such a way that the
polar heads of the molecules were in contact with the surrounding
water, while the nonpolar tails were shielded from the surrounding
water by aggregating in the interior of the micelle.
1.2 Surfactant-Containing Aggregates
It is well known that two immiscible liquids, e.g., oil and
water, can form a macroscopically clear, homogeneous mixture upon
addition of a third liquid (dispersing agent) which is miscible with
both the liquids (2). This has been conventionally represented by a
ternary phase diagram as shown in Figure 1-1. The single phase
region in the diagram is usually considered as a simple molecular
solution. However, when surface-active molecules, e.g., surfactants
1

2
C
Figure 1-1. A schematic ternary phase diagram representing the
formation of a homogeneous mixture of two immiscible
liquids (A & B) upon addition of a third liquid (C)
which is miscible with both the liquids. Shaded area
represents the two-phase region. P is the plait point.

3
or detergents, are used as the dispersing agent, the single phase
region may consist of raicrodomains of the dispersed phase and complex
association structures of surfactant molecules. Such a single phase
region is often referred as a MmicroemulsionM (3). Figure 1-2
represents a schematic ternary microemulsion phase diagram composed
of microdomains with various possible association structures of sur¬
factants. At low surfactant concentrations, normal and inverted
micelles are usually expected. At high surfactant concentrations,
water-in-oil (w/o) and oil-in-water (o/w) microemulsions may exist
when the system contains a considerable amount of oil and water. At
even higher surfactant concentrations, a liquid crystalline phase may
be observed. One special character of such a system is that dif¬
ferent structures may exist in a single-phase region without a phase
separation during structural transition.
1.3 Equilibrium Aspects of Surfactant-Containing Aggregates
The simplest type of micelles is a spherical one. Micelles are
observed only at surfactant concentration just above the "critical
micellization concentration" (CMC) and usually have an aggregation
number of the order of 100. The "CMC" can be defined as a relatively
small range of concentrations separating the limit below which virtu¬
ally no micelles are detected and above which virtually all addi¬
tional surfactants form micelles Increasing surfactant concentration
will alter the size and shape of micelles. Thus, with increasing sur¬
factant concentration one may observe various structures such as ran¬
domly oriented cylindrical micelles, hexagonally packed cylindrical
micelles or lamellar micelles (4-16).

4
SURFACTANT
Figure 1-2. A schematic ternary phase diagram of an oil-water-
surfactant microemulsion system consisting of
various association microstructures: A, normal
micelles or o/w microemulsions; B, reverse (or
inverted) micelles or w/o microemulsions; C,
concentrated microemulsion domain; D, liquid
crystal or gel phase. Shaded areas represent the
multi-phase regions and the clear area is the
single-phase region.

5
It should be stressed that the aggregation of surfactants in an
aqueous solution arises not from the attractive forces between the
nonpolar tails, but is due to the cohesiveness of water molecules.
The surrounding water, while accepting the polar surfactant heads,
squeezes out the nonpolar surfactant tails. This "hydrophobic”
effect, together with the repulsion between the surfactant head
groups, appears to determine the formation, structure and stability
of micelles (6,9,17-29). There have been many theories proposed for
micellization process, including phase separation models (30-32),
statistical mechanical models (33-34), thermodynamic models
(14,28,34-45), a Monte Carlo simulation (31), shell models (46-47),
geometric models (48-49) and a variation theory (50). All these
theories were concerned with the prediction of micellar equilibrium
properties, most notably the CMC, mean aggregation number, structure
and size distribution. Surfactants also form reverse micelles in non¬
polar media, but the process of micelle formation in nonpolar media
is significantly different from that in aqueous solutions (51-64). It
is known that the presence of water promotes the formation of reverse
micelles.
Microemulsions are isotropically clear, thermodynamically stable
dispersions of oil and water consisting of microdomains stabilized by
surfactant films. The most basic types of microemulsion structure are
oil-in-water (o/w) and water-in-oil (w/o) droplets. More details
about the formation, structure, and properties of microemulsions as
well as their structural relationship to micelles will be reviewed in
chapter 2.

6
1.4 Dynamic Aspects of Surfactant-Containing Aggregates
In addition to the equilibrium picture of surfactant solutions
described above, it is noted that surfactant aggregates are in
dynamic equilibrium with surfactant monomers in the solution. In
fact, micelles and microemulsions should be viewed as dynamic struc¬
tures. They are thermodynamically stable, but there are constant
formations, dissolution and (shape and size) fluctuations of these
aggregates--in the words of Winsor, "micelles are of statistical
character, and it is important to guard against a general picture of
micelles as persistent entities having well-defined geometrical
shapes." (9, page 3). Figure 1-3 schematically depicts a dynamic
equilibrium between surfactant monomers and micelles in an aqueous
solution. A micelle may constantly pick up a surfactant monomer at
rate constant k from the bulk and may also lose a monomer at rate
constant k . The critical micellization concentration actually
represents the equilibrium point of this dynamic process following
the relation CMC = k /k+ (65). The rate of micelle formation and
dissolution as well as the average life-time of micelles depend on
the reaction rate of this dynamic process.
An examination of two review articles on amphiphile aggregation
in aqueous solvents (66) and in nonpolar solvents (67) reveals that
the equilibrium and thermodynamic aspects of surfactant solutions
have received extensive attention ovr the past fifty years, but the
study on the dynamic aspect of these systems is still in its early
stage. However, some initial explorations of kinetics and dynamic
aspects of micelles and microemulsions have greatly advanced our
basic understanding of these systems during the past decade.

7
Figure 1-3. A schematic diagram depicting the dynamic monomer-
micelle equilibrium in a micellar solution. Surfactant
monomers keep exchanging between the micelles and+
the bulk solvent with association rate constant k
and dissociation rate constant k . The symbol "+"
represents the counterions of the surfactants.

8
1.4.1 Application of Fast Relaxation Techniques to
the Dynamic Study of Surfactant Solutions
The main objective of the kinetic study is to understand the
rate and mechanism of a chemical reaction when the reaction at
equilibrium relaxes back to its original or new equilibrium state
after a small perturbation on the thermodynamic parameters of the
system. Experimental techniques, such as stopped-flow, pressure-
jump, temperature-jump and ultrasonic relaxation, have existed for a
long time in conventional studies of fast chemical kinetics (68).
Research groups in Germany, Sweden and France as well as in Japan
have applied these techniques to study surfactant systems. These
studies have led to important conclusions about the aggregation
kinetics and dynamic structure of surfactant solutions. They will be
discussed in two categories as the kinetics of micellization and
dynamics of microemulsions respectively.
1.4.2 Kinetics of Micellization
The first documented attempt to measure the rate of micelle
breakdown was by Jaycock and Ottewill (69) in 1964. Using a
stopped-flow technique, they found that the breakdown of anionic
(SDS) and cationic (alkylpyridinium salts) micelles was rapid with
half-life on the order of 10 milliseconds. They also found qualita¬
tively that the rate of micelle breakdown could be expedited by
increasing the temperature, decreasing the surfactant chain length or
by decreasing the size of the counterion. However, the foundation of
relaxation kinetics of micellar solutions was established by a paper
(65) jointly published in 1976 by three research groups in France,
Germany and Sweden. A number of studies (70-117) on micellization

9
kinetics have been reported during the past ten years. It appears
that, through a combination of research efforts in both theoretical
and experimental aspects, the kinetic study has culminated into one
of the most fruitful approaches for the fundamental understanding of
micellar systems.
It is now well established that the relaxation of pure micellar
solutions is composed of two processes. One fast process is associ¬
ated with the exchanging of surfactant monomers between the micelles
and the bulk solution with a fast relaxation time x ^ in the order of
microseconds. The slow process is related to the micelle formation
and dissolution with a slow relaxation time I ^ in the order of mil¬
liseconds. Figure 1-4 schematically illustrates these two relaxation
processes. At low surfactant concentrations, micelles are formed
as described by equation
\ [1.1]
A
n
through a step-wise association process
[1.1]:
A1 + A1
A1 + A2
A1 + A3
A. + A . T-
1 n-1

10
Figure l-¿
(microseconds)
Slow
^(milliseconds)
A schematic diagram describing the mechanisms of two
relaxation processes observed in a pure dilute
surfactant solution. The fast relaxation process
is associated with the exchange of surfactant
monomers between the micelles and the bulk solution.
The slow relaxation process is related to micelle
formation and dissolution process .

11
while the overall reaction is
+
k
n A. A [1.2]
I n
k"
where A^. denotes a surfactant monomer and A^ is an aggregate contain¬
ing n surfactants. Figure 1-5 represents a schematic size distribu¬
tion curve of aggregates in a micellar solution. The relaxation times
T j and T ^ were formulated by Aniansson et al. (65) through an anal¬
ogy to a heat conduction process as follows:
1/ Tj - k"/ a2 + (k'/n)a [1.3]
1/ t2 = + a 2a/n)_1 [1.4]
a = (C - A1)/A1 [1.5]
where k is the dissociation rate constant of one surfactant monomer
from a micelle; ais the half-width of the Gaussian distribution of
micelles; n is the mean aggregation number of micelles; a is the
ratio of surfactants in micellar state to that in monomer state; A^
is the free monomer concentration in the bulk phase; C is the total
surfactant concentration; R is the rate-determining resistance of
micelle nucleus formation.

NUMBER DENSITY
12
Figure 1-5. A schematic size distribution curve of aggregates
in a micellar solution

13
Many experimental results obtained on micellar systems are to a
large part well described by the above theory. By applying this
theory to the experimental results, it is possible to determine two
statistical parameters of micellar solutions, namely the average
residence time of a monomer in micelles and the average life-time of
micelles. By plotting 1/ vs. a in equation [1.3], one can deter-
- - 2
mine k /n from the slope and k / a from the intercept. Using the
value of n obtained from other techniques (e.g., light scattering,
fluorescence or osmotic pressure measurements), one can obtain both
the kinetic (k ) and statistical (n, a ) parameters of the micelles.
The mean residence time of a surfactant monomer in micelles is equal
to n/k , and the life-time of micelles T.. is obtained from the fol-
M
lowing equation (113):
Tm = T 2na o + 02a/") 1 [1.6]
When surfactant concentration is much greater than the CMC, T__ is
M
approximately equal to n Thus, the study of the fast relaxation
process provides information about the residence time of a surfactant
monomer in micelles, while the measurement of slow relaxation time
allows an estimation of average life-time of micelles. The experimen¬
tally determined values of various kinetic and statistical parameters
of sodium alkyl sulfate micelles are listed in Table 1-1.
According to Aniansson and Wall theory, the controlling step in
the micellar aggregation process is the formation of micelle nuclei,
which has been conceptualized in analogy to a resistance offered to a
pseudo-stationary flow of heat from one block to the other through a

Table 1-1 Values3 of Kinetic and Statistical Parameters
of Sodium Alkylsulfate Micelles at 25° C
Surfactant
CMC (mM)
n
a
k (see
-1)
n/k
(sec )b
T (sec)C
M
C.H..SO.Na
6 1J 4
420
17
6
1.32
X
109
13
X
Hf9
C8Hi7S°4Na
130
27
1.0
X
108
0.27
X
io'6
:iOH21SVad
33
41
9
X
io7
0.45
X
io'6
0.35 x 10'3
:ilH23SVa
16
52
4
X
io7
1.3
X
io'6
:i2H25S04Na
8.2
64
13
1
X
io7
6.4
X
io’6
72 x 10‘3
:i4H29S°4Na
2.05
80
16.5
9.6
X
io6
8.3
X
io'6
3.4
:.,H,,S0. Nae
16 33 4
0.45
100
11
6
X
104
1.7
X
io'3
37
a. All values are taken from Aniansson et al., J. Phys. Chem., 80, 905 (1976)
b. Average residence-time of surfactant monomers in micelles
c. Average life-time ( n T^) of micelles at critical micellization concentration
d. Measured at 40° C
e. Measured at 30° C

15
wire during the equilibration process of micelles (65). Any additive
which alters the micellar nucleus state will concomitantly influence
the micellar aggregation process. Kinetic measurements can provide
information about the micellar nuclei even though these species are
present in such small concentrations (10 ^ to 10 ^ M as reported
in reference 75) that they can usually be neglected in the mass bal¬
ance equation, and can not be detected by other techniques. The
experimental results have provided a new insight into the thermo¬
dynamics and kinetics of aggregation processes. In fact, it is due to
our better understanding of micellization kinetics that we are in the
position to vary the life-time of micelles over several orders of
magnitude by an appropriate choice of the chain length of surfac¬
tants, additives and counterions.
In addition to the above-mentioned step-wise association pro¬
cess, micelles can also form through a reversible coagulation-
fragmentation process of submicellar aggregates for ionic surfactants
at sufficiently high concentrations or for nonionic surfactants. This
process can be written as
A. + A.
1 J
' i+j
[1.7]
where A^, A^ are submicellar aggregates. A review article by
Kahlweit states clearly that "ionic micelles should be considered as
charged colloidal particles. At low counterion concentrations the
electrostatic repulsion prevents the coagulation of submicellar
aggregates so that micelles grow by incorporation of monomers only.
At high counterion concentrations, however, this reaction path is

16
bypassed by a reversible coagulation of submicellar aggregates. With
nonionic systems, both reaction paths compete right from the CMC
onwards." (83, page 2069). The slow relaxation time of this reversi¬
ble coagulation process is formulated as follows (83):
1/ T00 = 3 nail + a2a/n) *(A /A )^ [1.8]
22 K o g go
where is the average mean dissociation rate constant of equation
[1.7] in the absence of potential barrier; A^ is the counterion con¬
centration; A is the counterion concentration at the onset of
go
coagulation of submicellar aggregates; and q is a complex function of
the charge of counterions and of the surface potential of submicellar
aggregates.
Besides the above theories for pure micellar solutions, a theory
for the kinetics of mixed micellar systems has also been proposed
(73,116). This theory predicts two fast and one slow relaxation
processes for two-component mixed micellar systems. Two fast relaxa¬
tion processes are associated with the monomer exchange between the
micelles and the bulk solution for each of the two components, while
the slow relaxation process is the same as in the case of pure micel¬
lar solutions. The application of this theory to mixed systems of
alcohols and surfactants has received considerable attention due to
its relevance to microemulsions. A series of reports by Zana et al.
(94-98) on the effect of alcohols on micellar solutions demonstrates
the strength of combining both static and dynamic studies in probing
a complex surfactant system. Their results have experimentally veri¬
fied the kinetic theory for mixed micellar systems.

17
Although most of the dynamic studies have been focused on relax¬
ation time measurements, the analysis of relaxation amplitude (88-
90,110-111) has also been shown to provide information about the
dependence of CMC and mean aggregation number of micelles on tempera¬
ture, pressure or surfactant concentration, depending upon the per¬
turbation method applied to the system.
1.4.3 Dynamic Aspects of Microemulsions
The dynamic aspects of microemulsions have been investigated in
recent years by NMR, electron-spin probe, fluorescence and chemical
relaxation techniques (112). The results of these studies suggest
that microemulsions are highly dynamic in nature. Many dynamic
processes are reported in microemulsions (112), some of which will be
discussed in chapter 2. Similar to a micellar solution, there exists
a fast exchange of surfactants and cosurfactants between the interfa¬
cial film and the continuous or the dispersed phase. However, the
dynamic process of most concern in our study is probably the
emulsification-coalescence equilibrium through which microemulsions
are formed. This is schematically shown in Figure 1-6. The decrease
of interfacial tension with the adsorption of surfactants and cosur¬
factants onto the interface induces the formation of emulsion dro¬
plets by self-emulsification or spontaneous emulsification. The big
emulsion droplets may break further into small microemulsion droplets
under proper thermodynamic conditions. On the other hand, small
microemulsion droplets can also coalesce to form bigger droplets,
leading to an ultimate phase separation of oil and water. By adjust¬
ing the thermodynamic conditions, this dynamic equilibrium can be
shifted toward the end favoring microemulsion formation.

18
Figure 1-6. A schematic diagram depicting the dynamic
emulsification-coalescence equilibrium for
the formation of microemulsions.

19
One of the long-recognized thermodynamic conditions for the for¬
mation of microemulsions is the ultralow interfacial tension. When
the interfacial tension is low, the system favors the expanding of
interfacial area to form small microemulsion droplets. However, two
additional parameters of the interfacial film, namely interfacial
curvature and elasticity, are also important which have not yet
received equal attention before. These two parameters are related to
the organization and fluidity of the interfacial film, which would in
turn influence the dynamic structures and interdroplet interactions
of microemulsions. In fact, much of the dynamic character of
microemulsions originates from the thermal fluctuations of interfa¬
cial films. Highly fluid interfacial films can result in strong
interdroplet interactions which may shift the emulsification-
coalescence equilibrium toward phase separation.
1.5 Scope
The major thrust of this dissertation is to explore the dynamic
aspects of micellar and microemulsion systems, with emphasis on the
influence of alcohols, oils and salinity on the systems. The dynamic
properties of these systems are studied mainly in the following three
aspects: 1. kinetics of aggregation and dissolution of surfactant
aggregates in aqueous solution; 2. fluidity and curvature of the sur¬
factant interfacial film; 3. aggregate-aggregate interactions. The
correlation between dynamic and equilibrium properties of the systems
has also been particularly noted. The study starts with simple aque¬
ous micellar solutions. With additives such as alcohols and oils

20
added to the solutions, the study extends to mixed micelles and
microemulsions.
Following a brief introduction to the general equilibrium and
dynamic aspects of surfactant solutions and a review on the develop¬
ment of micellization kinetics of simple aqueous micellar solutions,
chapter 2 reviews the formation, structure and properties of
microemulsions.
The influence of alcohols on the equilibrium parameters of
sodium dodecyl sulfate (SDS) aqueous micellar solutions, i.e., the
CMC and the degree of counterion dissociation of micelles, is
reported in chapter 3. The factors which affect the CMC and the ther¬
modynamic stability of micelles are delineated. The solubilization
site of alcohols in micelles and its influence on the properties and
structure of the micelles are also discussed. Chapters 4 and 5
present experimental results on the effects of alcohols and oils on
the dynamic parameter, namely the slow relaxation time T^, of SDS
micelles. The results are basically explained by the change of
micelle nucleus concentration and the alteration of micellization
kinetics by the additives. A concept which distinguishes the thermo¬
dynamic stability from the kinetic stability of micelles is proposed.
Chapters 6 and 7 focus on the microemulsion system. Theoretical
aspects of the effects of the spontaneous curvature and elasticity of
interfacial films on the solubilization and phase equilibria of oil-
external microemulsions are presented in chapter 6. The effect of
both interfacial parameters on aggregate-aggregate interactions and
its consequences on solubilization in microemulsions are discussed.
Chapter 7 reports the experimental verification of the proposed

21
theory and further delineates the influence of the molecular struc¬
ture of various components of microemulsions on the interfacial cur¬
vature and elasticity, and its consequences on the solubilization and
phase equilibria of oil-external microemulsions. Some phenomenologi¬
cal guidelines for the formulation of microemulsions with maximum
solubilization capacity are suggested. Chapter 8 demonstrates the
use of reaction kinetics of AgCl precipitation as a probe for the
dynamic structure of microemulsions. Finally, Chapter 9 concludes
this dissertation with conclusions and recommendation for future stu¬
dies .

CHAPTER 2
REVIEW OF THE LITERATURE: FORMATION, STRUCTURE
AND PROPERTIES OF MICROEMULSIONS
2.1 Introduc tion
In 1943, Hoar and Schulman (118) first described a microemulsion
as a transparent or translucent system formed spontaneously upon mix¬
ing oil and water with a relatively large amount of ionic surfactant
together with a cosurfactant, e.g., an alcohol of medium chain length
(C^ to C-y). The system contained dispersion of very small oil-in¬
water (o/w) or water-in-oil (w/o) droplets with radii in the order of
o
100-1000 A. Figure 1-2 shows schematically these two basic microemul¬
sion structures. Since Hoar and Schulman's report, considerable
interest and attention have been focused on microemulsions. This can
be attributed to the fact that microemulsions possess special charac¬
teristics of relatively large interfacial area, ultralow interfacial
tension and large solubilization capacity as compared to many other
colloidal systems. These special features offer great potential for a
wide range of industrial and technological applications, e.g., terti¬
ary oil recovery, detergency, catalysis, drug delivery, etc.
In general, the formation of microemulsions involves a combina¬
tion of three to five components, namely, oil, water, surfactant,
cosurfactant and salt. The chemical structure of surfactant, cosur¬
factant and oil strongly influences a microemulsion phase diagram
22

23
(119-121). In fact, the complexity and diversity in properties,
structures and phase behavior of microemulsions have always posed a
persistent challenge for many theoretical and experimental research¬
ers. During the past decade, scientific literature on microemulsions
has grown at a fast pace. Several books, symposium proceedings and
review articles have been published (122-136). An exhaustive cover¬
age of all aspects of microemulsions is virtually impossible in this
chapter. Hence, the review will focus only on some fundamental ques¬
tions and some recent developments of microemulsions which are of
particular scientific interest or technological relevance.
At present, there exists no precise, or commonly agreed-upon,
definition of microemulsions. As a matter of fact, there has been
much debate about the terminology of "microemulsions," and as a
consequence, many other terms such as "swollen micelles" or "solubil¬
ized micelles" have been suggested (135). The debate centers on dis¬
tinguishing microemulsions from a true micellar solution (137-139).
Historically, microemulsions were defined from a phenomenological
viewpoint, i.e., the observation of a homogeneous, transparent and
low viscosity system containing a considerable amount of dispersed
phase with the presence of suitable surfactant and cosurfactant. At
very low volume fraction of dispersed phase, however, the system
actually resembles a true micellar solution. The transition between
these two structures generally shows no apparent break in many of the
physical properties of pure surfactant systems (140), but may exhi¬
bit a discontinuity for commercial mixed surfactant systems (141).
Based on a temperature dependence study of photon correlation spec¬
troscopy, Zulauf and Eicke (142) have established a clear transition

24
from Aerosol-OT reverse micelles in iso-octane to w/o microemulsions
at a water to Aerosol-OT molecular ratio about 10. But the relation¬
ship between normal micelles and o/w microemulsions is not as
straightforward. It has been shown that the kinetics of solubiliza¬
tion of oil is much slower for o/w microemulsions than that for nor¬
mal micelles (143). Hence, the key to this long-arguing problem prob¬
ably lies more in kinetic or dynamic measurements of the system
rather than in static measurements.
In spite of the controversy mentioned above, the designation of
a clear isotropic single-phase region in a phase diagram as
microemulsions does offer practical convenience in terminology. In
our opinion, a microemulsion can be defined phenomenologically as a
thermodynamically stable, isotropically clear dispersion of two
immiscible liquids, consisting of microdomains of one or both liquids
stabilized by an interfacial film of surface-active molecules.
2.2 Spontaneous Emulsification and Thermodynamic
Stability of Microemulsions
Two most fundamental questions in dealing with a microemulsion
are probably the mechanism of microemulsion formation and its thermo¬
dynamic stability as compared to a conventional emulsion, i.e., a
macroemulsion. A macroemulsion, upon standing, has been known to
coalesce and eventually to separate into an oil and water phase due
to a lack of thermodynamic stability (144). However, it has been
pointed out that some emulsion systems may be thermodynamically
unstable but could exhibit long term stability for practical purposes
(124). This has been referred to as "kinetic stability" of the sys¬
tem due to a high energy barrier for coalescence between droplets

25
(124). The distinction between thermodynamic stability and kinetic
stability of a system is probably only a matter of concern from a
thermodynamic rather than an operational point of view.
2.2.1 Ultralow Interfacial Tension
One of the early, important contributions from Schulman and
coworkers was the realization that the stabilization of microemul¬
sions required a low solubility of surfactant (or surfactant mixture)
in both oil and water phases (145), resulting in the adsorption of
the surfactant at the water-oil interface to lower the interfacial
tension. This can be described by the well-known Gibbs adsorption
isotherm for multiple-component systems (146):
d/ = - Z T. dy. = - Zr-RTdUna.) [2.1]
i i i i i 1
where y is the interfacial tension, p^ is the surface excess of com¬
ponent i (amount of component i adsorbed per unit area), y ^ is the
chemical potential of component i, and a^ is the activity of the
solute i. Equation [2.1] basically dictates that the increase of
surfactant activity a. in the solution would result in a decrease of
interfacial tension if the surface excess of the surfactant is posi¬
tive. Moreover, the addition of a second positively adsorbed surfac¬
tant to the system would always cause a further decrease in interfa¬
cial tension. Hence, it has been proposed that the role of cosurfac¬
tant, together with the surfactant, is to lower the interfacial ten¬
sion down to a very small--even a transient negative--value at which
the interface would expand to form fine dispersed droplets and

26
subsequently adsorb more surfactants and cosurfactants until their
bulk concentration is depleted enough to make interfacial tension
positive again. This process, known as "spontaneous emulsification,"
forms the microemulsion.
The concept of transient negative interfacial tension and its
relation to spontaneous emulsification have been proposed and experi¬
mentally examined for some time (147-150). The value of this concept
is to emphasize the importance of ultralow interfacial tension for
the formation and thermodynamic stability of microemulsions. In fact,
the mechanism of microemulsion formation has been analyzed by Ostrov¬
sky and Good (151) based on a dynamic equilibrium process in which
the rate of self-emulsification is equal to the rate of coalescence
of microemulsion droplets. The analysis established a boundary of
interfacial tension between a thermodynamically stable microemulsion
and an unstable macroemulsion. For interfacial tensions lower than
-2
10 dyne/cm, stable microemulsions can be obtained. In other ther¬
modynamic models (152-159), lower inter facial tensions in the order
-4 -5
of 10 to 10 dyne/cm have been employed to satisfy the stable con¬
dition of microemulsions.
It is known that some surfactants, e.g., many double-chain sur¬
factants and nonionic surfactants, can form microemulsions without
the addition of a cosurfactant (160-161). Although this has been
attributed to different abilities of surfactants in lowering the
interfacial tension (133), it seems that additional factors besides
the ultralow interfacial tension may have to be considered for a com¬
plete explanation. In fact, the interfacial bending instability
resulting from the thermal fluctuations of interface and the

27
dispersion entropy of droplets in the solution may also contribute
significantly to the formation of microemulsions when the interfacial
tension is low.
2.2.2 Interfacial Curvature, Fluidity and Entropy
The formation of small microemulsion droplets requires a bending
of the interface. It has been shown by Murphy (162) that the bending
of an interface requires work to be done against both interfacial
tension and bending stress of the interface. Although always
present, the bending stress is important only for very low interfa¬
cial tension or highly curved interfaces. This can be described
schematically by Figure 2-1. At an equilibrium condition with very
low interfacial tension, an interface would assume an optimal confi¬
guration and curvature, known as the spontaneous curvature 1/R^, at
which the bending energy of the interface is minimized. Further
bending the interface away from this spontaneous curvature will cause
an increase in bending energy, which can be represented by a constant
K, known as the curvature elasticity (or bending elasticity) of the
interface. The constant K with the unit of energy actually dictates
the ease of interfacial deformation. A large K value corresponds to
a "rigid" interface for which large energy is required to bend the
interface. A small K value represents a "fluid" interface for which
little energy is necessary for bending. Hence, K is also called the
"rigidity constant" of the interface. When K is close to k^T, where
k^ is the Boltzmann constant, the interface is subject to a bending
instability resulting from thermal fluctuations.

28
*
Curvature Elasticity
(Rigidity Constant)
Figure 2-1. A schematic diagram for spontaneous curvature and
' curvature elasticity of an interfacial film.
The filled circles represent oil molecules penetrating
into the interfacial film.

29
Safran and Turkevich (163) have expressed the interfacial free
energy F^. of microemulsion droplets in terms of both interfacial ten¬
sion and bending energy for an uncharged interface:
FT = n [ 4 ttXR2 + 16 irK(l - R/R )2] [2.2]
1 o
where n is the number density of droplets, ) is the interfacial ten¬
sion, R is the droplet radius and Rq the radius of spontaneous curva¬
ture (or the natural radius). Equation [2.2] only contains the ener¬
getic term, and the entropic term of interface will be discussed
later. Equation [2.2] is applicable to ionic w/o or nonionic
microemulsions where the electrostatic energy can be neglected. The
-14
value of K has been found to be in the order of 10 erg for
microemulsions (164); hence the bending energy term is important only
when y is close to zero. Accordingly, Murphy (162) has concluded
that a planar interface having a low but positive interfacial tension
could nevertheless be unstable with respect to thermal fluctuations
if the reduction in interfacial free energy due to bending exceeds
the increase in free energy due to expansion of the interface.
Therefore, he suggested that the bending instability at low interfa¬
cial tension might be responsible for spontaneous emulsification.
The preceding discussion focused on the effect of thermal fluc¬
tuations on a "fluid" interface (small K). Although the role of mem¬
brane "fluidity" for the formation of microemulsions has been noted
earlier (148), its significance is better elucidated by recent
theories and experimental results (164-166). It has been shown
(164-165) that when K is larger than kfeT, oil, water and surfactant

30
may form a birefringent lamellar phase, and only when K is small,
isotropic disordered microemulsions are obtained. A lamellar
birefringent phase is often observed in the vicinity of a microemul¬
sion phase in the phase diagram (167). The addition of cosurfactant
is found to increase the fluidity of the interface, leading to a
structural transition from birefringent lamellar phase to isotropic
microemulsions (164,166). In practice, the fluidity of an interface
can be increased by choosing a surfactant and co-surfactant with
widely different sizes of the hydrocarbon moiety (148), or by setting
a temperature so that there is a balance between hydrophilic and
lipophilic properties of the surfactant (168).
The thermal fluctuations of a fluid interface lead to an
increase in the entropy of interfacial film. The entropy of such a
fluctuating interface has been approximated by the mixing entropy of
oil and water (124). The decrease in free energy of the system due
to this dispersion entropy may exceed the increase of free energy
caused by newly created interfacial area due to emulsification, thus
resulting in spontaneous emulsification and stabilization of a
microemulsion. This has been quantitatively accounted for on the
basis of phenomenological thermodynamics by many researchers (152—
159). Be cause excellent reviews on various thermodynamic models of
microemulsions have been published (124,133), only certain important
concepts and results will be described here.
Ruckenstein and Chi (152) have expressed the Gibbs free energy
change of microemulsion formation by three terms:
AGm(R) = Agl +ag2 +ag3
[2.3]

31
where A is the interfacial free energy including a positive term
due to creation of uncharged interface and a negative term due to the
formation of electric double layer; A G^ is the free energy of inter¬
droplet interactions composed of a negative term due to van der Waals
attraction and a positive term due to repulsive double layer interac¬
tion; A G^ is the entropy term accounting for dispersion of
microemulsion droplets in the continuous medium. From equation
[2.3], the condition for spontaneous formation of microemulsions with
the most stable droplet size (R ) at a given volume fraction of
dispersed phase may be obtained:
( 3Agm/3r)r=r* = 0 [2-41
(32AG /3R2) . > 0 [2.5]
m k—k "
Equations [2.4] and [2.5] indicate that a negative, minimum
A GAR ) is required to obtain a stable microemulsion as shown in
curve A of Figure 2-2. Curve B in Figure 2-2 represents a kineti-
cally stable macroemulsion providing that the height of energy max¬
imum is significant, and curve C corresponds to an unstable emul¬
sion. Figure 2-3 shows the influence of interfacial tension on the
formation of microemulsions. When interfacial tension is less than 2
-2
x 10 dyne/cm, a stable microemulsion can be formed. Figure 2-4
shows the individual contribution of the three terras in equation
[2.3] to the stability of microemulsions. The dispersion entropy
predominantly contributes to the thermodynamic stability of
microemulsions. Rosano and Lyons (169) using a titration method have

32
Figure 2-2. A schematic illustration of the Gibbs free energy
change of microemulsion formation AG as a function
of droplet radii R. Curve A shows a stable micro¬
emulsion with droplet radius R* at the minimum AG .
Curve B shows a kinetically stable emulsion and M
curve C an unstable emulsion •

33
a Gm (cal-cm 3)
Figure 2-3. The influence of interfacial tension on the formation
of microemulsions. Small but positive values of
interfacial tension can result in a stable microemulsion.

34
0,001
o
-0,001
The contribution of AG^, AG^ and AG^ to the free
energy of microemulsion formation
Figure 2-4.

35
shown that the formation of microemulsions is indeed entropically
driven. Ruckenstein's model further predicts a phase inversion from
one type of microemulsions to another, i.e., w/o to o/w, as well as a
phase separation (152-157).
As a criterion for the formation of a thermodynamically stable
dispersion system with low interfacial tension, an inequality has
been proposed (151):
- (d lny)/(d InR)— 2 [2.6]
Though the form of this inequality may differ depending on different
thermodynamic treatments (151), many analyses do agree upon a similar
trend that for microemulsions the average equilibrium radius of dro¬
plets increases with decreasing interfacial tension (159,170), but
the reverse is true for macroemulsions (151).
To recapitulate the discussion so far, it is concluded that the
spontaneous formation of microemulsions with decrease of total free
energy of the system can only be expected if the interfacial tension
is so low that the free energy of the newly created interface can be
overcompensated by the dispersion entropy of droplets in the medium.
The bending instability resulting from the thermal fluctuations of
interface with low tension and high fluidity could be responsible for
spontaneous emulsification. Two necessary conditions for the forma¬
tion of microemulsions are as follows:
(1) Large adsorption of surfactant or surfactant mixture at the
water-oil interface. This can be achieved by choosing a surfactant
mixture with proper hydrophilic-lipophilie balance (HLB) for the

36
system. One can also employ various methods to adjust the HLB of a
given surfactant mixture, such as adding a cosurfactant, changing
salinity or temperature etc.
(2) High fluidity of the interface. The interfacial fluidity
can be enhanced by using a proper cosurfactant or an optimum tempera¬
ture.
The role of cosurfactant in microemulsion formation is to (a)
decrease the interfacial tension; (b) increase the fluidity of the
interface; and (c) adjust the HLB value and spontaneous curvature of
the interface leading to the spontaneous formation of microemulsions.
2.3 Geometric Aspects and Structure of Microemulsions
Two types of most commonly encountered microemulsions are o/w
and w/o globular droplets as shown in Figure 1-2. Some theories such
as mixed (or duplex) film theory (147-148,171-172), "R" theory (173)
and the concept of hydrophilic-lipophilic balance of surfactant
(174-175), have long been proposed in attempt to delineate the fac¬
tors which determine the formation of a specific structure, i.e., w/o
or o/w, for a given water-oil-surfactant system. Recently, a
geometric model concerning the surfactant packing at the interface
has also been proposed (170,176). All these theories define certain
parameters which can dictate the curvature of a given interfacial
film and hence predict the corresponding structure. Since reviews of
these theories are in the literature (124,141,177), only the
geometric model will be discussed.

37
2.3.1 Geometric Packing Considerations in Atnphiphile Aggregation
Basically, the geometric model emphasizes the importance of
geometric constraints in the packing of amphiphiles at the interface
for determining the structure and shape of amphiphilic aggregates.
Following the concept of duplex film, which was first proposed by
Bancroft (178) and Clowes (179), and later applied to microemulsions
by Schulman (147), the model essentially considers the interfacial
film as duplex in nature, i.e., the polar heads and hydrocarbon tails
of amphiphiles are acting as separate uniform liquid interfaces, with
water hydration in the head layer and oil penetration in the tail
layer. The key element of describing the geometric packing of sur¬
factants at the interface is a packing ratio defined as the ratio of
cross-sectional area of hydrocarbon chain to that of polar head of a
surfactant molecule at the interface, v/a 1 , where v is the volume
o c1
of hydrocarbon chain of the surfactant, a^ is the optimal cross-
sectional area per polar head in a planar interface, and 1^ is
approximately 80-90% of the fully extended length of the surfactant
chain (176).
The direction and the degree of interfacial curvature are basi¬
cally a result of this packing ratio and are further influenced by
differential tendency of water to swell the head area and oil to
swell the tail area. It is intuitively clear that a greater cross-
sectional area of tail than that of head (v/a 1 >1) will favor the
o c
formation of w/o droplets, while a smaller cross area of tail than
that of head (v/a 1 < 1) would favor the o/w droplets. A planar
o c r
interface requires v/a 1 =1 which leads to the formation of
o c

38
lamellar structure. Figure 2-5 schematically depicts the above
descript ion.
Assuming that the optimal head area a^ will not change with
interfacial curvature, Mitchell and Ninham (176) suggested a neces¬
sary geometric condition for the existence of o/w droplets,
1/3 < v/aQlc < 1 [2.7]
Equation [2.7] predicts the formation of (1) normal micelles for
v/aQlc < 1/3; (2) o/w droplets for 1/3 < v/aQlc < 1; and (3) w/o
(inverted) droplets for >1. It should be pointed out that
the increase in packing ratio v/a^^ corresponds to an increase in
o/w droplet size, but to a decrease in w/o droplet size. The boun¬
dary at v/aQlc = 1 indicates a structural transition from o/w to w/o
droplets. The structure and molecular mechanism of this phase inver¬
sion domain remain poorly understood and will be discussed next.
Similar geometric criteria have also been proposed to describe the
structure of biological lipid aggregates (180). These results seem
to suggest that the geometric packing of amphiphiles plays an impor¬
tant role in determining the structure and shape of aggregates.
One of the advantages of this geometric model is that the pack¬
ing ratio can quantitatively account for the HLB of a surfactant. A
low HLB value in the range of 4-7 favoring w/o emulsions corresponds
to v/aQlc > 1, while a high HLB value in the range of 9-20 favoring
o/w emulsions corresponds to v/aQlc < 1. Further, taking the
geometric packing term into account in a thermodynamic model can
serve as a simple approach to establish a unified thermodynamic

Increasing Chain Area and
Oil Solubility of Surfactant
v/ao'c K 1
Oi1-In-Water
Chain
m-
t
Head
v/a0lc = 1
Water Solubility of Surfactant
v/a0lc > 1
Figure 2-5. A schematic diagram representing the interfacial curvature of o/w and
w/o microemulsions and phase inversion based on the geometric model

40
framework for amphiphilic aggregation (176). In addition, the con¬
cept of the geometric model can easily account for the influence of
salt, cosurfactant and oil on interfacial curvature. For a simple
water-oil-surfactant system, a surfactant with bulky head group and
relatively small tail area, like some single-chain surfactants, tends
to form o/w droplets. To obtain w/o droplets in this case, one has
to employ cosurfactant (e.g., medium-chain alcohols), high salinity,
or oil with a smaller molecular volume or a higher aromaticity in the
system. The incorporation of cosurfactant in the interface is
expected to increase the mean hydrocarbon volume per surfactant
molecule without affecting appreciably either aQ or 1^ (176). The
addition of salt is expected to decrease head area a^ due to the
suppression of electric double layer. Oil with smaller molecular
volume or higher aromaticity can enhance the penetration of oil into
the surfactant layer thus increasing the surfactant hydrocarbon
volume (176,181-183). All these effects tend to increase the packing
ratio, hence favoring the formation of w/o droplets. On the other
hand, a surfactant with a laterally bulky hydrocarbon part and a
relatively small head group, like some double-chain surfactants,
favors the formation of w/o droplets. This can explain why Aerosol-
OT (sodium bis-2-ethyl hexyl sulphosuccinate) forms w/o microemul¬
sions spontaneously without the addition of a cosurfactant.
The effect of temperature on packing ratio is difficult to
predict due to a lack of understanding of all forces in the system.
However, experimental data for biological lipid and non-ionic surfac¬
tant systems seem to suggest an increase in v/a 1 with increasing
o c
temperature (176). This can be explained partly by the decrease in

41
water hydration of the head group (decreasing aQ) at elevated tem¬
perature (184). One thus expects a growth of normal micelles formed
by nonionic surfactants with increasing temperature due to increasing
v/aolc until the cloud point (185), beyond which phase separation
occurs. On the other hand, flocculation of micelles due to attrac¬
tive interaction between micelles at elevated temperature has also
been observed (186-188). At even higher temperatures, known as the
phase inversion temperature (PIT), phase inversion from normal to
inverted micelles occurs (161,189-190). At this PIT, one expects
v/a 1 =1 and thus a zero curvature,
o c
2.3.2 Non-Glcbular Domain and Microemulsion Structure
in Phase Inversion Region
Apart from the consideration of geometric packing presented
above, two additional geometric constraints have to be observed for
the existence of globular structure in a ternary phase diagram (191).
First, there exists an upper limit of 0.64 as the maximum volume
fraction of dispersed droplets in the solution according to a simple
random close-packing model of hardspheres (192). Second, there must
also exist a lower limit of the polar head area a below which elec-
o
trostatic repulsion between polar heads increases. Such a limit
imposes a lower bound on the size of w/o droplets because decreasing
size requires a decreasing polar head area aQ and/or an increasing
hydrocarbon volume v according to the geometric model. But this con¬
straint does not apply to o/w droplets because aQ increases as the
droplet size diminishes.

42
Taking these two constraints into account, Biais et al. (191)
have identified some domains which cannot have any globule in the
ternary phase diagram shown in Figure 2-6(a). Figure 2-6(b) shows
the experimentally determined region where no globules are observed
(193). Instead, a lemellar structure has been found in region 2. In
region 1, the solution probably consists of small hydrated soap
aggregates solvated by alcohol molecules, and dispersed in the oil
medium (193).
It has also been proposed, as shown in Figure 2-6(a), that at
least two different mechanisms of phase inversion are possible (191).
Path 1 indicates a continuous transition from w/o to o/w with an
intermediate region in which o/w and w/o droplets may coexist
(190,194-195), or a bicontinuous structure has been suggested (196-
197). A discontinuous transition is also possible along path 2
through a structure that cannot be spheres. Usually, a birefringent
lamellar structure has been observed in this case (193).
According to the prediction of the geometric model, zero curva¬
ture is expected at phase inversion, thus justifying the existence of
lamellar structure along the path 2. However, the rationale for the
continuous transition along path 1 is not so obvious. In fact, the
mechanism and structure of this continuous phase inversion remain
poorly understood. Talmon and Prager (198) have proposed a statisti¬
cal mechanical model of bicontinuous structure to account for this
continuous phase inversion without a priori concerning about the
geometric features of aggregates. It was based on a Voronoi tessela-
tion followed by random segregation of oil and water domains. Subse¬
quently, de Gennes et al. (165,199) proposed a modification by

43
SURFACTANT
Figure 2-6. Nonglobular microemulsion domains (a) Two nonglobular
domains due to the close-packing constraint (hatched
area) and the limitation of minimum head area (dotted
area). Two possible mechanisms of phase inversion
are shown.

44
Butanol/SDS = 2
Water Toluene
Figure 2-6. Continued, (b) An experimentally determined nonglobular
domain (dotted area) in water/toluene/butanol/SDS
microeraulsion system.

45
taking a cubic lattice model instead of the Voronoi tesselation. It
was shown (165) that a persistence length £ ^ can be defined as the
characteristic length of the water-oil interface. The value of £ ^
increases exponentially with increasing curvature elasticity K of the
interface. An isotropic microemulsion phase can exist when K and
consequently are small; otherwise, periodic ordered structures
such as lamellae are expected. This result elucidates the importance
of fluidity and thermal fluctuations of interface for the formation
of microemulsions. It further delineates the correlation between
isotropic random microemulsion phase and periodic ordered phase of
lyotropic nematics. In addition, the physical meaning of elementary
size of a system without well-defined geometry such as the bicontinu-
ous structure has been clarified. The bicontinuous structure (196-
197) is envisioned as containing continuous interpenetrating domains
of both oil and water with neither one surrounding the other. Though
the equilibrium mean curvature of the interface is zero, complying
with the prediction of geometric model, the interface is constantly
subject to thermal fluctuations resulting in continuous sinusoidal
bending with no specific preference toward either water or oil
phases. It should be mentioned that in addition to the spherical and
bicontinuous structures discussed above, other microemulsion struc¬
tures, such as cylinders and lamellae, have also been proposed (200).
Phase inversion phenomena bear important technological
relevance. In general, one can obtain phase inversion by changing a
large number of variables in a systematic manner. Of greatest impor¬
tance among these changes are to increase the volume fraction of
dispersed phase, to vary the salinity of the system, and to adjust

46
the temperature. When salinity is varied, a middle-phase microemul¬
sion with equal solubilization of brine and oil can be obtained at
phase inversion salinity--the so-called "optimal salinity." This has
important implications for tertiary oil recovery because maximum
solubilization and ultralow interfacial tension can be obtained at
this optimal salinity. More details of this will be discussed later.
Shinoda and Kunieda (161) have also established that maximum solubil¬
ization can be obtained for nonionic surfactant systems at phase-
inversion temperature (PIT). Maximum or optimal detergency is often
obtained at the vicinity of PIT, i.e., the cloud point (201-202).
2.3.3 Design Characteristics of Microemulsions
Based on all the preceding discussions, it can be concluded that
the geometric features (or HLB) of a surfactant play an important
role in determining the formation and structure of microemulsions.
To design a microemulsion, the use of surfactant or surfactant mix¬
ture is required to lower the interfacial tension according to equa¬
tion [2.1]. But the addition of alcohol is not a theoretical require¬
ment, although alcohol is often used to fluidize the interfacial film
(decrease K). Actually, one can also obtain a fluid interfacial film
by using a double- or branched-chain surfactant at temperature above
the thermotropic phase transition temperature (203). However, when a
cosurfactant is not used in microemulsions, it is a necessary but not
a sufficient condition that surfactant hydrocarbon volume v, effec¬
tive chain length lc and head group aQ should satisfy the relation,
v/aQlc = 1, as the elementary design characteristic for a simple
three-component, namely oil, water and surfactant, microemulsion

47
system (160). Several other variables such as the chemical nature of
cosurfactant and oil, salinity and temperature can alter the packing
ratio. Thus many parameters are available for manipulation in design
and formulation of microemulsions.
2.3.4 Shape Fluctuations and Structural Dynamics of Microemulsions
Thus far, our discussion has focused mainly on the equilibrium
structure and character of microemulsions. It should be mentioned
that, on one hand, the thermal fluctuations of interface result in a
thermodynamic stability of microemulsions, but on the other hand a
highly dynamic character of microemulsions also results due to ther¬
mally induced size and shape fluctuations (polydispersity) of spheri¬
cal microemulsions (204-205). In fact, a microemulsion should be
viewed as a dynamic structure (204). They are thermodynamically
stable, but there is a constant coalescence, break-down and deforma¬
tion of microemulsion droplets. The picture of microemulsions as
persistent entities having definite geometric shape is not accurate.
The structural dynamics of microemulsions have been investigated
by a variety of techniques and methods, such as nuclear magnetic
resonance (NMR), electron spin resonance (ESR), chemical relaxation
techniques, chemical reaction or fluorescence quenching kinetics in
microemulsions (112), and quasi-elastic light scattering (206-207).
The results of NMR and ESR studies confirm that there exists a con-
- 8
stant and fast exchange (characteristic time on the order of 10
-9
10 second) of microemulsion components (e.g., surfactant and cosur¬
factant) between the interfacial film and the continuous phase (112).
This corroborates the view that the interfacial film of

48
microemulsions is highly fluid. Further, the content of microemul¬
sion droplets, specially w/o droplets, is found to be rapidly
exchanged between the droplets through collisions and formation of
"transient dimers." This is evidenced by studying the kinetics of
chemical reactions and fluorescence quenching in microemulsions
(112,208-211). The formation of dimers has been attributed to "sticky
collisions" between droplets resulting from attractive interdroplet
interactions as suggested by neutron and light scattering studies
(212-215). Such an exchange process and formation of dimers have
important relevance to the chemical reactions occurring in microemul¬
sions. This will be demonstrated in chapter 8. The study of dynamic
aspects of microemulsions has actually advanced the fundamental
understanding on the stability, fluidity of interface, interaction
forces and collision rate of microemulsion droplets.
2.4 Solubilization and Phase Equilibria of Microemulsions
2.4.1 Solubilization and Structure of Microemulsions
Solubilization is one of the most salient features of the
microemulsion system from which most applications stem. Many early
studies of solubilization reported in a classic book by Laing et al.
(216). are based on simple soap (micellar) solutions, i.e. the abil¬
ity of surfactants to increase the solubility of hydrophobic com¬
pounds in water. Since Marsden and McBain (217) published one of
the very first phase diagrams illustrating solubilization phenomena
in a solution, the field of solubilization has expanded considerably.

49
Figure 2-7 presents a series of schematic ternary or pseudoter¬
nary (in which two components are grouped at the same vertex)
microemulsion phase diagrams. These diagrams show the changes of
general features of microemulsions when varying the alcohol chain
length, and varying the surfactant from single-chain to double-chain,
or from ionic to nonionic surfactant. Each clear microemulsion phase
region represents a solubilization area with a specific structure.
The two mechanisms of phase inversion from o/w (L^) to w/o (L^)
microemulsions described earlier can be seen in Figure 2-7. The con¬
tinuous phase inversion is often observed when a short-chain cosur¬
factant is used (K is very small), resulting in a large connecting
homogeneous solubilization area (Figure 2-7(a) and 2-7(c)). A
discontinuous phase inversion is seen in most cases with and
regions separated by some intermediate liquid crystal regions.
The factors which determine solubilization have not been com¬
pletely delineated. However, based on current theories and under¬
standing of solubilization (156-157,163,198-199,216,218-223), some
important parameters can be identified on a qualitative basis. It
has been shown that three solubilization sites are possible in a sur¬
factant aggregate (224-227). Taking a normal micelle as example,
hydrocarbons and other nonpolar compounds are thought to be incor¬
porated in the micelle interior (swollen micelles, Figure 2-8(a)).
Some solubilizate molecules may distribute themselves among the sur¬
factant molecules at the interface (Figure 2-8(b)). Polar solubil¬
izate molecules may adsorb at the micellar surface (Figure 2-8(c)).
Here the discussion of solubilization is limited only to the first
case, which is relevant to the formation of swollen micelles or
microemulsions.

50
Butanol/SDS = 2 b Pentanol/SDS = 2
Sodium Caprylate d Sodium Caprylate
Figure 2-7. Schematic ternary (or pseudoternary) phase diagrams of
various microemulsion systems. represents a normal
micelle or o/w microemulsion region. shows a reverse
micelle or w/o microemulsion region. M represents a
middle-phase microemulsion. B refers to anisotropic
phases, a & b are based on reference 119; c & d on
reference 244; f is shown at phase inversion temper¬
ature. Note that detailed liquid crystalline regions
are not shown

51
AOT f Nonlonlc Surfactant
Figure 2-7. Continued.

52
Figure 2-8. A schematic view of three possible solubilization
sites in surfactant aggregates, namely (a) the
micelle interior; (b) the palisade layer; and
(c) the micellar surface.

53
From a simple geometric calculation, it can be shown that the
total solubilization volume V in a microemulsion is equal to:
V = A R/3 [2.8]
where At is the total interfacial area and R is the radius of dro¬
plets. is related to the total emulsifier concentration in a sys¬
tem. At constant total emulsifier concentration, the solubilization
is directly related to the droplet radius and hence the curvature of
the interface (163,199,219-222). Therefore, solubilization depends
on the structure of microemulsions. Equation [2.8] predicts that
solubilization is large when R approaches infinity (zero curvature).
This explains the maximum solubilization observed at phase inversion
region as discussed earlier. Further, one often observes a smaller
solubilization area in o/w (L^) than in w/o (l^) microemulsions as
shown in Figure 2-7. This can be attributed partly to the highly
curved interface (large aQ) associated with o/w droplets resulting
from strong electric repulsion between polar heads and from strong
water hydration of polar heads (for nonionic surfactants). Increas¬
ing salinity or temperature (for nonionic surfactants) can decrease
aQ and consequently decrease the curvature, thus increasing the solu¬
bilization. It is also generally observed that that w/o microemul¬
sions form more readily than o/w microemulsions (161,228).
The above analysis focuses only on the influence of interfacial
curvature (or bending energy) on solubilization. The interaction
between microemulsion droplets can also influence the stability,
structure and hence the solubilization of microemulsions

54
(165,223,229). The long-range electrostatic repulsive force in aque¬
ous micellar solutions at higher surfactant concentrations can lead
to a structural transition from isotropic micelles to an anisotropic
ordered structure such as hexagonal or lamellar phase (230). On the
other hand, attractive force between droplets can cause coagulation
or coalescence between droplets, and consequently a phase separation
of microemulsions (165,223,229). Coagulation of o/w droplets can usu¬
ally be obtained by increasing the salinity of the system (231-232),
while increasing the fluidity of interface leads to coalescence of
w/o droplets (233-234). In any of the above events, a corresponding
change in solubilization is usually observed.
Apart from the above viewpoint that solubilization depends on
structure and properties of microemulsions, solubilization itself
also induces changes in shape, size and structure of microemulsions
(235-238). Hence, solubilization, structure and properties of
microemulsions are all interrelated.
2.4.2 Phase Equilibria of Microemulsions
When the limit of solubilization of a microemulsion is reached,
phase separation occurs and the microemulsion phase can coexist in
equilibrium with other phases. The phase equilibria of microemul¬
sions are conventionally described by a phase diagram with tie lines
as shown schematically in Figure 2-7(d).
According to the Gibbs phase rule, the degrees of freedom of a
given system at constant temperature and pressure are equal to:
C - P
[2.9]

55
where C is the number of components, and P is the number of phases in
the system. Thus, a general four-component microemulsion system,
namely oil-water-surfactant-cosurfactant system, can be constituted
by one, two, three or four phases in equilibrium. Consequently, the
approach of studying microemulsions becomes a matter of choice
depending on the problem of concern. The most popular approach is to
study the one-phase microemulsion region. However, the study of two-
and three-phase equilibria is important for understanding the stabil¬
ity and interaction forces in microemulsions. This will become more
clear as our discussion proceeds. Such a phase-equilibrium approach
is also useful for determining the composition of phase boundary.
At this time, there is very little known about four-phase
equilibria of microemulsions (239-240), hence only two- and three-
phase equilibria will be discussed. At least three types of
two-phase equilibria in microemulsions have been elucidated: (i)
Microemulsions in equilibrium with excess internal phase (i.e., w/o
microemulsions with water or o/w microemulsions with oil). This type
of phase equilibria is driven by the bending stress (or curvature) of
the interfacial film (163,199,219-222), and the phase separation
occurs due to the resistance of interfacial film to bending for
growth of microemulsion droplets. (ii) Two isotropic microemulsions
phases (containing high and low density of droplets respectively)
coexist. This phase separation is driven by attractive interdroplet
interactions (165,223,229). Critical-like behavior and sometimes a
critical point may be observed in this case (2,233-234,241-243).
(iii) Both w/o and o/w microemulsion phase coexist (221). This phase
equilibrium is driven by the balance of hydrophilic and lipophilic

56
property of a surfactant (i.e., equal solubility of surfactant in
both oil and water). Experimentally, one often observes this type of
phase equilibria at very low surfactant concentrations (244). At
sufficiently high surfactant concentrations, birefringent mesophases
are often present between w/o (L^) and o/w (Lt) microemulsions
(discontinuous phase inversion); hence no direct phase equilibrium
between w/o and o/w is observed. It has further been shown that the
first two types of phase equilibria together can give rise to three-
phase equilibria of microemulsions (i.e., microemulsions in equili¬
brium with both excess oil and water) when both bending stress and
attractive force act in parallel upon the system (163,165,199,245).
Some theoretical treatments of the above-mentioned phase equilibria
of microemulsions can be found in the literature (163,165,199,219—
223,229,245). It can be concluded that the study of phase equilibria
leads to a better understanding of the stability of microemulsions
and serves as a simple measure to assess the driving force for phase
separation of microemulsions. This is important for the design and
formulation of microemulsions.
2.4.3 Phase Behavior of Winsor-Type Microemulsion Systems
The most studied phase equilibria in microemulsions are probably
the Winsor type microemulsions (246-248) using a salinity scan as
shown in Figure 2-9(a). One can prepare such a system by mixing
equal volumes of brine and oil with a proper surfactant and cosurfac¬
tant. By increasing the salinity, one observes a progressive change
in phase diagram and behavior as described by Figure 2-9(a) and 2-
9(b).

57
s s s
Increasing Salinity â–  *
Figure 2-9. A schematic presentation of a typical Winsor-type
microemulsion showing the progression of phase
diagrams, phase volumes and interfacial tensions by
salinity scan. M, W, 0 represent microemulsion,
excess water and excess oil phases respectively.
represents the interfacial tension between the
microemulsion and excess oil phases, and y^ is
the interfacial tension between the microemulsion
and excess water phases .

58
In the low salinity region, the Winsor I system represents a
lower-phase o/w microemulsions in equilibrium with excess oil. In
the high salinity region, the Winsor II system consists of an upper-
phase w/o microemulsion in equilibrium with excess brine. It is
clear that both Winsor I and Winsor II phase equilibria are driven by
the bending stress of interfacial films.
In the intermediate salinity region, the Winsor III system is
composed of a middle-phase microemulsion in equilibrium with both
excess oil and brine. The optimal salinity is defined as the salin¬
ity at which equal volumes of brine and oil are solubilized in the
middle-phase microemulsion. The structure of this middle-phase
microemulsion has not been determined conclusively. Based on the
data of ultracentrifugation, Hwan et al. (195) proposed that the
middle phase is a o/w microemulsion near the boundary close to low
salinity region, and a w/o microemulsion near the boundary close to
high salinity region. Thus, a middle-phase microemulsion at the
optimal salinity would represent a continuous phase inversion from
o/w to w/o structure. A bicontinuous structure (196) has been pro¬
posed for the middle-phase microemulsion at optimal salinity and has
been widely examined both experimentally and theoretically
(194,223,249-253).
It is the attractive force between microemulsion droplets that
leads to a transition of both Winsor I and Winsor II to Winsor III
microemulsions (223). The transition from Winsor I to Winsor III
microemulsions has been attributed to the coacervation of normal
micelles (232), while the transition from Winsor II to Winsor III
microemulsions is associated with the percolation phenomena of w/o

59
droplets (254-259). Both these transitions have also been associated
with critical phenomena (241). Thus the phase equilibria of Winsor
III systems are governed by both attractive forces between droplets
and interfacial bending stress.
Apart from the conventional salinity scan, the transition of
Winsor type systems from o/w to w/o structure can also be produced by
changing any of the following variables in a systematic way
(232,245,253): (1) increasing the alkyl chain length or molecular
weight of surfactant; (2)increasing the surfactant concentration; (3)
increasing the aromaticity of oil; (4) decreasing the oil chain
length; (5)increasing the alcohol chain length (more oil soluble) or
concentration; (6) increasing the temperature for nonionic surfactant
system or decreasing the temperature for ionic surfactant system; (7)
decreasing the number of hydrophilic groups (e.g., ethylene oxide) of
nonionic surfactant. All these changes may be accounted for by a
corresponding change of packing ratio v/aQlc according to the
geometric model. Some important properties of middle-phase
microemulsions and their relation to tertiary oil recovery remain to
be discussed later.
2.4.4 Pseudophase Hypothesis and Dilution Method
All the phase equilibria discussed so far refer to the equili¬
bria between "macroscopic" phases. However, it is a well accepted
concept today that a bulk homogeneous microemulsion phase consists of
three microscopic domains, namely a dispersed domain separated from a
surrounding continuous domain by a domain of interfacial film. The
three-compartment model (147) of microemulsions treats each

60
microdomain as a "microscopic" phase in equilibrium with the other
two. Components of microemulsions, such as surfactant and cosurfac¬
tant molecules, will partition in all three domains under an equili¬
brium condition. Since it is assumed that equilibria between the
microdomains obey the same thermodynamic laws as the equilibria
between macroscopic phases (191), each domain has been referred as a
thermodynamic "pseudophase." This is the essence of the pseudophase
model (191,260) upon which many thermodynamic frameworks (124,261-
262) for micellar and microemulsion systems have been proposed.
Based on the pseudophase model and some simple geometric con¬
siderations, Biais et al. (191) have justified the existence of
dilution lines and the use of a dilution method for w/o microemul¬
sions. A dilution line in a pseudoternary phase diagram represents a
locus along which the volume of continuous phase of a raicroemulsion
can be increased without significantly altering the size, shape and
composition of the droplets. The existence of dilution lines is
important for the structural study of elementary microemulsion dro¬
plets by scattering techniques or centrifugation. Since the data
obtained from these experiments are themselves a function of droplet
concentration, a dilution of droplets and extrapolation to zero dro¬
plet concentration are often employed in experiments to exclude the
concentration dependence. Further, a dilution procedure is also used
to obtain information about interactions between droplets (124,263).
By diluting a w/o microemulsion, one can also determine the composi¬
tion of each pseudophase. Most importantly, the distribution of
alcohol between continuous and interfacial domains can be determined,
which by no means can be obtained from other methods.

61
One of the great difficulties in diluting a microemulsion is to
ensure the constancy of structure and composition of the droplets
during dilution. In the course of dilution, water in the dispersed
phase to surfactant ratio has been kept unchanged to ensure a con¬
stant droplet size. A dilution procedure, first proposed by Bowcott
and Schulman (145) and modified by Graciaa (264), can be implemented
as follows: first, oil is added to a transparent microemulsion until
turbidity occurs, then the transparency is reinstated by adding
alcohol together with a certain amount of water. By repeating this
titration many times and plotting the volume of added alcohol versus
that of added oil, one can obtain a titration curve as shown in Fig¬
ure 2-10. Only at a correct alcohol/water ratio corresponding to that
in the continuous phase, a linear dilution line can be obtained
(curve b in Figure 2-10). The dilution line can be described by the
following equation:
V = kV + rV [2.10]
aso
where V , V and V are the volume of surfactant, added alcohol, and
sao 7 7
added oil respectively. Assuming that the dispersed phase contains
only water and that the surfactant molecules only partition at the
interface, r gives the volumetric ratio of alcohol to oil in the con¬
tinuous phase and k provides the volumetric ratio of alcohol to sur¬
factant at the interface. The composition of all pseudophases can
thus be deduced (124,191,264).
The validity of this dilution procedure has been examined exper¬
imentally using neutron scattering (265). It was concluded that the

62
Figure 2-10. Dilution curves of a water/SDS/butanol/toluene
microemulsion. The curvature in curves a & c
indicates a change of continuous phase and
droplet composition of microemulsions during
dilution. Only the linear curve b corresponds
to the dilution line.

63
dilution method only applies to a microemulsion with well-defined
droplet structure exhibiting weak interdroplet interactions. As a
result, the use of dilution method is limited to systems with small
volume fraction of droplets (266). At higher volume fraction of
dispersed phase, such as a middle-phase microemulsion which can not
be described by a droplet structure, the dilution method fails. Many
o/w microemulsion systems cannot be diluted unless enough salt is
added to screen the electric repulsive force between droplets (124).
It may also be pointed out that a dilution line in a pseudoter¬
nary phase diagram always corresponds to a demixing line where a
phase separation from one-phase to two-phase occurs (124). No dilu¬
tion line can be observed in a one-phase region. The dilution line
should also be a straight line due to the constancy of composition
and structure of droplets during dilution. Further, dilution can only
be applied to the demixtion (or phase separation) of microemulsions
resulting from the interfacial bending stress, not from the interac¬
tions between droplets.
2.5 Experimental Studies and Properties of Microemulsions
2.5.1 Experimental Techniques for Characterizing Microemulsions
Microemulsions have been studied using a great variety of tech¬
niques. The shape, size, structure and many other physico-chemical
properties of microemulsions have been determined for various systems
(267-275). A widely used method for structural study of microemul¬
sions is the scattering method, including static and dynamic light
scattering (263,276), small angle neutron and X-ray scattering

64
(277-281). These scattering techniques not only provide detailed
structural information about microemulsions, but also measure the
interactions between droplets which can influence the structure, pro¬
perties and phase behavior of microemulsions (282). Some other tech¬
niques used in structural studies of microemulsions include sedimen¬
tation and ultracentrifugation (195,283), electron microscopy
(232,284-286), positron annihilation (287-288), static and dynamic
fluorescence methods (210,235,289), and NMR (290-291). The tech¬
niques probing the dynamics of microemulsions are NMR (181,272), ESR
(164,194), ultrasonic absorption (292-293), electric birefringence
(294-295) etc. The measurements for various properties of microemul¬
sions include conductivity and dielectric measurements (258,296-297),
viscometry (241,249,283), interfacial tension and ellipticity meas¬
urements (298-299), density and heat capacity measurements (268-269),
and vapor pressure measurements (275) etc. It is not intended here
to describe elaborately these techniques or measurements and the
information thereby obtained because reviews on some of these tech¬
niques are available in the literature (112,123-124,131,133-134).
Many of these techniques are complementary. A result obtained from
one technique often requires a comparison with other techniques to
avoid possible artifacts associated with each technique. The remain¬
ing discussion will be devoted to describing some important proper¬
ties of microemulsions which are of technological relevance, and some
applications of microemulsions.

65
2.5.2 Middle-Phase Microemulsions and Ultralow Interfacial Tension
The middle-phase microemulsion has been widely studied due to
its relevance to tertiary oil recovery processes (300). After the
primary and secondary oil recovery, a large amount of oil remains
trapped as oil ganglia in the porous rocks of the oil reservoir due
to capillary forces (301). A surfactant solution is then injected
into the reservoir to mobilize the oil ganglia by lowering the inter¬
facial tension between the oil and water phases. In tertiary oil
recovery, a lowering of oil-water interfacial tension from about 20-
-2 -3 .
30 dynes/cm to at least 10 -10 dyne/cm is required under practical
reservoir conditions (232). The formation of in-situ middle-phase
microemulsions with sufficient solubilization of oil and brine in the
reservoir by the injected surfactant solution can fulfill this
requirement.
It has been shown that as salt concentration approaches the
optimal salinity, the solubilization parameter of microemulsions
(defined as the ratio of volumes of solubilized phase to that of the
surfactant, V /V and V /V ) increases in both lower- and upper-phase
O S W S rr r
microemulsions as shown in Figure 2-9(c). At the same time, interfa¬
cial tension between the microemulsion phase and the excess phases
decreases as shown in Figure 2-9(d). Apparently, interfacial tension
is related to the solubilization parameter of microemulsions. A
higher solubilization parameter corresponds to a lower interfacial
tension. At the optimal salinity, equal solubilization of brine and
oil in microemulsions as well as equal interfacial tension of the
microemulsion phase toward both excess oil and water phases are

66
observed. These are the most important properties of middle-phase
microemulsions as related to tertiary oil recovery. Other properties
of middle-phase microemulsions such as conductivity and viscosity can
be found in the literature (254).
Some empirical rules have been proposed to predict the optimal
salinity for a given oil and surfactant system (302-305), but the
precise mechanism responsible for the ultralow interfacial tension is
not well established. The study of low interfacial tension systems
can be divided into two regimes (124,232): (i) Two-phase system with
low surfactant concentrations (0.1%-2% by weight). This is basically
a micellar system. (ii) Three-phase (Winsor type) system with high
surfactant concentrations (2%-10%), containing a middle-phase
microemulsion. In both cases, the low interfacial tension has been
attributed to the presence of a thin adsorbed surfactant and/or
cosurfactant layer (Langmuir film) with high surface pressure at the
interface (306-307). This can be described by the Gibbs adsorption
isotherm of equation [2.1]. It has also been proposed that a
surfactant-rich phase at the interfacial region containing liquid
crystalline structures may be responsible for the low interfacial
tension observed in some systems (308). However, in high surfactant
concentration regime near the optimal salinity (S*), extremely low
interfacial tensions of y,_. below S* and y_w above S* (see Figure 2-
WM OM
9(d)) have been attributed to a thick diffuse interfacial region
associated with critical phenomena (124,234,241), and the ultralow
interfacial tension has been described satisfactorily by the critical
scaling laws (234).

67
Several theoretical models (165,170,229,252,309-310) have been
proposed to predict the low interfacial tension between two bulk
phases in which micelles or microemulsion droplets are present and a
surfactant monolayer layer is adsorbed at the interface between the
two bulk phases. For most two-phase systems, the result seems to
confirm that low interfacial tension can be accounted for by the
presence of a surfactant layer at the interface. The value of inter¬
facial tension is mainly influenced by the curvature (or size) of
micelle or microemulsion droplets. However, for the critical diffuse
interface of a middle phase microemulsion near the optimal salinity,
the dispersion entropy and interactions of droplets may become dom¬
inant in determining the interfacial tension. Theoretical prediction
of interfacial tension becomes less satisfactory in this case.
Although many microscopic properties such as the interfacial
curvature, dispersion entropy and interactions of microemulsion dro¬
plets can influence the interfacial tension as predicted by many
theoretical models, the presence of microemulsion droplets in two-
phase systems is not required in maintaining the low interfacial ten¬
sion once the equilibrium between two bulk phases has been reached.
It has been shown (234) that the interfacial tension of a two-phase
system, say a Winsor I microemulsion system, remains unchanged after
diluting continuously the o/w microemulsion phase by brine (but sur¬
factant concentration has to remain above the critical micelle con¬
centration in the aqueous phase). This conclusion is also valid for
Winsor II system (234). These striking results seem to further con¬
firm the role of a surfactant layer at the interface in obtaining a
low interfacial tension. It is not clear at this time, however,

68
whether the presence of middle-phase microemulsion structure is
important for maintaining the ultralow interfacial tension of Winsor
III system because the dilution method can not be applied.
2.6 Novel Applications
Microemulsions also offer a great variety of technological,
industrial and biomedical applications. Some advantages of the
microemulsion technology are its spontaneous formation (easy to
prepare), thermodynamic stability (long shelf-life time), isotropi¬
cally clear appearance (easy to monitor spectroscopically), low
viscosity (easy to transport and mix), molecularly ordered interface
(easy to control the diffusivity as membrane), large interfacial area
(accelerate surface reactions), low interfacial tension (flexible and
high penetrating power), and large mutual solubilization of water and
oil (thus possess both hydrophilic and lipophilic characteristics).
It is these special characteristics that many applications of
microemulsions are based on. Some of potential engineering applica¬
tions of microemulsions are (1) enhanced oil recovery; (2) lubrica¬
tion, metal cutting fluid; (3) detergency; (4) improved combustion
efficiency of fuels; (5) novel heat transfer fluid; (6) corrosion
inhibition; (7) media for chemical reactions. Some potential biomed¬
ical applications of microemulsions include (1) agricultural spray;
(2) improved radiation detection fluid; (3) cosmetic and health-care
products; (4) drug-delivery systems; (5) blood substitutes and organ
preservation fluid. Surveys on some of these applications can be
found in the literature (3,131,133,311-317).

69
In conclusion, it is clear that there has been a rapid develop¬
ment and better understanding of microemulsions and their applica¬
tions since their introduction decades ago. Today, microemulsions
still offer worthwhile scientific challenges for researchers. Many
novel applications of microemulsions will probably emerge in the com¬
ing years.

CHAPTER 3
EFFECTS OF ALCOHOLS ON THE DYNAMIC MONOMER-MICELLE
EQUILIBRIUM AND CONDUCTANCE OF MICELLAR SOLUTIONS
3.1 Introduction
Alcohols are the most commonly used additives in micellar solu¬
tions to form various solubilized systems or microemulsions for a
wide range of industrial applications. As mentioned in chapter 1,
there exists a dynamic equilibrium between monomers and micelles in
surfactant solutions. The addition of alcohols influences the surfac¬
tant monomer concentration and the rates of the dynamic equilibrium
process. Many studies (318-342) concerning the effects of alcohols on
the equilibrium and thermodynamic properties of micellar solutions
such as CMC, aggregation number, structure and counterion binding,
etc. have been reported. Singh and Swarup (318) have found that the
CMC goes through a minimum upon addition of alcohols from propanol to
hexanol in sodium dodecyl sulfate (SDS) and cetyItrimethylammonium
bromide solutions. Methanol and ethanol also show a similar change
(343-344). Since the Gibbs free energy change for the addition of one
surfactant monomer to a micelle is proportional to RT In (Xq), where
is the CMC in mole fraction units, the lower the CMC, the more
negative will be the Gibbs free energy change and hence the more
stable the micelle. Thus, it can be asserted that the addition of
alcohols thermodynamically stabilizes the micelles at low alcohol
70

71
concentrations, but destabilizes them at high alcohol concentrations.
The objective of this chapter is to examine systematically the effect
of alcohol chain length on the equilibrium aspect of micellar solu¬
tions, specifically the critical micellization concentration and the
counterion dissociation of micelles. The information obtained will be
used for comparison with the effect of alcohols on the dynamic
aspects of micelles presented in chapters 4 and 5.
3.2 Experimental
Sodium dodecyl sulfate (SDS) was used as supplied by B.D.H.
(purity _ 99%). All normal alkanols with purity above 99% were used
directly without further purification. Absolute ethanol was of USP
200 proof (reagent grade) from Florida Distillers Corporation. The
critical micellization concentration was determined at 20°C using
electrical conductance method by diluting the micellar solution with
a mixed solvent of water and alcohol. The electrical conductance was
measured at 1000 Hz using Beckman conductivity bridge.
3.3 Results and Discussions
Figure 3-1 shows the change of electrical conductance of micel¬
lar solutions as a function of SDS concentration in the presence of
pentanol. The CMC can be determined from the break point of two
linear conductance curves. The CMC of SDS at 20°C was found to be 8.5
mM, in agreement with the literature (345). Figure 3-2 reports the
change of CMC of SDS with addition of various alcohols from ethanol
to hexanol. The change of CMC exhibits a minimum for alcohols from
butanol to hexanol.

SPECIFIC CONDUCTIVITY ( 10 S/cm
72
Figure 3-1.
Specific conductivity of SDS solutions as a
function of SDS concentration with addition
of pentanol

73
Figure 3-2.
The change of critical micellization concentration of
SDS with alcohol concentration

74
In addition to the information about CMC, the ratio between the
slopes of the two linear conductance curves above and below the CMC
(Figure 3-1) can also provide information about the degree of coun¬
terion dissociation from the micelles (346-347). This can be under¬
stood from the following equations. The total surfactant concentra¬
tion in a solution can be written as:
C = C + nC [3.1]
o m
where C is total surfactant concentration, C is the monomer concen-
o
tration, C^ is the micelle concentration and n is the mean aggrega¬
tion number of micelles. Assuming a complete counterion dissociation
of surfactant monomers, the total free counterion concentration in
the solution is:
C = C + anC [3.2]
go m
where C^ is the total free counterion concentration, a is the degree
of counterion dissociation from micelles. Substituting equation [3.1]
into [3.2], one obtains:
C = C + a(C - C )
go o
[3.3]
The specific conductivity of a micellar solution is equal to:

75
K = X+C + X'C + X C
sp o g 0 0 mm
where K is the specific conductivity (S/cm) of the solution,
sp r
X and X are the equivalent conductivity of the counterion
0 m
factant monomer anion and micelle respectively. Substituting
tions [3.1] and [3.3] into [3.4], one obtains:
[3.4]
, sur-
equa-
K = ( X
sp o
X")C + ( o X + X /n)(C - C )
0 0 0 m o
[3.5]
Equation [3.5] can be simply written as:
K = X c +
sp o o
X'
m
(C - C )
o
[3.6]
where X^ is the equivalent conductivity of a surfactant monomer, and
X^ is the apparent equivalent conductivity of a surfactant in
micelles. It is clear that Xq is equal to the slope of the conduc¬
tance curve below the CMC and corresponds to the slope above the
CMC shown in Figure 3-1. The ratio of these two slopes gives:
W Xn = ( aK + + O
mo 0 m q o
[3.7]
Since the mobility of a counterion is expected to be greater than a
surfactant monomer anion or a micelle, by assuming a X >> X /n and
0 m
X0 » XQ, equation [3.7] reduces to:
A / X„
[3.8]
Figure 3-3 reports the counterion dissociation of SDS micelles as a
function of pentanol concentration. The addition of pentanol to the

76
PENTANOL CONCENTRATION ( mM )
Figure 3-3. The change of degree of counterion disso¬
ciation of SDS micelles with pentanol
concentration

77
micellar solution increases the counterion dissociation of micelles.
The a value obtained for pure SDS micelles has been found to be
around 0.41. The reported literature values range from 0.14 to 0.54
(348).
3.3.1 Effect of Mixed Solvents on CMC
Short-chain alcohols are usually known as cosolvents which are
highly miscible with water. An extensive review of solvent effect on
amphiphilic aggregation by Magid (349) has indicated that the con¬
tinuous addition of cosolvents into aqueous micellar solutions usu¬
ally leads to higher CMC values (but may be proceeded by an initial
CMC depression for many penetrating cosolvents at low concentra¬
tions), smaller aggregation number and eventually a break-down of
micelles. In the literature, two factors have generally been proposed
to account for the influence of cosolvents on micelle formation. The
first one is related to the comicellization of cosolvents with sur¬
factants. Despite the high miscibility of cosolvents with water, many
cosolvents are known to penetrate into micelles (349). The effect of
such penetrating solvents has been analyzed by Zana et al. (94)
mainly in two aspects: an increase in distance between surfactant
head groups (steric effect) and a decrease in the dielectric constant
of micellar palisade layer. The CMC decreases as a result of dilution
of micellar surface charges (94). A quantitative analysis (326) has
shown that the factor governing the CMC depression is the mole frac¬
tion of the alcohol in the micellar phase, independent of the chain
length of alcohols. The chemical potential of micelles decreases
because the electrical potential of micelles decreases upon alcohol

78
penetration (326). However, a recent paper by Manabe et al. (350)
argued that the solubilized alcohols in micelles cause an increase in
the degree of ionization of the micelles but have little influence on
electrical potential at the micellar surface due to a compensation
effect from the dissociated counterions. Hence, the depression of CMC
has been attributed to an increase in mixing entropy of mixed
micelles due to comicellization (330,351).
The second factor deals with the change of structure and proper¬
ties of water upon the addition of cosolvents. The hydrophobic asso¬
ciation and micelle formation have been interpreted in term of the
structure of water. When the cosolvent is a structure-breaker, CMC
usually increases (hydrophobic interaction decreases), whereas the
CMC decreases when the cosolvent is a structure-maker (349). Frank
and Ives (352-353) have
reviewed
the structural
properties
of
alcohol-water mixtures.
Various
physico-chemical
properties
of
short-chain alcohols and water mixtures often show maxima or minima
at low alcohol fractions, suggesting a maximum structure promotion of
water by alcohols, followed by a structure disruption at higher
alcohol fractions. Hence, the decrease of CMC at low alcohol concen¬
trations shown in Figure 3-2 can be partly ascribed to the structure
promotion of water (333,349). The increase of CMC and break-down of
micelles at very high alcohol concentrations is connected with the
disruptive effect of alcohols on the structure of water, and the
decrease of dielectric constant of water.
However, the alcohol concentrations reported in Figure 3-2 is
probably not high enough to produce a disruptive effect on the struc¬
ture of water. Hence, the increase of CMC observed in Figure 3-2 is
probably due to a third factor which will be discussed later.

79
3.3.2 Effects of Alcohols on the Conductance
of Micellar Solutions
According to equation [3.5], the specific conductivity of a
micellar solution mainly depends on total surfactant concentration,
surfactant monomer concentration and the degree of counterion disso¬
ciation of micelles. At low surfactant concentrations near the CMC,
the first term in equation [3.6] is important and hence the conduc¬
tance is mainly attributed to the surfactant monomers. At higher sur¬
factant concentrations, micelles may contribute predominantly to the
conductance. Figure 3-4 shows the change of electrical conductance of
10 mM SDS (slightly above the CMC, 8.5 mM) with addition of alcohols
from methanol to heptanol. The change of conductance is similar to
the change of CMC shown in Figure 3-2, and the alcohol compositions
at the conductance minima (160 mM butanol, 62 mM pentanol and 24 mM
hexanol respectively) roughly coincide with that of CMC minima (160
mM butanol, 73 mM pentanol and 30 mM hexanol respectively). This sug¬
gests that the change of conductance upon addition of alcohols actu¬
ally reflects the change of surfactant monomer concentration (or CMC)
in 10 mM SDS solution.
Figure 3-4 can be replotted in Figure 3-5 as a function of
alcohol concentration partitioning in the micelles. The fraction of
various alcohols partitioning in SDS micelles can be obtained from
the literature (354). Figure 3-5 shows that all the conductance data,
except that of heptanol, roughly fall in a v-shape curve, and the
conductance minima of butanol to hexanol occur at about the same
alcohol concentration present in the micellar phase. This result
supports the assertion (326) that the mole fraction of alcohol in the

80
Figure 3-4. The change of specific conductivity of 10 mM SDS
with alcohol concentration

Specific Conductivity (10 JS/cm)
81
Alkanol Concentration (mM)
Present in the Micellar Phase
Figure 3-5. The change of specific conductivity of 10 mM SDS with
alcohol concentration present in the micellar phase.
The symbols used are the same as that in Figure 3-4 for
various alcohols. The filled symbols represent the
conductivity minima shown in Figure 3-4.

82
micellar phase is the governing factor for determining the surfactant
monomer concentration or the CMC. The surfactant monomer concentra¬
tion of 10 mM SDS can further be plotted as a function of
alcohol/surfactant ratio in the micellar phase as shown in Figure 3-
6. It shows that the minimum surfactant monomer concentration (or
CMC) occurs at about 2-3 alcohol/surfactant ratios in the micellar
phase, independent of alcohol chain length from butanol to hexanol.
It is noted that the 2-3 alcohol/surfactant ratios have also been
reported for various alcohol-surfactant systems, at which maximum
stability or performance of the system have been observed (355). The
result has been interpreted by the closest geometrical packing
between the alcohols and the surfactants at the optimal ratios (355).
The ratios have also been found to be the optimal compositions of
inter facial films in many microemulsion systems. The minimum CMC at
the optimal alcohol/surfactant ratios shown in Figure 3-2 is probably
due to the closest geometrical packing of the surfactants and
alcohols in the mixed micelles.
As surfactant concentration increases, the contribution from
micelles to the conductance of a micellar solution will increase as
compared to the contribution from surfactant monomers. Since the
addition of alcohol increases the counterion dissociation of micelles
as reported in Figure 3-3, the conductance will consequently increase
if the micelles contribute predominantly to the conductance. Figure
3-7 indeed supports the above statements. Upon addition of pentanol,
the decrease in surfactant monomer concentration tends to decrease
the conductance at low SDS concentrations. However, such a decrease
will be offset by an increase in the conductance due to the

83
Figure 3-6. The change of surfactant monomer concentration
in 10 mM SDS as a function of alcohol/surfactant
molar ratio in the micellar phase. The filled
symbols correspond to the conductivity minima
shown in Figure 3-4.

84
increasing counterion dissociation of micelles with the addition of
pentanol. This counteracting effect increases with increasing SDS
concentration. At about 22 mM SDS, these two opposite effects com¬
pensate each other, resulting in no change in the conductance with
addition of small amount of pentanol as shown in Figure 3-7. This
explanation can further be confirmed by a simple calculation. Based
on equation [3.6], the change of specific conductivity of a micellar
solution can be written as:
dK = ( X - x' )dC + C d( X - X' ) + Cd X'
sp omooom m
[3.9]
To observe no change in the conductance, dK
to have:
sp
= 0 and it is necessary
(X - x’ )dC + C d( x - X' ) + Cd x' = 0 [3.10]
omooom m
The total surfactant concentration at which no conductance change is
observed upon addition of pentanol can be calculated from equation
[3.10]. For pure SDS solution, C = 8.5 mM, X = 53.8 and =
o o m
22.3. Upon addition of 10 mM pentanol, Cq = 7.7 mM, Xq = 54.8 and
^ = 23.5. Then C = 22.4 mM is obtained from equation [3.10] at
m
which no change in the conductance is expected with addition of small
amount of pentanol. This is in agreement with Figure 3-7. At even
higher SDS concentrations, a direct increase in the conductance is
observed upon addition of pentanol since the micelles contribute
predominantly to the conductance.

SPECIFIC CONDUCTIVITY ( TO"* S/cm )
85
Figure 3-7. The change of specific conductivity of various SDS
solutions as a function of pentanol concentration.
The points of conductivity minima are indicated by
arrows. The inset figure shows the correlation of
SDS concentrations with pentanol concentrations at
the conductivity minima.

86
3.3.3 Formation of Swollen Micelles by Alcohols
As shown in Figure 3-4, heptanol exhibits distinctly different
effect from that of butanol to hexanol on the conductance. Since the
change of conductance reflects the change of surfactant monomer con¬
centration, the surfactant monomer concentration decreases with addi¬
tion of heptanol, instead of exhibiting a minimum as in the case of
butanol to hexanol. This can be attributed to the formation of swol¬
len micelles by heptanol. Figure 3-8 shows the schematic models of
swollen micelles. The added alcohols in micellar solutions partition
initially in the palisade layer of the micelles as shown in Figure
3-8(b). The penetrating alcohols may replace some surfactants to
form mixed micelles and provide steric shielding as well as dilution
effects on the micellar surface charges (94), thus resulting in an
increase in counterion dissociation from micelles. However, at
higher alcohol concentrations, alcohols start penetrating into the
hydrophobic core of micelles (98) to form swollen micelles as dep¬
icted in Figure 3-8(c), causing an increase in the micellar volume
and the aggregation number of surfactant in micelles. Increasing
alcohol chain length enhances the swelling of the micelles to
decrease the hydrocarbon chain density in the hydrophobic core (Fig¬
ure 3-8(d)). The formation of swollen SDS micelles by heptanol has
been evidenced by fluorescence method (341). Russell et al. (341)
have found that the aggregation number of SDS micelles first
decreases and then increases as the heptanol/SDS ratio increases.
The increase in hydrophobic volume of micelles can result in a
decrease in the curvature of micellar surfaces and a concomitant

87
Figure 3-8. Schematic models of swollen micelles
(a) a pure micelle; (b) alcohols are solubilized
in the palisade layer close to the micellar
surface. Some surfactants may be replaced by the
alcohols, providing steric shielding and
dilution effects on the surfactant head groups;
(c) alcohols are solubilized in the hydrophobic
core of micelles at sufficiently high alcohol
concentrations, resulting in an increase in the
volume of the hydrophobic core and a decrease
in interfacial curvature (swollen micelles);
(d) long-chain alcohols can swell the micelles
immediately; (e) nonpolar oils are solubilized
in the hydrophobic core of the micelles •

88
closer packing of surfactant head groups. This will lead to a recom¬
bination of counterions to the micelles and a consequent decrease in
the conductance. The result shown in Figure 3-9 supports this asser¬
tion. Since micelles contribute primarily to the conductance of 100
mM SDS solution, the electrical conductance is expected to increase
due to increasing counterion dissociation from micelles with addition
of alcohols from propanol to octanol. The decrease of conductance
with addition of methanol and ethanol can be explained by the
decreasing dielectric constant of water. At sufficiently high alcohol
concentrations, swollen micelles are formed and consequently the con¬
ductance decreases. Table 3-1 lists the alcohol concentrations and
the alcohol/surfactant ratios in the micellar phase at the conduc¬
tance maxima observed in Figure 3-9. The alcohol/surfactant ratio
required to swell the micelles decreases with increasing alcohol
chain length. Note in Figure 3-9 that swollen micelles form right
upon the addition of decanol.
Table 3-2 tabulates the initial slopes of conductance increase
for various alcohols shown in Figure 3-9. The ratios between the
slopes (first column in Table 3-2) can provide at first approximation
the relative partitioning of different alcohols in 100 mM SDS
micelles. By taking the partitioning of hexanol in the micelles as 1,
the relative partitioning of butanol and pentanol in the micelles
obtained from the conductance measurements is close to the literature
values as indicated in the parenthesis of the first and tne second
columns in Table 3-2. The discrepancy in propanol is due to the high
solubility of propanol in water, which may significantly alter the
dielectric constant of water, hence the conductance measurement does

SPECIFIC CONDUCTIVITY ( 10'¿ S/cm
89
Figure 3-9. The change of specific conductivity of 100 mM SDS
as a function of alcohol concentration. The inset
figure shows the enlarged details of low alcohol
concentration region. The filled points represent
turbid solutions.

Table 3-1 Alcohol/Surfactant Ratios in the Micellar Phase at the Onset
of Formation of Swollen Micelles in 100 mM SDS Solution
Fraction'3 of Alcohol
Concentration Partioning in the Alcohol/Surfactant
of Alcohols (M)
Micellar Phase
RatioC ii
butanol
0.68
0.44
3.0
pentanol
0.47
0.77
3.6
hexanol
0.234
0.92
2.2
heptanol
0.099
0.96
1.0
octanol
0.051
0.985
0.5
a. Total alcohol concentration at the conductance maxima reported in Figure 3-9.
b. The fraction of total added alcohol present in the micellar phase.
All values taken from Stilbs, P. , J. Colloid Interface Sci., 87, 385 (1982).
c. Assuming the surfactant monomer concentration in the aqueous phase
is negligible; hence all surfactants are present in the micellar phase.

91
Table 3-2 Initial Slopes of Conductance Increase in Figure 3-9
Fraction0 of Alcohol
Slope
(10 S/cm M)
Partitioning in the
Micellar Phase
Slops
(10 S,
propanol
0.59
(0.08)b
0.32 (0.348)d
1.84
butanol
2.9
(0.394)
0.44 (0.48)
6.53
pentanol
5.2
(0.718)
0.77 (0.837)
6.75
hexanol
7.3
(1)
0.92 (1)
7.95
heptanol
6
(0.82)
0.96 (1.04)
6.29
octanol
4.1
(0.56)
0.985(1.07)
4.19
a. Increase in the specific conductivity per mole of total
added alcohol. The data point of pure 100 mM SDS was not
included in the calculation of initial slopes.
b. The values in the parentheses represent the relative ratios
between slopes by taking the slope of hexanol as 1.
c. Taken from Stilbs, P. , J. Colloid Interface Sci., 87, 385 (1982).
d. The values in the parentheses represent the relative ratios
between the fractions of alcohols partitioning in the micellar
phase by taking hexanol as 1.
e. Increase in the specific conductivity per mole of alcohol
partitioning in the micellar phase.

92
not provide correct estimation for alcohol partitioning. The rela¬
tive partitioning of heptanol and octanol in the micelles has been
underestimated by the conductance measurement because the increase in
conductance due to alcohol penetration may be offset by a decrease of
conductance resulting from the swelling of micelles by heptanol and
octanol. When one compares the slopes of the conductance increase
per mole of alcohols partitioning in the micellar phase (third column
in Table 3-2), it can be seen that butanol, pentanol and hexanol
exert roughly the same effect on the conductance, indicating that the
alcohol/surfactant ratio in the micellar phase is the governing fac¬
tor for counterion dissociation from micelles, independent of alcohol
chain length at least from butanol to hexanol.
It may also be pointed out that for many nonpolar solubilizates
like hydrocarbon oil, solubilization would take place in the hydro-
phobic core of micelles (Figure 3-8(e)), and hence swollen micelles
are formed immediately upon the addition of these solubilizates.
Some data of the solubilization of oil in SDS micelles will be
presented in chapter 5.
The formation of swollen micelles may be the initial step for
the formation of oil-in-water (o/w) microemulsions. Conductance
measurements can reveal such a structural transition from simple
micelles to swollen micelles or o/w microemulsions. Figure 3-10
shows a maximum conductance at the onset of o/w microemulsion forma¬
tion, similar to the case of swollen micelle formation. The initial
increase in the conductance is due to alcohol partitioning at the
interfacial films of macroemulsions, resulting in an increase in
interfacial fluidity and interfacial curvature ( decrease in droplet

SPECIFIC CONDUCTIVITY ( 10~2 S/cm
93
Figure 3-10. The change of specific conductivity as a function of
pentano1 concentration in 20 ml 0.5 M SDS solution
containing 1 ml dodecane. Note that the maximum
conductivity corresponds to the onset of clear oil-
in-water microemulsion formation.

94
size). At the onset of microemulsion formation, further addition of
alcohol only swells the droplets, and hence decrease the conductance.
3.4 Conclusions
The conclusions obtained from this chapter can be summarized as
follows:
1. The dynamic monomer-micelle equilibrium is very sensitive to the
addition of alcohols. The alcohol/surfactant ratio in the micel¬
lar phase is an important factor for monomer-micelle equili¬
brium, independent of alcohol chain length from butanol to hex-
anol.
2. The penetration of alcohols into the palisade layer of micelles
(formation of mixed micelles) can decrease the surfactant mono¬
mer concentration due to the increase in the mixing entropy of
mixed micelle formation. It can also increase the counterion
dissociation from the micelles due to the steric shielding and
dilution effects on the micellar surface charges.
3. The alcohol/surfactant ratios of 2 to 3 in the micellar phase
appear to be the optimal values at which the maximum stability
of micelles occurs as evidenced by the minimum CMC value. The
maximum stabilizing effect at the optimal ratios is attributed
to the closest geometric packing of the surfactants and alcohols
in the micelles.
4. The conductance of a micellar solution is determined by both
surfactant monomers and micelles. At low surfactant

95
concentrations (close to the CMC), contribution from the former
is important, while at high surfactant concentrations, contribu¬
tion from the latter dominates. For low surfactant concentra¬
tions, the conductance exhibits a minimum as a function of
alcohol concentration. At higher surfactant concentrations, the
conductance increases immediately upon the addition alcohols.
5. At sufficiently high alcohol concentrations or long alcohol
chain length, solubilization of alcohols takes place in the
hydrophobic core of the micelles, leading to the formation of
swollen micelles. The formation of swollen micelles often
results in a decrease in surfactant monomer concentration and a
recombination of counterions to the micelles due to a closer
packing of surfactant head groups on less curved micellar sur¬
faces, thus decreasing the electrical conductance of the micel¬
lar solution. The formation of swollen micelles is an initial
step in the structural transition of a simple micellar solution
to an o/w microeraulsion.

CHAPTER 4
DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS: EFFECTS OF
SHORT-CHAIN ALCOHOLS AND POLYMERS ON MICELLE STABILITY
4.1 Introduction
As introduced in chapter 1, studies on the kinetics and dynamic
properties of micellar solutions have received considerable attention
in recent years. A number of reports (65,69-117) employing various
chemical relaxation techniques, such as stopped-flow, temperature-
jump, pressure-jump and ultrasonic absorption, have provided a new
insight into the dynamic aspect and micellization kinetics of surfac¬
tant solutions. The theoretical frame work, first developed by Ani-
ansson and Wall (71,73) and supplemented by Kahlweit (83-84) and Hall
(93), successfully correlates the dynamic parameters of a micellar
solution with its thermodynamic or equilibrium parameters, and hence
makes the dynamic study of micellar solutions informative and unique.
Some review articles on dynamics of surfactant solutions has
been published (83,112,356). It appears that the kinetics of pure
micellar solutions have been well established. But very few studies
(93-98,356) focus on the effect of solubilizates (e.g., alcohols and
oils) on the dynamic properties of micellar solutions. It has been
concluded in chapter 3 that the addition of alcohols thermodynami¬
cally stabilizes the micelles at low alcohol concentrations, but des¬
tabilizes them at high alcohol concentrations. However, this is
96

97
opposite to the results obtained from some dynamic studies of mixed
micellar solutions (95). The slow relaxation time has been found
to decrease upon the addition of alcohols (95), indicating a shorter
life-time and a less stable micelle. Such a paradox between the
equilibrium and dynamic studies of mixed micellar solutions stimu¬
lated us to examine systematically the effect of alcohols and some
other additives on the dynamic properties of micellar solutions, and
re-evaluate the concept of micelle stability.
4.2 Experimental
The surfactant (SDS) and alcohols used are the same as that in
chapter 3. The nonionic polymer polyvinylpyrrolidone (PVP) NP - K90
(average molecular weight 360,000) was supplied by GAF Corporation.
l-ethyl-2-pyrrolidinone was from Aldrich Chemical Company. In
preparing a surfactant-polymer solution, a stock polymer solution at
the desired polymer concentration (weight of polymer/100 gm water)
was prepared first. Then the stock was used as solvent to prepare a
surfactant solution at the desired surfactant concentration. The
relaxation time T^ was measured using pressure-jump apparatus with
conductivity detection from Dia-Log Corporation, West Germany. The
basic principle of pressure-jump is to increase the pressure of the
sample cell until a brass diaphragm bursts at about 140 bar. The
pressure in the sample cell will suddenly drop to the atmospheric
pressure, and the relaxation time can be obtained from the change of
conductivity of the sample with time. All measurements were carried
out at 20 C.

98
4.3 Results and Discussions
The fast relaxation time T ^ was too fast to measure by our
pressure-jump apparatus, hence only the slow relaxation time T ^ has
been investigated. Figure 4-1 shows the slow relaxation rate con¬
stant 1/ of a 10 mM SDS solution upon the addition of normal
alkanols from methanol to pentanol. The concentrations of all
alkanols are limited to the CMC-decreasing region reported in chapter
3. The slow relaxation rate constants are found to increase for all
alcohols except pentanol, in which a reverse trend is observed. This
indicates that short-chain alcohols from methanol to butanol labilize
the micelles. In contrary to this interpretation, the equilibrium
data on the change of CMC suggests that these alcohols stabilize the
micelles. When the alcohol concentrations are extended to much
higher values where the CMC increases, the rate constant 1/t ^ of 100
mM SDS still increases monotonically as shown in Figure 4-2 for all
alcohols except pentanol. Based on the dynamic property of micellar
solutions reported in Figures 4-1 and 4-2, alcohols from methanol to
butanol can be categorized as short-chain alcohols which labilize the
micelles; and alcohols from pentanol onwards as medium- and long-
chain alcohols.
4.3.1 Labilizing Effect of Short-Chain Alcohols on Micelles
It is apparent that in the multi-equilibrium, step-wise associa¬
tion process of micelle formation described in equation [1.1], each
aggregate is in dynamic equilibrium with monomers. The fast relaxa¬
tion process can readily be understood as an overall process involv-

i/r2 ( io sec'
99
Figure 4-1.
The change of the slow relaxation rate constant
of 10 mM SDS as a function of alcohol concentration

100
CONCENTRATION OF 1 - ALKANOLSt lo’mM)
Figure 4-2. The change of the slow relaxation rate constant of
100 mM SDS as a function of alcohol concentration

101
ing the chemical equilibria between monomers and aggregates. However,
the origin of the slow relaxation process is not as explicit. It
actually arises from the fact that the formation of small aggregates
(often referred as oligomers or premicellar aggregates), being ther¬
modynamically unstable (358), are in extremely low concentration and
thus limit the relaxation flux of micelles. The slow relaxation time
is thus a measure of the ease of micelle nucleus formation.
The increase of 1/T ^ in Figure 4-1 and 4-2 suggests that the
addition of short chain alcohols actually favors the formation of
small aggregates and thus eases the micelle nucleus formation. This
can be qualitatively explained in parallel with the discussion in
chapter 3 about the effect of alcohols on equilibrium properties of
micellar solutions. In the CMC-decreasing region, the addition of
alcohol decreases the chemical potential of micelles leading to a
decrease of monomer concentration. It also decreases the surfactant
aggregation number of mixed micelles resulting in a greater number
density of micelles in the solution. This has been confirmed experi¬
mentally by a fluorescence study (322) and explained as the result of
the replacement of surfactant molecules by alcohol molecules in the
mixed micelles. In the CMC-increasing region, the diminishing hydro-
phobic driving force disfavors the formation of large micelles and
consequently produces more small aggregates in the solution. The net
result of all is a continuous increase in the population of small
aggregates and hence a monotonic increase of 1/ T^ upon the addition
of short-chain alcohols.
This can further be illustrated by the theory of Aniansson and
Wall (71). The simplest equation for the slow relaxation process

102
takes the form of
1/ T ^ = n2 (A^) * k“ (1 + where the subscript r denotes the aggregation number at the minimum
of the theoretical distribution curve of micelle population shown in
Figure 1-5; A^ is the number density of micelles at the distribution
minimum (i.e., the concentration of micelle nuclei); k” is the disso-
r
ciation rate constant of a surfactant molecule from a micelle
nucleus, and the rest of the parameters assume the same definition as
that of equation [1.4]. According to equation [4.1], the rate of
slow relaxation process is a function of aggregation number (n),
2
monomer concentration (A^), surfactant concentration (1 + a a/n),
the dissociation rate constant k and the concentration of micelle
r
nuclei (A^). One often observes orders of magnitude change in 1/
when a small amount of additives is added (65). Such a drastic change
in the rate of slow relaxation process is not likely to be attributed
to any of the above parameters except A^_ (65,95). This is partly due
to the simple physical constraint of a system, i.e., many parameters
such as aggregation number of micelles can not change by orders of
magnitude upon small perturbation. Hence the slow relaxation process
is primarily a function of the number density of micelle nuclei (or
oligomers) in the solution: the more oligomers in the solution, the
more rapid is the formation and dissolution of micelles.
For experimental data analysis, the theory of kinetics for mixed
micellar solutions (73,116) is too complex to use. However, when the
additive concentration is relatively low, the kinetic theory applied

103
to pure micellar solutions can be extended to the case of mixed
micelles, provided that all the parameters are not changed signifi¬
cantly by additives. For 10 mM SDS solution, the equation of slow
relaxation process can be written as (65,95)
1/ T2 = k+ Kr"2 A^~1 n2 ( 1 + a2a/n)'1 [4.2]
where k+ is the association rate constant of a surfactant molecule
into a micelle; K is the average equilibrium constant of the associa¬
tion processes in the oligomer region. As shown by Yiv et al. (95),
it is possible to estimate how the r and K values change upon addi¬
tion of propanol in terms of two limiting cases. The value of K for
pure dilute SDS micellar solution is first calculated by using the
_ 3
parameters as follows (65): T 2 = 2.32 x 10 sec, n = 64, CMC = 8.5
x 10 3 M, k = 1.2 x 10^M ^sec CT = 13 and r - 7, K is thus
obtained as 3.51 M Upon the addition of propanol, k+ is prac¬
tically constant due to the fact that the association of a surfac¬
tant molecule to a micelle is close to a diffusion-controlled process
(75). Moreover, there is no sizable change in CMC and there is only
a small change (less than 10%) in aggregation number (320) for pro¬
panol concentrations up to 0.1 M. Hence, it can be assumed that
all parameters do not change upon addition of propanol except r and
K. Approximate 30% and 130% increases in 1/ were observed in Fig¬
ure 4-1 in the presence of 0.03 M and 0.1 M propanol, respectively as
compared to pure 10 mM SDS at 20° C. Assuming K does not change as
the first limiting case, r = 6.93 and 6.76 are obtained for 0.03 M
and 0.1 M propanol respectively. As the second limiting case, K is

104
calculated to be 3.69 and 4.16 for 0.03 M and 0.1 M propanol respec¬
tively, provided that r does not change. The increase in K indicates
the stabilization of micelle nuclei by the alcohol.
It is clear from the above calculation that a small variation of
r or K can result in a large change in 1/ tYiv et al. (95) have
pointed out the physical significance of these two limiting cases:
the first corresponds to the case where no alcohol partitions into
the species at the minimum of the size distribution function, whereas
the second limiting case implies that all the species are assumed to
contain alcohol and therefore are stabilized by the alcohol. In
actuality both r and K may change simultaneously and hence make the
data analysis difficult.
4.3.2 Influence of Alcohol Chain Length and Surfactant
Concentration on Labilizing Effect of Alcohols
Since the plot of log 1/ T ^ versus the concentration of alcohols
is linear as shown in Figures 4-1 and 4-2, the comparison of the
slopes can provide the information about the effect of alcohol chain
length on labilizing the micelles. Figure 4-3 shows the plot of the
slopes in Figures 4-1 and 4-2 as a function of alkyl chain length of
alcohols. For 10 mM SDS, the labilizing power of alcohols increases
with alkyl chain length up to L = 3, then levels off at L = 4.
Considerable hydrocarbon/water contact in micelles has been sug¬
gested from some theoretical considerations of micelles (359-362). A
few instances of experimental evidence (363-368) support the view
that water does penetrate into the micellar surface, but opinions
differ with respect to the exact depth to which water really

Figure 4-3.
A plot of the labilizing power of alcohols as a function of alcohol
chain length. Slopes in Y-axis are from Figures 4-1 and 4-2.
105

106
penetrates. Accordingly, the existence of a loosely packed palisade
layer with considerable water hydration or penetration on the micel¬
lar surface has been suggested. The thickness of the palisade layer
may be equivalent to about three carbon-carbon bond length as sug¬
gested from the CMC measurement (94) and the partial molal volume
measurement of the solubilized alcohols in micelles (354). The
abrupt change in the slopes from propanol to butanol shown in Figure
4-3 may be attributed to this water penetration effect. It is likely
that short-chain alcohols with three carbons or less mainly partition
in the palisade layer, where the microenvironment experienced by
alcohols is probably not much different from the bulk water. The
exchange of alcohols between bulk phase and micellar phase is
extremely fast and without much hindrance. The measured relaxation
rate is mainly due to the micellization kinetics of surfactant
molecules. However, as the alcohol chain length increases to four
carbons or more, alcohols start penetrating into the hydrocarbon core
of micelles. The exchange of alcohol between the micelles and the
bulk solution will then be hindered and slowed down (357), and there¬
fore the relaxation rate of mixed micelles reflects partly the rate
of solubilization of alcohols into micelles. This can partly explain
the negative slope observed in the pentanol case of Figures 4-1 and
4-2. Another possible explanation is that short-chain alcohols can
stabilize micelle nucleus species by partitioning into them.
Alcohols of longer chain length can only be solubilized in the
micelles without partitioning into the nuclei, because the solubili¬
zation of longer alcohols probably requires a more hydrophobic
environment like the micellar interior. The environment of micellar

107
nuclei is probably close to that of the loose water-hydrated palisade
layer at the micellar surface.
Both curves in Figure 4-3 can be best fit into second order
equations by least-square method:
S = 0.32 - 0.07L + 0.33L2 [4.3]
S = 0.69 -0.075L + 0.215L2 [4.4]
-3 . . .
where S x 10 is the slope of linear lines in Figures 4-1 and 4-2,
and L is the number of carbon atoms of alcohols. Equation [4.3] is
for 10 mM SDS and equation [4.4] for 100 mM SDS. Two points emerge
from this result. First, the effect of alcohols in labilizing
micelles increases with increasing alkyl chain length up to propanol.
This can be explained by the increasing partition of alcohols in the
micelles and the stronger disruptive effect on water structure as the
alkyl chain length increases (369). Secondly, the nonlinearity of the
curve implies a varying power of each additional methylene group of
the alcohol for labilizing the micelles : the deeper the penetration
of methylene group into the micelle, the higher is its labilizing
power. Each additional methylene group probably also experiences a
progressively changing environment from more hydrophilic (bulk water)
to more hydrophobic (hydrocarbon core). Equations [4.3] and [4.4] can
be rewritten as
S' = (S - 0.32)/L = -0.07 + 0.33L
[4.5]

108
S' = (S - 0.69)/L = -0.075 + 0.215L [4.6]
The plot of S' vs. L gives a straight line as shown in Figure
4-4. The term S' can be interpreted as the average labilizing power
per methylene group of an alcohol assuming all methylene groups exert
the same effect on micelles. Figure 4-4 suggests that the average
labilizing power per methylene group is lower in 100 mM SDS than in
10 mM SDS for all alcohols. One could attribute this to the lower
alcohol/surfactant ratio of 100 mM SDS than that of 10 mM SDS. How¬
ever, this reasoning should be discarded in view of the result shown
in Figure 4-5, in which the effect of propanol on the slow relaxation
rate constant of various SDS concentrations are reported. The slope
decreases as the concentration of SDS increases from 10 mM to 50 mM,
then keeps constant when the concentration of SDS doubles from 50 mM
to 100 mM, and increases thereafter as the concentration further
increases.
It may not be easy to interpret such abnormal change completely
without ambiguity. However, it has been shown that two extrema exist
in the plot of 1/ T2 versus SDS concentration: a maximum around 20 mM
SDS (65) and a minimum at about 180 mM SDS (83). This trend is very
similar to that in the change of labilizing power of propanol versus
SDS concentration shown in Figure 4-5. It seems that higher labiliz¬
ing power of alcohols always occurs in the region where 1/
increases. The increase in 1/ T^ indicates a decreasing resistance
for micelle formation. The first 1/ x2~increasing region at SDS con¬
centrations lower than 20 mM has been interpreted in terms of the
decreasing r, or the increasing A^_ with
increasing surfactant

0 12 3
Alkyl Chain Length of Alkanols, L
Figure 4-4. A plot of average labilizing power per methylene group of alcohols
as a function of alcohol chain length
109

110
CONCENTRATION OF I-PROPANOL (lo’m/vO
Figure 4-5. The change of the slow relaxation rate constant of
SDS at various concentrations as a function of
propanol concentration

Ill
concentrations (76). The high labilizing power of alcohols in this
region is probably due to the partitioning of alcohols into the
micelle nuclei. However, as the surfactant concentration further
increases, the population of micelles is expected to outgrow that of
micelle nuclei, and hence a diminishing concentration ratio of nuclei
to micelles results. A smaller labilizing effect on 50 mM SDS is
then expected due to less partitioning of alcohols into the micelle
nuclei. The fact that no change in the slopes of 50 mM and 100 mM
SDS, even under the condition of twice less alcohol concentration in
micelles expected for 100 mM SDS solution as compared to 50 mM SDS
solution, strongly supports our previous assertion that the slow
relaxation process is more dominated by the parameters related to
micelle nuclei rather than the parameters related to micelles. The
second 1/ T^-increasing region at SDS concentrations higher than 180
mM has been explained by the reversible coagulation of submicellar
aggregates (83). It is suspected that the addition of alcohols leads
to a fluidization of micellar interior (328) and a higher degree of
ionization of micelles (higher counterion concentration), which con¬
sequently accelerate the mutual coagulation of submicellar aggre¬
gates .
4.3.3 The Concept of Micelle Stability
In an analogy to a nucleation process, the rate-determining step
in the process of micelle formation and dissolution has been found to
be the micelle nucleus formation (75-76,88). The study of thermo¬
dynamics of micelle formation regards the picture of micelles as a
thermodynamically stable, statistical entity existing at the most

112
probable size with a statistically distributed population, while the
study of kinetics of micellar solutions emphasizes the concept that
such statistical entity is actually in dynamic equilibrium with mono¬
mers. Each micellar aggregate may fluctuate in size around the vicin¬
ity of its mean value by picking up or releasing some monomers at a
time as reflected by the fast relaxation process, but a complete for¬
mation or dissolution of a micelle by passing through the micelle
nucleus formation steps is a much slower process than the former as
reflected by the slow relaxation process. The latter is the govern¬
ing factor in controlling the life-time of micelles. The difference,
by orders of magnitude, in the rate of both processes can be under¬
stood partly from the energetics of both reactions. As calculated by
Aniansson et al. (65) from the temperature dependence of relaxation
rate, the activation enthalpy for the exchange of monomers has been
found to be -4.3 Kcal/mole, while the activation enthalpy for the
nucleus formation is around 33 Kcal/mole for SDS micelles.
The conventional thermodynamic analysis via CMC measurement only
leads to a conclusion about the "thermodynamic stability" of micelles
in terms of how surfactants distribute between the monomer and micel¬
lar state; while the dynamic study leads to the concept of "kinetic
stability" of micelles depicting the average life-time of a micellar
aggregate. It is possible that on one hand the addition of short
chain alcohols increases the thermodynamic stability of micelles,
thus driving more surfactants into micellar state, while on the other
hand it can decrease the kinetic stability of micelles leading to
faster formation and dissolution of micellar aggregates in the solu¬
tion. In addition to the possible change of r and K in equation

113
[4.4] with the addition of alcohols as analyzed previously, the
labilizing effect of alcohols may also be attributed to the decreas¬
ing activation energy of micelle nucleus formation. A decrease of
activation energy from 37.5 Kcal/mole to 36.4 Kcal/mole and 34
Kcal/mole for 10 mM SDS upon the addition of 20 mM and 140 mM pro¬
panol respectively has been observed.
4.3.4 Micelle Nucleus Formation as the Rate-Limiting Step:
Evidence from the Polymeric Additives
One could argue that since the rate-limiting step of micelle
formation depends on the rate of nucleus formation, one may observe
only a single relaxation process in micellar solutions if there was
no constraint on the formation of micelle nuclei. As a matter of
fact, 1/ of 100 mM SDS is too fast to measure on pressure-jump
apparatus at propanol concentrations higher than 1.5 M (Figure 4-2).
It may be proposed that the slow relaxation time T ^ may
progressively merge with the fast relaxation time at higher pro¬
panol concentrations. The surfactant aggregates at this stage may
resemble the oligomers which are formed through a continuous associa¬
tion process (77), in contrast to a true micellization process. The
major difference between these two processes is that no minimum
exists between monomers and aggregates in the distribution curve of
micelle population for continuous association process.
In addition to short-chain alcohols, the presence of polyme.j in
micellar solutions also expedites the slow relaxation process (370).
The polymer effect can actually be utilized to further delineate
the mechanism of slow relaxation process in a micellar solution.

114
Figure 4-6 show the change of 1/ for 100 mM SDS upon the addition
of a nonionic polymer polyvinylpyrrolidone (PVP). It is surprising to
observe a three orders of magnitude increase in 1/ as PVP concen¬
tration increases from 1% to 8 % (w/w). In contrary to this experi¬
mental observation, a decrease in 1/ by two orders of magnitude
may be theoretically expected as far as the diffusion of surfactant
is concerned. This is because that the viscosity of the solution
increases by two orders of magnitude as concentration of PVP
increases from 1% to 8% (371), a decrease by the same orders on k+ in
equation [4.2] is thus anticipated assuming a simple Stoke-Einstein
diffusion relation (372). This contradiction poses an interesting
problem for the mixed surfactant-polymer systems.
It is well known that surfactants interact with polymers in many
systems (373-378). The specific association between surfactants and
polymers leads to a decrease of free surfactant monomer concentration
in the solution, and induces a surfactant-polymer complex formation.
Despite a lack of complete understanding about the exact nature and
structure of this surfactant-polymer complex, experimental evidence
strongly indicates (373-378) that the complex resembles the charac¬
teristics of mixed micelles between the surfactants and polymers, and
may even mimic the true micelles of pure surfactant solutions at high
surfactant concentrations. Three regions with distinct surfactant-
polymer binding are often identified experimentally upon increasing
surfactant concentration in a polymer solution (373-378): very little
surfactant-polymer binding in region I, specific stoichiometric bind¬
ing of surfactants onto polymer in the form of clusters of premicel-
lar aggregates in region II, and true micellization in region III.

115
CONCENTRATION OF PVP(% w/w)
Figure 4-6. The change of the slow relaxation rate constant
of 100 mM SDS as a function of PVP concentration

116
The boundary between region I and II denotes the effective CMC
of the surfactant with the presence of polymers. The effective CMC
is always found to be smaller than that of pure surfactant, suggest¬
ing the formation of mixed micelles in region II. Cabane (376-377)
has characterized the structure of mixed micelles in region II
(referred as stoichiometric aggregates) based on NMR and neutron
scattering studies of SDS-poly(ethylene oxide) systems. The essen¬
tial features are the follows: (i) each stoichiometric aggregate con¬
tains a single macromolecule with associated SDS molecules; (ii) the
SDS molecules in stoichiometric aggregates are clustered in subunits
which are small spheres similar to the micelles formed in pure SDS
solution at low ionic strength; (iii) these subunits are adsorbed on
the polymer strands. No penetration of polymer through the hydrocar¬
bon core is observed, hence the adsorption occurs on water-micelle
interface and involves weak binding. The overall picture of the com¬
plex is that the polymer is wrapped around micelles in a random coil
block. The formation of such stoichiometric aggregates continues
through region II up to region III where the stoichiometric binding
saturates. In region III, the excess surfactants form regular
micelles in equilibrium with the stoichiometric aggregates of region
II. This picture of surfactant-polymer complex is expected applicable
to SDS-PVP system due to the existence of similar transitions of
three regions (373-374).
It is clear irom the above description that the presence of
polymer does not alter the micellization characteristics of SDS sig¬
nificantly, hence the steep increase of 1/ in Figure 4-6 should be
interpreted in terms of the effect of polymer on micellization

117
kinetics. In analogy to a heterogeneous nucleation process, PVP is
likely to act as a nucleating agent for the surfactants, stabilizing
the micelle nuclei in the solution, thus accounting for the increase
of 1/ i2« Moreover, the formation of stoichiometric aggregates in
region II may lead to a smaller aggregation number (369) as well as a
greater mutual coagulation of micelles, and consequently to a faster
relaxation rate, simulating the case of pure micellar solution with
SDS concentration higher than 180 mM (83). This is suggested in light
of Cabane's picture regarding the structure of surfactant-polymer
complex (377). The greater mutual coagulation between micelles
partly results from the large configuration entropy associated with
the polymer chain. The activation energy of 100 mM SDS in the pres¬
ence of 2% PVP has been measured to be 24 Kcal/mole, close to 23
Kcal/mole of activation energy of pure 250 mM SDS. It has also been
observed that the activation energy below 180 mM SDS, where the slow
relaxation process is mainly controlled by step-wise association, is
always around 33-37 Kcal/mole. This value is obviously higher than
the activation energy of the slow relaxation process controlled by
mutual coagulation of micelles. This substantiates the conjecture
that the slow relaxation process in the presence of high PVP concen¬
tration is dominated by mutual coagulation of micelles.
This conjecture has further been examined by comparing the
labilizing power of propanol on SDS micelles in the presence of 1%
PVP with that of pure SDS micelles. The results are tabulated in
Table 4-1. It is interesting to see that unlike the complex change in
pure SDS micelles, the labilizing effect of propanol is monotonically
decreasing with increasing surfactant concentration in the presence

118
Table 4-1 Comparison of Labilizing Effect of Propanol
on SDS Micelles with/without 1% PVP
Slope of log (1/ T 2) vs. Propanol Concentration
Concentration
of SDS CmM)
SDS3 + Propanol
no PVP
SDS^ + Propanol +
1% PVP
10
3
--
50
1.9
6.4
100
1.9
2.7
180
3.65
—
250
4.3
2.4
a. Al^ values were obtained directly from the slopes multiplied by
10 in Figure 4-5.
b. All values were the slopes obtained by plotting log(l/ T^) of SDS
micellar solution in the presence ^f 1% PVP vs. propanol
concentration and multiplied by 10 .

119
of 1% PVP. This confirms our observation that the effect of alcohol
is strongly dependent on the mechanism and the rate of slow relaxa¬
tion process. Here the presence of 1% PVP has turned the slow relaxa¬
tion process into more of coagulation-controlled process, hence a
much greater effect of alcohol on 50 mM SDS is observed than that of
pure 50 mM SDS. As surfactant concentration increases, more regular
micelles will form from the excess surfactants, then the slow relaxa¬
tion process is progressively shifting back to partly step-wise asso¬
ciation controlled. This explains the monotonic decrease of slopes
with increasing surfactant concentrations.
At very high polymer concentrations, the complication of cross¬
over of the polymer coil may occur (377). However, as far as micell-
ization kinetics is concerned, the model described above should still
be valid although two additional factors must be taken into account.
Upon increasing PVP concentration, the amount of stoichiometric bind¬
ing increases, thus decreasing the population of regular micelles
formed in region III. It is possible that at very high PVP concentra¬
tion, most SDS molecules are present in the form of stoichiometric
aggregates which are of highly labile structure. Secondly, Fishman
and Eirich (373) have shown that increasing the polymer concentration
at constant surfactant concentration increases the degree of coun¬
terion dissociation of SDS, and hence increases the ionic strength of
the solution. Since the configurational entropy of polymer dominates
the system at high ionic strength (377), the interaction between
adsorbed micelles may then turn from repulsion into attraction. Both
factors can actually labilize micelles significantly, thus accounting
for the steep increase of 1/ T2 at high PVP concentrations as shown
in Figure 4-6.

120
With regard to how the degree of polymerization influences our
conclusion, Bloor et al. (370) has reported that at fixed PVP concen¬
tration, an increasing effect of PVP on labilizing micelles was
observed with decreasing molecular weight of polymer. This may be
correlated with the decreasing number of SDS subunits in a single
stoichiometric PVP coil because of the increasing number of polymer
coil in solution with decreasing molecular weight. An experiment has
been performed at the extreme case of degree of polymerization equal
to 1 by adding 1% 1-ethy1-2-pyrrolidinone, a water soluble monomer of
PVP, into 100 mM SDS solution. Interestingly a drastic increase in
1/ from 1.59 sec * of pure 100 mM SDS to 22.2 sec ^ is observed,
while a lesser increase to 8.9 sec ^ is observed upon addition of 1%
PVP (M.W.= 360,000) into 100 mM SDS. This can be viewed as the
cosolvent effect of PVP monomers on micelles, similar to the case of
short-chain alcohols. Unlike PVP polymers which wrap around the
micelles without penetrating into the micellar core (377), PVP monon-
ers are expected to penetrate into the micelles.
In summary, it has been shown that both polymers and cosolvents
can decrease the kinetic stability of micelles. Unlike the cosolvent
which may greatly alter the structure of micelles and the hydrophobic
driving force of the system, the addition of low concentration poly¬
mer may exert minimum perturbation on micelles. Thus the change of
1/ upon addition of polymers strongly support the concept about
the rate-limiting of micelle nucleus formation. Consequently, the
surfactant-polymer system can also serve as a model system to study
how the kinetic stability of micelles influence the properties of
micellar systems. Interestingly, there has been some indication

121
(379) that the kinetic stability of micelles influences the solubili¬
zation rate of a micellar solution. Thus a further study along this
line can lead to a better understanding of both fundamental and
applicational aspects of micellar systems.
4.4 Conclusions
The effect of short-chain alcohols on the dynamic properties of
SDS micellar solutions has been investigated. The conclusions from
this study are as follows:
1. Short-chain alcohols from methanol to butanol labilize micelles
as reflected by the increase in the slow relaxation rate con¬
stant 1/ T^.
2. The labilizing power of alcohols increases with increasing
alcohol chain length from to C^, and levels off at C^. This
result is in agreement with the experimental observation that a
palisade layer of about three carbon-carbon bond length may
exist on the micellar surface, which is penetrated by water.
3. The labilizing power of alcohols also varies with SDS concentra¬
tion: more pronounced effect of alcohols on micelles was
observed at higher SDS concentrations. The labilizing effect of
alcohol is related to the mechanisms of slow relaxation process
and micelle nucleus formation, irrespective of the ratio of
alcohol molecules per micelles.
4. The overall concept of micelle stability includes two aspects:
the "thermodynamic stability" and the "kinetic stability" of

122
micelles. The "thermodynamic stability" of micelles concerns
the distribution of surfactants between monomer and micellar
state; while the "kinetic stability" of micelles is related to
the life-time of micellar aggregates, which is dependent on the
rate of formation of micelle nuclei.
5. The addition of PVP up to 8% (w/w) in micellar solution can
increase 1/ by three orders of magnitude. The role of PVP in
micellar solutions has been interpreted as a nucleating agent
for micelle nucleus formation, thus supporting the concept that
the rate-limiting step in the surfactant aggregation process is
the formation of micelle nuclei.

CHAPTER 5
DYNAMIC PROPERTIES OF MICELLAR SOLUTIONS:
EFFECTS OF MEDIUM- AND LONG-CHAIN ALCOHOLS AND OILS
5.1 Introduction
The effects of short-chain alcohols and polymers on the dynamic
properties of micellar solutions were discussed in Chapter 4. Based
on the slow relaxation time measurements, alcohols from methanol to
butanol have been identified as short-chain alcohols which labilize
the micelles. The labilizing effect of short-chain alcohols depends
on the surfactant concentration and the mechanism of the slow relaxa¬
tion process. Short-chain alcohols exert greater labilizing effect on
reversible coagulation - fragmentation kinetics than on step-wise
association kinetics of micelles. In this chapter, the effect of
medium- and long-chain alcohols (C,. to C^) as well as oils will be
reported. The materials and methods used in this study have been
described in Chapter 4.
5.2 Results and Discussions
5.2.1 Slow-Down of Step-Wise Association Kinetics by Alcohols
Figure 5-1 shows the change of slow relaxation rate constant of
100 mM SDS micellar solution as a function of alcohol concentration.
Unlike the short-chain alcohols which labilize the micelles, alcohols
from pentanol to nonanol increase the slow relaxation time and hence
123

1/T2 (SEC"
124
Figure 5-1.
The change of the slow relaxation rate constant
of 100 mM SDS as a function of alcohol concentration

125
the average life-time of micelles at low alcohol concentrations.
Recall from chapter 4 that the change of 1/ is primarily attri¬
buted to the change of number density of micelle nuclei (A^) in the
solution. Short-chain alcohols (methanol to butanol) are considered
to decrease the aggregation number of the surfactant in the mixed
micelles. This leads to an increase in and a possible decrease in
r. The reverse change of 1/ in the presence of medium-chain
alcohols (pentanol onwards) as compared to short-chain alcohols seems
to suggest that A^_ should decrease with addition of pentanol and
longer alcohols. However, it has been shown (322) by fluorescence
measurements that alcohols from ethanol to octanol decrease the
aggregation number of micelles at least in the low alcohol concentra¬
tion region, thus favoring the formation of small aggregates. This
contradiction between the fluorescence results and the proposed
change in A^_ makes the explanation for the observed decrease of 1/
in Figure 5-1 difficult.
An examination of Figure 5-1 reveals that the alcohol chain
length seems to play an important role in determining the change of
1/ . Butanol sets the boundary of two regimes exhibiting opposite
changes in 1/ . It is plausible that alcohols shorter than butanol
may partition loosely in the palisade layer close to the micellar
surface, whereas alcohols with longer chain length may start
penetrating into the hydrocarbon interior of the micelles. One possi¬
ble explanation of the result in Figure 5-1 is that short-chain
alcohols can partition into the micellar nuclei, thus stabilizing
them (increasing A^_). However, alcohols with longer chain length can
only be solubilized in the micelles. As discussed in chapter 3, such

126
solubilization can stabilize the micelles, driving more surfactant
monomers into micelles, and consequently may decrease the micelle
nucleus population. The result in Figure 5-2 seems to support the
above conjecture. Figure 5-2 reports the change of 1/ of 10 mM
and 100 mM SDS with constant surfactant/alcohol ratio of 10 except 10
mM SDS with dodecanol to hexadecanol concentration at respective
solubilization limit. For 100 mM SDS, 1/ decreases with increas¬
ing alcohol chain length and then levels off (or increases slightly)
from dodecanol to hexadecanol. For 10 mM SDS solution, the addition
of 1 mM alcohols decreases 1/ sharply from hexanol to decanol.
However, upon adding 1 mM dodecanol, tetradecanol and hexadecanol,
the 10 mM SDS solution appears cloudy and a new slow relaxation pro¬
cess with relaxation time T ^ much longer than T ^ is observed. The
cloudiness and new relaxation process is attributed to the unsolubil¬
ized alcohols in crystal form with adsorbed surfactants. Hence, the
solubilization limit of dodecanol to hexadecanol in 10 mM SDS
micelles can be determined by the onset of T ^ relaxation process,
which has been determined to be around 0.5 mM, 0.25 mM and 0.02 mM
for dodecanol, tetradecanol and hexadecanol respectively. The abrupt
increase in 1/ of 10 mM SDS containing hexadecanol is then related
to the extremely low solubilization of hexadecanol in the micelles.
Furthermore, 1/ has been found to remain roughly unchanged as hex¬
adecanol concentration increases from 0.02 mM to 1 mM. This indicates
that the unsolubilized hexadecanol does not serve as a nucleating
agent for the micelle nuclei like polymers as reported in chapter 4.
Hence, medium- and long-chain alcohols seem to influence the slow
relaxation process through their solubilization in the micelles.

(1/T2)/(1/T2)
127
ALCOHOL CHAIN LENGTH
Figure 5-2. The relative change of 1/x in reference to that
of pure SDS micelles as a function of alcohol
chain length. The filled symbols have different
alcohol concentration from that of open symbols.

128
Unlike short-chain alcohols, they may not exert direct effect on the
micellar nuclei.
5.2.2 Transition in Micellization Kinetics with
Addition of Alcohols
As alcohol concentration increases, 1/ exhibits a minimum as
shown in Figure 5-1, similar to the change of 1/ with SDS concen¬
tration (83). A minimum 1/ at about 180 mM pure SDS solution has
been reported (83). The increase of 1/ at SDS concentrations
higher than 180 mM has been attributed to the reversible
coagulation-fragmentation process of submicellar aggregates. Accord¬
ing to equation [1.8], this reversible coagulation process mainly
depends on the total counterion concentration in the solution. A
critical counterion concentration A^q depending on each specific sys¬
tem must be reached for the onset of coagulation of submicellar
aggregates. Recall from chapter 3 that the penetration of alcohols
into micelles leads to a higher degree of counterion dissociation of
micelles. Thus, the reversible coagulation kinetics of submicellar
aggregates can occur at SDS concentrations lower than 180 mM if an
alcohol is added to the solution. This explains the transition of
micellization kinetics from step-wise association to reversible
coagulation shown in Figure 5-1 solely by the change of A^ with addi¬
tion of alcohols. However, the addition of alcohols may alter the
geometric packing of surfactants and increase the fluidity of the
micelle interior (328) which may facilitate the fragmentation of
micellar aggregates. Hence, the parameters such as 8 , A and q in
o’ go
equation [1.8] which are more related to the intrinsic properties of

129
micellar aggregates may also be changed by addition of alcohols and
hence contribute to the transition of micellization kinetics. Unfor¬
tunately, more quantitative analysis of the experimental data is hin¬
dered due to the existence of too many adjustable parameters and a
lack of information about how the degree of counterion dissociation
of micelles changes with alcohol concentration.
5.2.3 Effect of Alcohol Chain Length on the
Transition of Micellization Kinetics
Figure 5-3 shows a linear variation of 1/ T2 values and alcohol
concentrations at 1/ T2 minima reported in Figure 5-1 with increasing
alcohol chain length. The alcohol concentration required to induce a
transition of micellization kinetics decreases by 10 mM for each
additional CH2 group on the alcohol hydrocarbon chain. The
alcohol/surfactant ratio in the micellar phase at 1/ minima is
calculated to be about 0.46, 0.46 and 0.3 for pentanol, hexanol and
octanol respectively by taking the fractions of alcohol partitioning
in micelles from Table 3-1. These ratios are much below the ratios
of the conductance maxima reported in Table 3-1 of chapter 3. The
lower alcohol/surfactant ratio of octanol indicates that octanol is
more effective in inducing the transition of micellization kinetics.
However, Table 3-2 indicated that octanol is less effective in
increasing the counterion dissociation of micelles than pentanol or
hexanol. This seems to suggest that the transition of micellization
kinetics can not be solely attributed to the change of counterion
concentration (A^) in the solution. The change in the intrinsic pro¬
perties of submicellar aggregates ( f^, A^q, q) may also contribute

130
Figure 5-3. The correlation of 1/t2 value and alcohol chain
length of each curve in Figure 5-1 with the
corresponding alcohol concentration at the
minimum point
ALKYL CHAIN LENGTH OF 1-ALKANOLS

131
to the transition of micellization kinetics. By linear extrapolation,
one would expect a minimum 1/ at about 70 mM butanol and 20 mM
nonanol as indicated in Figure 5-3. However, this is not observed in
Figure 5-1. The monotonic decrease in 1/t ^ with addition of nonanol
can be explained by the formation of swollen micelles, which will be
discussed later.
The linear variation of each minimum 1/ value with alcohol
chain length can be fit into the following equation:
log(l/ T 2) = 1.26 - 0.227L [5.1]
where L is the alcohol chain length from C,. to Cg. For each addi¬
tional methylene group (C^) on the alcohol hydrocarbon chain,
log( 1/ T2 ) decreases by 0.227. If the change of 1/ T^ is solely
attributed to the change of in equation [3.1], the micelle nucleus
concentration is expected to decrease by about 1.7 folds with each
additional methylene group on alcohol.
As SDS concentration increases, the alcohol concentration
required for the transition of micellization kinetics decreases as
shown in Figure 5-4. At SDS concentrations greater than 180 mM, the
addition of hexanol increases 1/ T2 directly. Figure 5-5 shows the
correlation of both the value of 1/ T2 and the SDS concentration at
each minimum point reported in Figure 5-4 wich the corresponding hex¬
anol concentration. The correlation between the SDS concentration
with the corresponding hexanol concentration required for transition
of micellization kinetics can be best fit into a second order equa¬
tion:

SEC
132
The change of 1/t2 of various SDS solutions as
a function of hexanol concentration
Figure 5-4.

133
CONCENTRATION OF 1-HEXANOL
Figure 5-5. The correlation of minimum 1/t^ value and SDS
concentration of each curve in Figure 5-4 with
the corresponding hexanol concentration at the
minimum point.
1 /r2 (SEC)

134
A = 195.8 - 1.5B - (8.33 x 10"3)B2 [5.2]
where A is the SDS concentration, B is the hexanol concentration.
The equation states that at about 196 mM SDS no hexanol is needed to
induce the reversible coagulation kinetics. Depending on the concen¬
tration of hexanol, each added hexanol molecule is equivalent to
about 1.5 to 3 SDS molecules in inducing the transition of micelliza-
tion kinetics from step-wise association to reversible coagulation
process.
The value of 1/ at each minimum point shown in Figure 5-5 can
be best correlated with the corresponding hexanol concentration by a
second order parabolic equation:
log (1/ T2) = -0.43 - (5.74 X lo'3) B + (2.46 x 10‘4) B2 [5.3]
According to this equation, the lowest possible value of 1/ is at
about 11 mM hexanol concentration. Furthermore, the lowest 1/ 1 ^
value is roughly equal to the minimum value of 1/ at pure 180 mM
SDS solution. This result can be understood as the following.
Increasing both SDS concentration and hexanol concentration will
decrease 1/ T^ initially. However, by combining both effects, the
lowest value of 1/ T^ will not be smaller than the 1/ of pure 180
mM SDS solution, because the addition of hexanol in SDS solutions
with concentrations greater than 180 mM will increase 1/ T^ directly.
Indeed, Figure 5-6 shows that the addition of ethanol to decanol in a
250 mM SDS solution directly increases 1/ T2 at least in low alcohol
concentration region. The increase in 1/ can easily be understood
from the enhanced counterion dissociation (increasing A ) due to

SEC
135
Figure 5-6. The change nf the slow relaxation rate constant
of 250 inM SDS as a function of alcohol concentration

136
penetration of alcohols into the palisade layer of the micelles. The
initial slopes of changes in 1/ increase with increasing alcohol
chain length from ethanol to octanol, and then decrease for nonanol
and decanol. The increase in the slopes is due to increasing alcohol
partitioning in the micelles with increasing alcohol chain length,
while the decrease in the slopes results from the tendency of
micelles to be swollen by long chain alcohols.
5.2.4 Formation of Swollen Micelles by Alcohols and Oils
with Resultant Slow-Down in Mice 11ization Kinetics
As seen in Figure 5-1 (hexanol) and Figure 5-6 (octanol to
decanol), following an increase in 1/ with increasing alcohol con¬
centrations, 1/ decreases at sufficiently high alcohol concentra¬
tions. The maximum 1/ also corresponds to a maximum electrical
conductance at the same alcohol concentration as shown in Figure 5-7
and Figure 5-8. The formation of swollen micelles with resultant
decrease in electrical conductance has been discussed in chapter 3.
The decrease in 1/ at high alcohol concentrations is apparently
related to a decrease in counterion concentration (A ) due to solu-
g
bilization of alcohols in the hydrophobic core of the micelles (swol¬
len micelles). This explanation is applicable to micellization
kinetics following the reversible coagulation-fragmentation process.
The formation of swollen micelles can also slow down the micelliza¬
tion kinetics which follows the step-wise association process. liie
addition of nonanol to 100 mM SDS as shown in Figure 5-1 is one such
example. This can be further illustrated by adding oil to a micellar
solution as reported in Figures 5-9 and 5-10. Nonpolar oil like

SPECIFIC CONDUCTIVITY ( 1(T2 S/cm
137
Figure 5-7. The change of specific conductivity of 100 mM SDS
as a function of hexanol concentration

138
O 20 40 60 80 100 120 140
ALKANOL CONCENTRATION ( mM )
Figure 5-8. The change of specific conductivity of 250 mM
SDS as a function of alcohol concentration

139
Figure 5-9. The change of the slow relaxation time of 100 mM
SDS as a function of oil concentration

SPECIFIC CONDUCTIVITY ( TO-2 S/cm
140
Figure 5-10. The change of specific conductivity of 100 raM SDS
as a function of oil concentration

141
hexane is known to be solubilized primarily in the micellar interior,
while aromatic oils like benzene and toluene may be solubilized first
in palisade layer, and then in micellar interior at higher solubil-
izate concentrations (380-383). They all decrease 1/ of 100 mM
SDS solution as indicated by Figure 5-9. Figure 5-10 also shows a
corresponding decrease in electrical conductance of the solution. The
decrease in 1/ T^ may be explained by the formation of swollen
micelles leading to a decrease in micelle nucleus concentration (Af).
Figure 5-11 reports a decrease of 1/ of 250 mM SDS solution with
addition of oils. However, benzene seems to have little effect when
compared to hexane or toluene in decreasing 1/ T
5.3 Conclusions
The effect of medium- and long-chain alcohols and oils on
dynamic properties of micellar solutions has been discussed. The
conclusions can be summarized as follows:
1. The effect of medium- and long-chain alcohols on dynamic proper¬
ties of micellar solutions depends on the micellization kinetics
and alcohol chain length. The addition of alcohols often slows
down the step-wise association kinetics, but expedites the
reversible coagulation-fragmentation kinetics.
2. Micellization kinetics can be changed from a step-wise associa¬
tion to a reversible coagulation-fragmentation process upon con¬
tinuous addition of medium-chain alcohols. The alcohol concen¬
tration required for the transition decreases with increasing
alcohol chain length.

1 /T
142
Figure 5-11. The change of the slow relaxation time constant
of 250 mM SDS as a function of oil concentration

143
3. Formation of swollen micelles by alcohols and oils partitioning
in the hydrophobic core of the micelles often slows down the
micellization kinetics. Hence, the relaxation technique,
together with electrical conductance measurement, can identify
the solubilization site of additives in micelles and the struc¬
tural transition from simple micelles to o/w microemulsions
(swollen micelles).

CHAPTER 6
SOLUBILIZATION AND PHASE EQUILIBRIA OF
WATER-IN-OIL MICROEMULSIONS: INFLUENCE OF
SPONTANEOUS CURVATURE AND ELASTICITY OF INTERFACIAL FILMS
6.1 Introduction
As reviewed in chapter 2, the studies on both dynamic and
equilibrium aspects of microemulsions have provided a better under¬
standing of the formation, properties and phase behavior of
microemulsions during the past decade. However, the task of formu¬
lating a microemulsion seems still an empirical problem in many
instances, despite the advances in the understanding of microemul¬
sions. Since solubilization is the most salient feature of the
microemulsion system from which most applications stem, the develop¬
ment of guidelines for formulation of microemulsions is important.
This is the main objective of this and next chapter.
Although solubilization in microemulsions has been attributed to
ultralow interfacial tension, the spontaneous curvature and elasti¬
city of interfacial films also play an important role in determining
the droplet size and phase behavior of raicroemulsions (163,219-
221,384-385). In this chapter, basic concepts and theory on how
these two parameters of interfacial films influence the solubiliza¬
tion and phase equilibria of water-in-oil (w/o) microemulsions are
presented, followed by experimental verification of the proposed
theory in the next chapter.
144

145
6.2 Basic Concepts and Theory
The solubilization in a microemulsion depends on the size and
stability of droplets. We can group many factors (386-387) which
affect the size and stability of microemulsion droplets into two sim¬
plified categories, namely the free energy of interfacial films and
the free energy of interdroplet interactions.
6.2.1. Interfacial Free Energy of Microemulsions
The importance of ultralow interfacial tension between two
immiscible liquids for spontaneous emulsification has long been
recognized (147). The conventional treatment of interfacial tension
for a spherical interface based on the capillarity theory regards the
interfacial region as a membrane of uniform tension and zero thick¬
ness separating the two bulk regions with uniform pressure. The posi¬
tion of this membrane is identified as the so-called "surface of ten¬
sion" (385,388). The concept of single "surface of tension" has been
found suitable for "macroscopic" systems. However, Hall and Mitchell
(388) pointed out that the treatment of single surface of tension is
inadequate for systems with low interfacial tension such as micelles
and microemulsions. They have suggested a treatment of two surfaces
of tension with tensions of opposite sign in an interfacial region
having a finite thickness. Hall and Mitchell's treatment actually
corroborates the old concept of du; .ex film (147,178-179) and "R"
theory of Winsor (173) proposed in an attempt to explain the prefer¬
ence of a given system to the formation of a specific type emulsion
(e.g., w/o or o/w). An alternative to the treatment of duplex

146
surfaces of tension is to consider a single surface of tension
together with a bending stress related to the change in surface free
energy with curvature as suggested by Murphy (162). He has shown that
to deform an interface, work has to be done against both the interfa¬
cial tension and bending stress of the interface. Although always
present, resistance to bending is important only for very low inter¬
facial tension or highly curved interfaces. This alternative, with
the merit of simplicity, has been widely adopted by many researchers
in modeling microemulsion systems (163,165,199,219-220,389-390,384-
385).
Accordingly, the total interfacial free energy of a w/o
microemulsion can be written phenomenologically as follows (163):
F, = n[4 TTR2y + 16 TTK(1 - R/R )2] [6.1]
1 o
where F^. is the intradroplet free energy per unit volume of a
microemulsion; the first term in the square bracket represents the
interfacial tension term and the second term in the bracket is the
interfacial bending stress; n is the number density of microemulsion
droplets; Rq is the radius of spontaneous curvature (or natural
radius) of the interface; R is the equilibrium radius of droplets; y
is the interfacial tension including the contribution from both the
interfacial tension of the bare oil/water interface and the surface
pressure of the surfactant film; and K with the dimension of energy
is the curvature elasticity of the interface describing the interfa¬
cial compressibility and flexibility (199). High values of K
correspond to a "rigid" interface for which large energy is required

147
to bend the interface, while small K values represent a "fluid"
interface for which little energy is necessary for bending. Hence K
has also been referred as the "rigidity constant" of an interface.
-14
The value of K has been estimated on the order of 10 erg for
microemulsions (391-393); therefore, the bending energy of the inter¬
face is important in determining the size and phase equilibria of
microemulsions only when the interfacial tension is close to zero. It
should be noted that the electrostatic energy term is neglected in
equation [6.1] for oil-external microemulsions.
6.2.2 Free Energy of Interdroplet Interactions
The above-mentioned interfacial free energy is expected to
influence the droplet size in a microemulsion system, while the
interaction potential between droplets can affect the stability of
the droplets and hence the phase separation process. In o/w
microemulsions, the interaction is basically one of long-range
Coulombic repulsions. Reducing electrostatic forces between droplets
by increasing the electrolyte concentration can lead to a coacerva-
tion or coagulation of droplets and a phase separation of microemul¬
sions (231). However, the interdroplet interaction in w/o microemul¬
sions has been found to be essentially of hard sphere type (212-
215,394-396). The interaction potential can be satisfactorily
described by a hard-sphere repulsive interaction (entropic) term plus
an attractive perturbation term (212-215,394-396). Thus, the inter¬
droplet free energy ^2^ Per un^c v°iuine of a microemulsion can be
written as follows (163):

148
F = (k, T/v ,)[x(lnx-l)+4x^+5x^+6.12x^+7.06x^ - (l/2)Ax^] [6.2]
2 b d
where x is the volume fraction of droplets; A is a constant charac¬
terizing the attractive interaction between the droplets; v^ is the
volume of a single microemulsion droplet; and is the Boltzmann
3
constant. Note that v = 4nR /3 and x = nv,.
d d
It should be pointed out that the interdroplet interaction in
many w/o microemulsions has been found to be about 100 times larger
than that of van der Waals force (214), hence the physical origin of
the attractive perturbation term has been attributed to the specific
molecular structure and curvature of the interfacial film (212-
213,215,391). It has been suggested that this short-range attractive
interaction between droplets is mainly determined by the degree of
overlapping of droplets upon collision. This overlapping occurs due
to a penetrable aliphatic layer on the droplet surface, which is con¬
stituted by a concentric spherical layer with thickness proportional
to the difference between the hydrodynamic radius and hard sphere
radius of droplets as determined by scattering techniques (213,395).
For single-chain surfactant system, the hard sphere radius is roughly
equal to the sum of radius of water core and the chain length of
alcohol (213,395). Hence the thickness of interpenetrable aliphatic
layer is proportional to the difference between the chain length of
the surfactant and the alcohol. It has been experimentally observed
(212-213,395) that the attractive interdroplet interaction increases
with decreasing alcohol chain length and increasing oil chain length
(increasing the thickness of interpenetrable layer), as well as
increasing droplet size (increasing the area of penetration).

149
Apart from the above view point of penetrable aliphatic layer
associated with an interfacial film, Auvray (391) has further pro¬
posed that a curvature effect can lead to an apparent attractive
interaction between w/o microemulsion droplets. This idea arises from
Safran's work (205) that a spherical microemulsion droplet is
unstwith respect to shape fluctuations which lower the bending
energy of the interfacial film at small R/Rq values. Thus, at large
values of R , the formation of dimers between droplets with small R
can decrease the interfacial bending energy. This explanation is in
agreement with an experimental observation (215) that the attractive
interactions between w/o droplets increase with increasing size of
the surfactant head group (corresponding to an increase in Rq). One
can also increase Rq and hence increase the attractive interdroplet
interaction by increasing alcohol partitioning at the interface (lim¬
ited to alcohols which can increase the interfacial fluidity),
increasing oil chain length and decreasing electrolyte concentration.
It is noted that interfacial fluidity is likely to increase
(i.e., decreasing K) with decreasing alcohol chain length (233),
increasing oil chain length, decreasing electrolyte concentration and
increasing alcohol partitioning at the interface. Such an increase in
interfacial fluidity results in inelastic collisions between dro¬
plets, leading to the formation of transient dimers and an enhanced
apparent interdroplet interaction. The transient processes of these
inelastic collisions and dimer formation may involve interpenetration
and deformation of droplets, with a resultant change in interfacial
curvature. Hence, it may not be easy to separate the individual con¬
tribution of the two above-mentioned mechanisms to the attractive

150
interaction among droplets. However, it may be plausible to consider
that interfacial fluidity (K) strongly influences the interactions
among microemulsion droplets.
6.2.3 Phase Equilibria of W/0 Microemulsions
When the limit of solubilization or stability of microemulsions
is reached, phase separation occurs and the microemulsion phase can
coexist in equilibrium with other phases. At least three types of
two-phase equilibria of microemulsions have been delineated: Type 1.
A microemulsion phase in equilibrium with an excess internal phase,
which is driven by the interfacial bending stress; Type 2. A
microemulsion phase in equilibrium with an excess external phase con¬
taining low density of microemulsion droplets; which is driven by an
attractive interdroplet interaction; Type 3. Coexistence of both w/o
and o/w microemulsion phase. It has further been suggested that when
both bending stress and attractive force act in parallel upon the
system, the first two types of phase equilibria together can give
rise to three-phase equilibria of microemulsions, i.e., microemul¬
sions in equilibrium with both excess oil and water phases. For more
details and references of these phase equilibria, refer to section
2.4.2
An understanding of the driving force for phase separation is
important for the formulation of microemulsions. Prior to laborious
physical measurements of interfacial elasticity by spin-labeling
resonance (393) or fluorescence methods (236,397-398), and of inter¬
droplet interactions by scattering techniques (213,394), one can
study the phase equilibria of microemulsions as a direct, qualitative

151
means to assess the physical origin of phase separation of microemul¬
sions .
6.2.4 Equilibrium Droplet Size and Solubilization
of W/0 Microemulsions
As indicated by equation [2.8], at constant interfacial area the
solubilization of a microemulsion is directly proportional to the
droplet size, and hence the curvature of the interfacial film.
Based on equations [6.1] & [6.2], the droplet size and solubili¬
zation of w/o microemulsions can be calculated as follows. For
microemulsions in equilibrium with excess water (type 1 phase equili¬
bria), it is required that the total excess free energy of the system
(F) be minimized with respect to both droplet radius (R) and volume
fraction of the dispersed phase (x) as
( 3f/ 3 R) = 0
x
[6.3]
( 3 F/ 9 x)R = 0
[6.4]
where the chemical potential of excess water has been set to zero.
The total excess free energy (F) per unit volume of a microemulsion
is given by the sum of intra- and inter-droplet free energy as:
[6.5]
Equations [6.3] & [6.4] have to be solved simultaneously in
order to obtain R and x. However, under simplified conditions, some

152
analytical solutions can be obtained. By assuming F^/nK << 1 and a
saturated (7=0), incompressible interfacial film (the area per
molecule will not change significantly with curvature), and further
neglecting the dependence of K on R, the equilibrium droplet radius
can first be obtained from equation [6.3] after some algebra as the
following:
R « R [1 + 3F./(32 TTnK)] [6.6]
o z
where higher order terms of F^/nK have been neglected. It can further
2 2
be shown that when R < R , we have (9 F/ 9 R ) > 0. Hence the radius
— o x
obtained from equation [6.3] is at the minimum free energy of the
system. Equation [6.6] dictates that if the interfacial tension is
very low, the equilibrium radius of microemulsion droplets depends on
three parameters, namely the natural radius Rq, the elasticity K of
the interfacial film and the free energy of interdroplet interactions
V
Equation [6.6] shows that at the limit of relatively rigid
interfacial films (large K), phase separation occurs at R=Rq and
hence solubilization is solely determined by the natural radius Rq.
The natural radius of an interfacial film can be described by a
geometric model proposed by Ninham and coworkers (180,399-400), which
has been discussed in section 2.3.1. The natural radius (R ) of a
o
surfactant film is essentially dependent on the packing ratio of sur¬
factant at the interface (401). For inverted structure (w/o),
increasing droplet size corresponds to a decreasing value of v/a 1 .
o c
In contrast, droplet size increases with increasing v/a 1 in a
o c

153
"normal" structure (o/w). Many factors such as the molecular struc¬
ture of oil, alcohol and surfactant, electrolyte concentration, tem¬
perature are expected to influence the packing ratio, and hence the
natural radius of an interfacial film. The detailed discussion of
some of these factors will be left to the next chapter.
Regarding how the elasticity of an interfacial film influences
the droplet size, equation [6.6] suggests that for small K values
(fluid interfaces) and < 0, R is smaller than Rq, suggesting that
phase separation would occur at smaller droplet radius than the
natural radius Rq. The mixing entropy (F^) of droplets also tends to
decrease the droplet radius below the natural radius. One possible
explanation for this result is that the increase in dispersion
entropy due to an increase in the number of droplets with a decrease
in the droplet size can largely offset the increase in energy result¬
ing from the bending of the interfacial film (221,384). In addition,
an increase in attractive interaction (A) between droplets will lead
to a further decrease in droplet radius R due to more negative value
of F^. The consequence is that when the interface is fluid, any
further increase in interfacial fluidity or attractive interdroplet
interaction tends to decrease the droplet radius below the natural
radius. This is consistent with Auvray's view that deviation of dro¬
plet size from the natural radius can result in an apparent attrac¬
tive interaction between droplets with fluid interfaces. When F^ >0,
R > Rq and the large interfacial bending stress leads to phase
separation of type 1. To reduce the interfacial bending stress and
increase the solubilization, one can increase the natural radius of
the interfacial film and/or decrease the K value.

154
In addition to the equilibrium droplet radius obtained from
equation [6.3], the analysis of equation [6.4] can provide informa¬
tion about the stability of microemulsion droplets. By substituting
equations [6.1] and [6.2] into equation [6.4] and assuming A is
independent of x, the following equation is obtained:
F./x + (k,T/v,)[lnx + 8x + 15x^ + 24.48x"^ + 35.3x^ - Ax]=0 [6.7]
1 d d
Equation [6.7] indicates that the mixing entropy of droplets compen¬
sates the interfacial free energy, resulting in a minimum free energy
of the system. At the limit of y=0 and relatively rigid interfacial
films, R=Rq an<^ Fj=0. The droplet volume fraction at the free energy
minimum of the system can then be obtained:
lnx + 8x + 15x^ + 24.48x^ + 35.3x^ - Ax = 0 [6.8]
Figure 6-1 plots the obtained volume fraction of droplets (x) as
a function of interaction parameter (A) from equation [6.8]. Note
that increasing attractive interdroplet interaction can stabilize the
system to higher volume fractions of droplets when the interface is
2 2
relatively rigid. For A < 21, it can be shown that ( 3 F/ 3x ) > 0
K
for all x values, indicating a minimum free energy of the system at
the obtained x as plotted in Figure 6-1. In fact, the interaction
parameter A often increases with an increase in the natural radius of
many microemulsion systems, hence the droplet volume fraction x actu¬
ally increases via an increase in droplet size with increase A,
resulting in a greater solubilizaiton in microemulsions. However,
2 2
when A > 21, negative values of ( 3 F/ 9x are found in the range

155
A
Figure 6-1. A plot of the volume fraction of microemulsion
droplets at the minimum free energy of the system
as a function of interdroplet interaction parameter
A. The scjeene^ area represents a two-phase region
where (3 F/3x )^< 0. The envelope of the screened
area is the spinodal line.

156
of x shown by the screened area in Figure 6-1. This suggests that at
sufficiently strong attractive interdroplet interaction, the
microemulsion droplets are destabilized, resulting in an early phase
separation of microemulsions at lower droplet volume fractions. The
phase separation of microemulsions will change from type 1 to type 2
containing a critical point. The critical volume fraction (xc) and
critical interaction strength (Ac) at the critical point can be
determined from ( 9^F/ 9x^)R=0 and ( g^F/ 3x^)R=0 to be xc=0.13 and
2 2
A =21.14. The spinodal line can be found from ( 9 F/ 9x ) =0 as
c k
1/x + 8 + 3Ox + 73.44x2 + 141.2x3 + A = 0 [6.9]
The spinodal line determined from equation [6.9] is the envelope
of the dotted area shown in Figure 6-1. It can be seen that as
interaction strength (A) increases, the two-phase region between two
spinodal points expands. Thus, a single-phase microemulsion may
enter a two-phase region by increasing the attractive force among the
droplets and hence decreasing the solubilization in microemulsions.
Since the attractive interaction between droplets is related to the
equilibrium droplet radius, interfacial fluidity and natural radius,
the critical interaction strength can be reached at smaller droplet
radius by increasing the interfacial fluidity or natural radius. The
decrease in solubilization of microemulsions is due to a phase
separation at smaller droplet radius.

157
6.3 Discussions
Although the two interfacial parameters, namely the natural
radius Rq and rigidity constant K, are defined phenomenologically and
expressed by two independent constants in equation [6.1], they are
somewhat interrelated in the real physical setting of many microemul¬
sion systems. For example, a lamellar birefringent structure having
large Rq and K can be fluidized upon the addition of short-chain
alcohols to form an isotropic structure, resulting in a decrease in K
with a concomitant change in Rq. It is also clear that the rigidity
of an interfacial film depends on the geometric packing and cohesive
interactions of surfactant and cosurfactant molecules at the inter¬
face. A change in Rq could possibly cause a resultant change in the
packing and configuration of the surfactant molecules, and a conse¬
quent change in K. In microemulsions, both alcohol and oil molecules
penetrate into the interfacial film, and thus are likely to vary both
Rq and K together. It may be proposed, based on the experimental
data presented in the next chapter, that elasticity parameter K often
decreases with increasing natural radius Rq by increasing oil chain
length or increasing alcohol partitioning at the interface. Unfor¬
tunately, very few attempts (402-403) have been made to correlate K
with the molecular parameters (e.g., microscopic elasticities and
cross-sectional areas of head and tail layers) of surfactants at the
interface. The precise connection between K and Rq at molecular
level is not well established on both theoretical and experimental
basis at the present time.

158
The above-presented theory is based on a simple phenomenological
and qualitative approach. However, the implication is important.
First of all, the solubilization of microemulsions mainly depends on
the droplet size at constant total interfacial area. Equation [6.6]
indicates that the size of microemulsion droplets is basically deter¬
mined by the radius of spontaneous curvature of the interfacial film,
and is further influenced by the interfacial elasticity, the mixing
entropy of droplets and attractive interdroplet interaction. For
relatively rigid interfaces (large K), the droplet size is close to
the natural radius. It would appear that the droplet size (or solu¬
bilization) always increases with increasing Rq according to equation
[6.6] and with increasing attractive interdroplet interaction A
according to equation [6.8]. As mentioned earlier that the increases
in both Rq and A may physically cause a decrease in K for many
microemulsion systems, it may be inferred that increasing interfacial
fluidity will increase the solubilization of microemulsions with
relatively rigid interfaces. However, as indicated by equation
[6.6], the attractive interdroplet interaction and interfacial
fluidity always exert a small negative correction factor to the
equilibrium radius, consequently offset the increase of solubiliza¬
tion due to increasing Rq. At sufficiently large Rq and small K, any
further increase in R and decrease in K will destabilize the
o
microemulsions, causing an early phase separation at even smaller
droplet size. The solubilization will then decrease and the phase
separation process will change from type 1 to type 2 as implied by
equations [6.8] and [6.9]. A maximum solubilization is therefore
obtained at optimal values of Rq and K where the bending stress of

159
rigid interfaces counteracts the attractive force of fluid inter¬
faces. This is schematically shown in Figure 6-2. The left side of
the solubilization maximum represents a region of rigid interfaces
dominated by the interfacial bending stress. The right side of the
maximum corresponds to a region of fluid interfaces and phase separa¬
tion results from attractive interdroplet interaction. The change in
the properties of the interface and the phase separation process can
be readily revealed by the distinct phase behavior of microemulsions,
i.e., the left side is of type 1 phase equilibria while the right
side exhibits type 2 phase equilibria. However, complications of
phase behavior may occur if the packing ratio of surfactant is close
to unity where a birefringent phase on either side of the maximum may
be observed.
In conclusion, a general phenomenological guideline for the for¬
mulation of w/o microemulsions can be proposed. To obtain a maximum
solubilization in a given w/o microemulsion, one could adjust the
natural radius and fluidity of the interface to optimal values at
which the bending stress and the attractive force of the interface
are both minimized. The study of phase behavior of microemulsions
can provide a quick assessment of the interfacial elasticity and cur¬
vature, and a guideline for their adjustment. In the next chapter,
experimental data will be presented to verify and elucidate the prin¬
ciple established above, and to further delineate how the molecular
structure of various components of microemulsions influences the
elasticity and natural radius of interfacial films.

160
Decreasing Curvature Elasticity K
Rigid Region
Increasing
Solubilization
due to Decreasing
Interfacial Bending Stress
with Increasing Natural
Radius and Decreasing
Interfacial Rigidity
Fluid Region
Decreasing
Solubilization
due to Increasing
Attractive Interaction
between Droplets
with Increasing
Interfacial Fluidity
By
Increasing Natural Radius R0
/ \
Increasing Oil Chain Length
Decreasing Alcohol Chain Length
Increasing Alcohol/Surfactcnt Ratio
Decreasing Electrolyte Concentration
V
r
Figure 6-2. A schematic diagram describing the effects
of spontaneous curvature and elasticity of
the interfacial film on solubilization of
w/o microemulsions. The black regions in
the phase equilibrium plots represent the
water phase.

CHAPTER 7
SOLUBILIZATION AND PHASE EQUILIBRIA
OF WATER-IN-OIL MICROEMULSIONS: EFFECTS OF
OILS, ALCOHOLS AND SALINITY
7.1 Introduction
In the previous chapter, basic concepts and theory about solu¬
bilization and phase equilibria of water-in-oil microemulsions were
presented. It was shown that the solubilization and phase equilibria
of oil-external microemulsions can be basically accounted for by two
phenomenological parameters of the system, namely the spontaneous
curvature and elasticity of the interfacial film. In this chapter,
some experimental data will be presented to verify and elucidate the
above-mentioned theory, and to further delineate the influence of
molecular structure of various components of microemulsions on spon¬
taneous curvature and interfacial elasticity. The chain length com¬
patibility effect observed in w/o microemulsions (404-405) will also
be discussed. As a result of this study, some phenomenological
guidelines for the formulation of microemulsions will be proposed.
7.2 Materials and Methods
Sodium stearate (LOT-CC-6581) was used as supplied by ICN Phar¬
maceuticals. Sodium dodecyl sulfate (SDS) was purchased from B.D.H.
(purity _ 99%). All alcohols and oils with purity above 99% were
used directly without further purification.
161

162
The solubilization limit of microemulsions was determined by
titration of microemulsions with water or brine at ambient conditions
until opaqueness occurred. The onset of opaqueness was defined as a
turbid, milky appearance of the system such that nothing can be seen
through. The bluish, translucent appearance of a system (Rayleigh
scattering) was not considered as the onset of opaqueness. However,
for the transition of isotropic microemulsions to a birefringent
phase, the boundary was determined as the onset of cloudiness due to
a lack of strong turbidity. The reproducibility of titration was
within + 0.1 ml. Birefringence was detected by viewing the sample
through two cross polarized plates.
The results of phase equilibria of samples were recorded after
48 hours' equilibration at ambient conditions. This equilibration
period was found to be sufficient for qualitative determination of
the type of phase equilibria, although true thermodynamic equilibrium
may require longer time. The type of phase equilibrium of a sample is
represented by a letter beside each data point in all figures as the
following:
1. Phase equilibria of microemulsions with excess water (referred
as type 1 phase equilibria) are labelled by "w". Phase separa¬
tion of this type is due to the interfacial bending stress of
rigid interfaces.
2. Phase equilibria of microemulsions with excess oil containing
low density of microemulsion droplets are labelled by "o"
(referred as type 2 phase equilibria). A critical point at which
two equal-volume, isotropic phases are in equilibrium is

163
labelled by "oc". Phase separation of this type is associated
with strong attractive interdroplet interactions of fluid inter¬
faces.
3. A transition from an isotropic microemulsion to a birefringent
phase is labelled by "b".
4. The existence of "gel-like" or solid surfactant precipitates
swollen by water is labelled by "p".
5. A special symbol "6" has been used to designate the phase
equilibria of microemulsions with excess oil in the second iso¬
tropic region (see text in section 7.3.2) where the microemul¬
sion structure is uncertain.
7.3 Effect of Oil Chain Length
7.3.1 Influence of Oil Penetration into Interfacial Films
The water solubilization and phase equilibria of w/o sodium
stearate microemulsions as a function of oil chain length are shown
in Figure 7-l(a). The change of electrical resistance of these solu¬
tions as a function of water concentration has been reported in
reference 404. The result suggests that the structure of these sam¬
ples is likely to be percolating w/o microemulsions (241). The
amount of solubilized water in the microemulsions increases with
increasing oil chain length up to dodecane, then decreases upon
further increasing the oil chain length. The condition of "chain
length compatibility" would predict a maximum solubilization at tri¬
decane (404). Furthermore, the phase separation of microemulsions

164
Figure 7-1. Solubilization of water as a function of oil chain
length in microemulsions containing 10 ml oil, 1 gm
sodium stearate and various amount of 1-pentanol,
(a) 8 ml; (b) 4 ml. The filled circles stand for the
first boundary, while the open circles for the second
boundary of solubilization. Benzene is plotted as oil
with chain length of 2.4 (filled square) and the
screened area indicates a clear birefringent phase.
The letter beside each data point represents the
type of phase equilibrium of each sample.

165
exhibits distinct patterns with varying oil chain length. For oils
longer than dodecane, an isotropic microemulsion phase in equilibrium
with excess oil phase (type 2) was observed, indicating an attractive
interaction between droplets for phase separation. For oils shorter
than dodecane, a birefringent liquid crystalline phase was observed
between dodecane and octane. Figure 7 in reference 404 best depicts
the patterns of the observed phase equilibria. For oils shorter than
octane, solid or gel-like surfactant precipitates were observed. The
equivalent alkane carbon number (EACN) of benzene has been calculated
to be 2.4 based on an extrapolation of solubilization data of octane,
hexane and pentane in Figure la using a second order equation.
Although the EACN of benzene may vary with the composition of
microemulsions, benzene will always be reported as an oil with carbon
number of 2.4 throughout for convenience.
The decrease of water solubilization in microemulsions with
increasing oil chain length (C^ to Cj^) in Figure 7-l(a) can be
attributed to the enhanced interdroplet attraction according to the
theory presented in chapter 6. It has been experimentally esta¬
blished that the attractive interaction between w/o microemulsion
droplets increases with increasing oil chain length (210,212,406-
407). This enhanced attraction is related to the penetration of oil
molecules into the interfacial film, and the resultant change in the
interfacial curvature and fluidity. Many experimental and theoreti¬
cal studies of microemulsions (181-183) and amphiphilic bilayers
(408-413) have suggested that oil penetration into the interfacial
film increases with decreasing molecular volume and increasing aroma¬
ticity of the oil. This penetration can increase the interfacial

166
mixing entropy, thus stabilizing the interfacial film. The oil pene¬
tration also swells the aliphatic layer of the surfactant film, caus¬
ing a higher spreading pressure at the surfactant tail/oil interface,
and consequently resulting in a more curved interface (smaller dro¬
plet size). Further, the oil penetration straightens the surfactant
chain (favor trans- conformation of the chain) and consequently lead
to a more rigid, hardened interfacial film (181,183,409,412-413).
All these factors serve as stabilizing forces for w/o microemulsion
droplets (245).
In contrast, less penetration of oil into the surfactant film
with increasing oil chain length results in a more flexible interface
and a greater natural radius (Rq) than that of short-chain oil. The
attractive interaction between droplets are thus increased due to
"sticky" collisions between droplets (208,233) and a decrease in
interfacial bending energy upon the formation of dimers (391). The
"stickiness" of collisions is proportional to the thickness and the
area of penetrable aliphatic layer, the fluidity of the interface
(212-213), and the ease of oil removal from the interface upon colli¬
sions (395). The last factor is very similar to the case of steric
stabilization in polymeric colloidal systems (414-415). One can con¬
sider the long-chain oil as a "poor" solvent for interfacial films,
resulting in attractive steric forces between microemulsion droplets
in analogy to that between polymer-coated particles (414-415).
7.3.2 Formation of Birefringent Phases
As shown in Figure 7-1(a), the water solubilization in the
microemulsions diminishes as oil chain length decreases from dodecane

167
to octane. Unusual phase transition of isotropic --> birefringent
--> isotropic --> turbid was observed when water was continuously
added to the solution. Similar phase transition has also been
reported in other w/o microemulsion systems (416-418,420-421). The
birefringent phase appears translucent and consists of fine liquid
crystalline dispersion, analogous to a neat phase in equilibrium with
phase in many lyotropic liquid crystalline systems (419). Ekwall
(419) has indicated that a neat phase (lamellae) often occurs when
the solubilization limit of isotropic w/o microemulsion phase (L^) is
exceeded. The "second" isotropic phase at higher water content
appears bluish and translucent; and has very narrow water solubiliza¬
tion range as compared to the "first" isotropic phase or the
birefringent phase. The transition of the second isotropic to turbid
phase will be referred to as the "second boundary" in contrast to the
"first boundary" of transition from the first isotropic to the
birefringent phase.
A possible structure of o/w microemulsions has been suggested
for some second isotropic phases (416). However, similar second iso¬
tropic region observed in AOT systems (421) indicates that the second
isotropic region connects with L2 (w/o) phase through a narrow pas¬
sage. The exact nature and structure of this second isotropic phase
probably still remain to be studied. One possible view is that the
second isotropic phase in Figure la represents a gradual and continu¬
ous phase inversion from w/o to o/w droplets as moving from dodecane
to octane.
In an attempt to delineate the relation between a microemulsion
and a lyotropic nematic phase, specific lamellar phases have been

168
studied (164,166-167,422-424). Unusual labile lamellar structure
with collective undulation mode has been observed in
SDS/pentanol/cyclohexane/water system (164,166-167). These lamellae
occur in the vicinity of a microemulsion phase in a phase diagram,
but contain less alcohol than that needed to form isotropic
microemulsions. The result of these studies emphasized the impor¬
tance of interfacial fluidity for the formation of microemulsions,
and suggested the role of alcohol (cosurfactant) as to increase the
interfacial fluidity resulting in a transition from periodically
organized structure (lamellae) towards random isotropic microemul¬
sions. The phase behavior of the same system upon mixing 0.4 gm SDS,
5 ml cyclohexane, 1 ml 1-pentanol and various amounts of water
together has been investigated. An analogous phase transition of iso¬
tropic --> birefringent --> isotropic --> turbid was observed. The
macroscopic similarity between the sodium stearate and SDS system
indicates that the solubilization and phase transition observed in
Figure 7-l(a) is probably related to the formation and stability of
birefringent lamellae. This argument can be substantiated by the
result in Figure 7-l(b). When the amount of pentanol decreases by
half, the pronounced solubilization maximum observed in Figure 7-1(a)
vanishes and the birefringent region enlarges. The height of
birefringent region decreases with increasing oil chain length. It
is clear that the stability of lamellae is influenced by the alcohol
concentration and oil chain length.
The periodic long-range orientational order of lamellae mainly
results from the solute-solvent as well as the solute-solute interac¬
tions (165). The penetration of solvent (oil) molecules into the

169
aliphatic layer of the interface (solute-solvent interaction) result¬
ing in more rigid surfactant chains (183,210,410,412-413). The
cohesive interactions in the aliphatic layer are thus increased,
leading to a long-range ordered and more rigid interface. This
partly accounts for the observation of the birefringent phase in the
short-chain oil region (CQ to C. ) in Figure 7-l(a). Furthermore,
the continuous decrease of water solubilization in isotropic
microemulsions with decreasing oil chain length from dodecane to hex¬
ane in Figure 7-l(a) can also be explained by the increasing oil
penetration in the interfacial film (solvation effect). The oil pene¬
tration increases the packing ratio v/aQlc, thus decreases the
natural radius. This conjecture has been verified both experimen¬
tally (425) and theoretically (219). It has been estimated that for
a surfactant film with v/aQlc value close to unity, v/a^l^ may
increase to about 1.5 - 2 in the presence of hexane, while it may
remain unchanged in the presence of tetradecane or hexadecane (160).
The above analysis focuses on the influence of oil on interfa¬
cial rigidity and curvature (v/a^l^. The effect of alcohol can be
analyzed in a similar way. Since many single-chain ionic surfactants
have a bulky polar head group (aQ) and a relatively smaller cross-
sectional area (v/lc) in the hydrocarbon chain, the packing ratio
v/a 1 is usually smaller than unity favoring the formation of oil-
o c
in-water droplets. It has been generally observed (419) that a
minimum amount of alcohol is required to form a neat phase in a
binary ionic amphiphile-water system. The addition of alcohol is
expected to increase hydrocarbon chain volume v (387,426), and may
decrease the area per polar head group aQ slightly (387,419), thus

170
increasing the packing ratio v/aQlc towards unity and leading to a
structural transition from normal micelles to lamellae. However, at
higher alcohol concentrations, large increase in v, and hence in
v/a 1 , may eventually favor the formation of "inverted" (w/o) struc-
ture. This can explain the smaller birefringent region (with higher
water solubilization in the microemulsions) in Figure 7 — 1(a) contain¬
ing higher alcohol concentration as compared to that in Figure 7-
1(b). Moreover, the partitioning of alcohol at the interface is
expected to disorder the surfactant film due to void formation
between the surfactant chains (427-428). It has also been found that
alcohol partitioning at the interface increases with increasing oil
chain length (212,404). Thus the diminishing of the birefringent
region can also be reasonable explained by the increasing fluidity of
the interface with increasing oil chain length and alcohol concentra¬
tion.
The uptake of water and oil by lamellae is probably the main
factor in determining the stability of a birefringent phase. Figure
7-1 shows that increasing oil chain length decreases the height
(water uptake) of lamellae. It has been generally observed that
fatty acid soap forms lamellae of non-expanding type (419). This
means that the incorporation of water in lamellae can result in a
marked rise in the interfacial area per polar head group as well as a
reduction in the thickness of bilayers (double hydrocarbon chain
layers) (419). The intake of water by lamellae is limited to the
point where the interfacial area per polar head group is about twice
as great as the cross section of the amphiphilic chain (v/a 1 = 1/2)
o c
beyond which a phase transition to a new phase will occur (419). It

171
appears that decreasing oil chain length can increase the water
uptake by lamellae due to a preferential penetration of short-chain
oils (higher value of v) as compared to hexadecane.
7.3.3 Chain Length Compatibility in W/0 Microemulsions
It may be questioned whether the pronounced solubilization max¬
imum at dodecane observed in Figure 7-1(a), complying with the condi¬
tion of chain length compatibility, is fortuitous. Upon decreasing
the hydrocarbon chain length of the surfactant, chain length compati¬
bility has also been observed for sodium myristate/pentanol system
(404). A system of SDS/pentanol has further been examined as shown in
Figure 7-2. The result is similar to that in Figure 7-1 and the con¬
dition of chain length compatibility is again observed. However,
when varying the chain length of alcohol, no solubilization maximum
at the point of chain length compatibility was observed for sodium
stearate system containing butanol (curve 1) and hexanol (curve 3)
shown in Figure 7-3. It can be inferred that the observation of
chain length compatibility in various systems is not incidental, but
requires specific conditions. Both alcohol chain length and concen¬
tration appear to be important factors for the chain length compati¬
bility effect.
It is interesting to note that when the condition of chain
length compatibility is obeyed, the liquid crystalline phase always
vanishes at the preferred oil chain length as shown in Figure 7-l(a)
and Figure 7-2(a). An explanation is postulated as the following.
The interfacial fluidity is likely to be influenced by the degree of
oil penetration (static factor) and the ease of oil removal (dynamic

172
Figure 7-2.. Solubilization of water as a function of oil chain
length in microemulsions containing 5 ml oil, 0.7 gm
sodium dodecyl sulfate and various amount of 1-pentanol:
(a) 4 ml;(b) 1.8 ml. See Figure 7-1 legend for details.

173
Figure 7-3. Solubilization of water or brine as a function
of oil chain length in microemulsions containing
10 ml oil, 1 gm sodium stearate and 8 ml different
alcohols: 1-butanol (curve 1); 1-pentanol (curve 2);
1-hexanol (curve 3); 1-pentanol with 0.2% (w/w)
salt (curve 4); and 1-pentanol with 0.25% (w/w)
salt (curve 5). The open symbols at oil chain
length of 2.4 represent benzene.

174
factor) from the interface upon collisions. Since the interpenetr¬
able collision between microemulsion droplets is accompanied by oil
removal (395), the interfacial fluidity may increase with increasing
ease of oil removal. It is likely that the environment experienced
by the oil molecules residing in the aliphatic layer of interface is
on average close to a hydrocarbon phase with equivalent chain length
of 1 -1 , where 1 is the surfactant chain length, 1 is the alcohol
s a7 s a
chain length. Any alkane shorter than lg-la will experience higher
average cohesive force in the aliphatic layer than that in the bulk
oil phase, thus having a tendency to reside in the aliphatic layer
and increasing the packing ratio as well as the rigidity of the
interface. This concept involving the cohesive forces between
molecules is important in light of a paper by Leung et al. (405). It
has been found (405) that the effect of oil on the solubilization of
w/o microemulsions is dependent on the collective properties of the
solvent (oil), not on the individual molecular structure of the oil.
A binary alkane mixture of Cg + can behave like oil at the
interface as far as the bulk cohesive forces are concerned, and the
heterogenity of oil chain length at the interface becomes immaterial.
The result of chain length compatibility may also imply that the
absorption of oil molecules into the interface (solvation effect)
increases significantly for oil shorter than the preferred oil chain
length. Similar absorption of alkanes in black lipid films has also
been reported (412-413).
The pronounced maximum of water solubilization in microemulsions
with the condition of chain length compatibility can be considered as
a result of counteracting effects of "interdroplet" interaction and

175
"intradroplet" interaction (386). The intradroplet interaction is
related to the cohesive force between adjacent hydrocarbon chains at
the interface and to the rigidity of the interface. Starting from a
short-chain oil, increasing oil chain length will gradually reduce
the cohesive interaction between hydrocarbon chains and the rigidity
of the interface due to decreasing oil penetration. The solubiliza¬
tion in microemulsions thus increases due to a decrease in packing
ratio v/aQlc and a consequent increase in natural radius (Figures 7-
1(a) & 7-2(a)). At longer oil chain length, the intradroplet
interaction gradually diminishes (thus interfacial fluidity
increases) and the attractive interdroplet interaction starts govern¬
ing the system. One consequently expects a decrease in solubiliza¬
tion in microemulsions with further increase in oil chain length. At
the point of chain length compatibility, these two interactions are
both minimized, hence resulting in a maximum solubilization. In
Figure 7-3, the interdroplet interaction dominates all the
microemulsions containing butanol (curve 1), while intradroplet
interaction governs all the microemulsions containing hexanol (curve
3), hence no chain length compatibility effect is observed due to the
absence of these two counteracting effects.
For some short-chain oils (C^ or shorter) and aromatic hydrocar¬
bon in Figure 7-l(a), phase separation of surfactant precipitates
swollen by water occurs. In many cases, a "gel-like" phase with
anisotropy was observed. It is generally known that the gel state is
intermediate between a liquid crystalline state with moderately
disordered hydrocarbon chains and a crystalline state with completely
ordered chains (419). It is postulated that the observation of

176
gel-like precipitates is due to the stiffness of surfactant chains
resulting from the large absorption of hydrocarbons at the interface.
The specific interaction between aromatic hydrocarbons and polar head
groups of amphiphiles (181,183,289,428) further enhances the hydro¬
carbon absorption. It was speculated (412-413) that interfacial ten¬
sion may consequently increase due to an increase in area per surfac¬
tant head group and possible hydrocarbon-water contact.
7.4 Effect of Alcohols
7.4.1 Effect of Alcohol Chain Length
As mentioned earlier, two effects emerge from decreasing alcohol
chain length in a microemulsion system. First, the hard sphere radius
of droplets decreases (241,395). Second, the interfacial fluidity
increases due to void formation in the aliphatic layer of the inter¬
facial film (427). Both effects lead to an increase in attractive
interdroplet interaction. Using fluorescence methods, Atik and Thomas
(211) have indeed found a faster ion exchange between water pools of
w/o microemulsions as alcohol chain length decreases, suggesting more
effective and inelastic collisions between droplets. In contrast,
increasing alcohol chain length may lead to an increase in hard-
sphere radius (241,395) and a rigid interface (429). The result of
phase equilibria in Figure 7-3 supports the above statement. It shows
that phase equilibria of microemulsions containing butanol are
totally driven by attractive interdroplet interaction (curve 1),
while for microemulsions containing hexanol, phase equilibria are all
governed by rigid interfaces (curve 3) as seen by the precipitate

177
formation. The decrease of solubilization in curve 1 from CQ to C,.
o 14
can be explained by the increasing attractive interdroplet interac¬
tion with increasing oil chain length, whereas the increase of solu¬
bilization in curve 3 is due to the increase in natural radius with
increasing oil chain length. It appears that a small change in
alcohol chain length can largely influence the interfacial elasticity
and hence the phase behavior of the microemulsions.
When salt is added to the water, solubilization in the
microemulsions decreases drastically in short-chain oil region as
shown in curve 4 and curve 5 of Figure 7-3. The phase equilibria are
typically of type 1, indicating that the system is governed by inter¬
facial bending stress. Several facts can be elucidated from this
result. First of all, the addition of salt can reduce the area per
head group (aQ) due to the effect of charge screening. Hence the
solubilization (or natural radius) of microemulsions decreases drast¬
ically due to a large increase in v/aQlc. The birefringent phase
also vanishes due to such an increase in v/a 1 . Furthermore, the
o c
rigidity constant K may also increase upon the addition of salt
(214). As oil chain length increases, the water solubilization
increases abruptly and shows a maximum at undecane for microemulsions
containing pentanol at 0.2% (w/w) salinity in curve 4 of Figure 7-3.
The abrupt increase in solubilization implies that the alcohol parti¬
tioning at the interface increases abruptly at the solubilization
maximum, resulting in an increase in the fluidity and natural radius
of the interface. The maximum solubilization shifts towards higher
oil chain length when salinity is increased from 0.2% to 0.25% in
curve 5 of Figure 7-3. This may be interpreted by tighter molecular

178
packing at the interface with higher salinity, which necessitates
longer oil chain length to induce alcohol partitioning at the inter¬
face (212,404). The proportionality between a^ and v at fixed 1^ in
the packing ratio v/a 1 also dictates the criterion that smaller a
o c o
at higher salinity requires a smaller v (less oil penetration), and
hence longer oil chain length to attain proper interfacial curvature
and elasticity.
With regard to how the solubilization changes with alcohol chain
length, Figure 7-3 indicates that for microemulsions with rigid
interfaces (oil chain length less than C^ in curves 1, 2 & 3), the
water solubilization is in the order of C^OH (0.2 % salinity) < C^OH
< C..0H < C^OH, which increases toward the direction of decreasing
alcohol chain length (increasing the interfacial fluidity). In con¬
trast, for microemulsions with fluid interfaces (oil chain length
longer than in curves 1, 2 & 4), the water solubilization is in
the order of C^OH < C..0H < C^OH (0.2% salinity), which increases
toward the direction of increasing alcohol chain length (increasing
interfacial rigidity). The effect of salinity is obviously
equivalent to increasing alcohol chain length. All these changes are
consistent with the theory presented in chapter 6. The above discus¬
sion is based on an assumption that total interfacial area for solu¬
bilization does not change drastically with alcohol chain length,
hence only the effect of natural radius and interfacial elasticity is
considered.

179
7.4.2 Effect of Alcohol Concentration
The effect of pentanol concentration on solubilization in
microemulsions containing hexadecane, dodecane, octane and benzene is
shown in Figure 7-4. The solubilization shows a maximum as a func¬
tion of pentanol concentration. The maximum is most pronounced for
the dodecane system due to chain length compatibility effect. It may
be inferred that chain length compatibility is probably one of the
necessary conditions for a system to achieve its highest possible
solubilization. Increasing alcohol concentration can increase the
alcohol partitioning at the interface, and consequently increase the
total interfacial area for solubilization. This will contribute to
an increase in solubilization. However, at sufficiently high alcohol
concentration, the fluidity of the interface increases (164,166,291)
to an extent that attractive interdroplet interaction starts dominat¬
ing the system, and hence the solubilization decreases. The attrac¬
tive interdroplet interaction is evidenced by the existence of
critical-like behavior (240-241). The dodecane system containing 17
ml pentanol is close to a critical point based on the phase equili¬
brium of two isotropic phases with equal volumes. The critical point
in microemulsions containing hexadecane was observed at about 16 ml
pentanol.
Friberg (386) has discussed the maximum water solubilization at
specific alcohol/surfactant ratio in terms of a diffuse electric dou¬
ble layer effect in a ternary surfactant-alcohol-water system. Since
the major effect of an electric double layer is on the molecular
packing and the curvature of the interface, his analysis is basically

180
Figure 7-4. Solubilization of water as a function of
1-pentanol concentration in microemulsions
consisting of 1 gm sodium stearate, 1-pentanol
and 10 ml different oil: (a) hexadecane; (b)
dodecane; (c) octane; (d) benzene. See
Figure 7-1 legend for details.

Water ( ml ) Water
181
Pentanol(ml)
0 2 4 6 8 10 12 14 16 18 20
Pentanol(ml)
Figure 7-4. Continued.

182
in line with the proposed explanation. The total alcohol/surfactant
molar ratio at the maximum water solubilization in Figure 7-4 is
approximately 23 and is apparently independent of oil chain length.
The solubilization in the microemulsions containing benzene
(Figure 7-4(d)) shows no sizable change with respect to alcohol con¬
centration This seems to imply that alcohol does not penetrate into
the interfacial film effectively. It has been generally observed
that w/o microemulsions consisting of aromatic solvents have always
smaller water solubilization than than that of nonpolar solvents
(e.g., alkanes) (181,289,425). The specific surfactant head group-
solvent interaction is considered to be the cause. This specific
interaction can result in a large absorption of solvent molecules
into the aliphatic layer of the interface (solvation effect) and
hence greater v/aQlc value. It is also speculated that the solvation
of surfactant polar heads by the solvent molecules may hinder the
partioning of alcohol at the interface and may even raise the inter¬
facial tension due to the increase in solvent-water contact
(289,409).
Figure 7-5 shows the water solubilization in microemulsions con¬
taining hexadecane as a function of surfactant concentration at con¬
stant alcohol/surfactant ratios. It has been suggested that such a
plot can provide the information about solubilization efficiency
(amount of solubilized water per surfactant molecule) from the slope,
and the amount of surfactant not partitioning at the interface from
the intercept on surfactant concentration axis (429-431). The result
in Figure 7-5 indicates that the solubilization efficiency of hexade¬
cane microemulsions is maximum at a ratio of 8 ml pentanol per 1 gm

183
Figure 7-5. Solubilization of water as function of
sodium stearate concentration in microemulsions
consisting of 10 ml hexadecane, 1-pentanol
and sodium stearate. The alcohol to surfactant
ratio (A/S) is kept constant in each plot:
(a) 2 ml pentanol/1 gm sodium stearate;
(b) 8 ml pentanol/1 gm sodium stearate;
(c) 16 ml pentanol/1 gm sodium stearate.

184
sodium stearate. A maximum water solubilization at this ratio has
been reported in Figure 7-4(a). However, the maximum solubilization
efficiency does not correspond to a maximum partitioning of surfac¬
tant at the interface as previously reported (429-431). Instead, the
result indicates that the total interfacial area, which is propor¬
tional to both surfactant and alcohol partitioning at the interface,
keeps increasing with increasing alcohol/surfactant ratio, despite
the decrease in solubilization efficiency. This result suggests that
the solubilization efficiency of microemulsions may not always be
proportional to the total interfacial area in the microemulsions. The
curvature and elasticity of the interface are probably more decisive
factors for solubilization.
The linearity in the plots of Figure 7-5 implies a constant
interfacial composition of microemulsion droplets when
alcohol/surfactant ratio is kept constant. Thus the droplet size
will not change, only the number density of microemulsion droplets
increases with total emulsifier concentration. The linearity of the
plots further suggests that the interdroplet attractive force does
not change with the number density of droplets in the solution, but
mainly depends on interfacial curvature (droplet size) and elasti¬
city.
7.5 Effect of Salinity
7.5.1 Optimal Salinity in Single-Phase W/0 Microemulsions
Figure 7-6(a) shows the brine solubilization as a function of
salinity in microemulsions containing isobutyl alcohol (IBA), sodium

185
Sal ¡ nit y f % w/w )
Figure 7-6. Solubilization of brine as a function of salinity
in microemulsions consisting of 10 ml dodecane,
1 gm sodium stearate and 4 ml different alcohols:
(a) isobutyl alcohol; (b) 1-pentanol. The filled
circles represent the second boundary. The filled
squares represent the boundary of transition from
turbid to the second isotropic phase. An imaginary
dotted line has been drawn between the first
isotropic and the second isotropic regions.

186
stearate and dodecane. A maximum brine solubilization was observed
at the salinity of 1.5%. This salinity will be referred to an
"optimal salinity" (432). The maximum brine solubilization is the
result of counteracting effects of attractive interdroplet interac¬
tion and interfacial bending stress. The decreasing brine solubili¬
zation with increasing salinity is due to the increase in interfacial
rigidity and geometric ratio v/aQlc with increasing salinity. A
decreasing alcohol partitioning at the interface with increasing
salinity may also contribute to the decrease of brine solubilization
(432-433) due to the decrease in total interfacial area. In the low
salinity region, the increase in brine solubilization with increasing
salinity can be explained by the increasing alcohol partitioning at
the interface (432). This enhanced alcohol partitioning is attributed
to salting-out effect (434). The addition of salt also reduces the
attractive interaction of fluid interfaces (396), and hence increases
the solubilization.
The term "optimal salinity" was originally defined as the salin¬
ity at which a middle-phase microemulsion solubilizes equal amounts
of oil and brine (435-436). The existence of middle-phase microemul¬
sions in equilibrium with excess oil and brine has been attributed to
both attractive interdroplet interaction and interfacial bending
stress (163,165). The reason for designation of this terminology to
a single-phase microemulsion system is self-explanatory when the
result of phase equilibria shown in Figure 7-6(a) is examined. Upon
addition of excess brine, a microemulsion in equilibrium with oil
(type 2 phase equilibrium) was observed for salinity smaller than or
equal to the optimal salinity (1.5%), while a microemulsion in

187
equilibrium with brine (type 1 phase equilibrium) was observed for
salinity of 2.5% and higher. Only at the salinity of 1.7% and 2%, a
transition of two-phase (type 1) to three-phase (coexistence of type
1 and type 2) and then to two-phase (type 2) equilibria was observed
upon continuous addition of excess brine. The three-phase equili¬
brium occurs in the vicinity of optimal salinity where the driving
force for phase separation changes from attractive interdroplet
interaction to interfacial bending stress. This corroborates the
view that three-phase equilibria are driven by both interdroplet
interaction and interfacial bending stress.
When IBA is replaced by pentanol, a strange phase behavior was
observed as shown in Figure 7-6(b). Only two-phase equilibria were
observed in all samples, and a narrow bluish, translucent single
phase region was observed where three-phase equilibria would have
been anticipated according to Figure 7-6(a). This single phase is
analogous to the second isotropic phase observed in the AOT system
(420-421). The reason for this observation is not clear. A comparison
between Figures 7-6(a) and 7-6(b) seems to indicate that it is
related to the vanishing of three-phase equilibria as a result of
increase in alcohol chain length. Since increasing alcohol chain
length can reduce the attractive interdroplet interaction, a clear
isotropic microemulsion may exist when the interfacial bending stress
has already diminished, but the interdroplet interaction is not
strong enough to induce phase separation. Further, a possible con¬
tinuous phase inversion from w/o to o/w structure may occur in the
second isotropic region. Similar second isotropic regions are found
in most microemulsions containing pentanol presented in Figures 7-7

188
and 7-8. However, they are not shown in the figures for clarity and
simplicity.
7.5.2 Effects of Alcohol and Oil on Optimal Salinity
As alcohol chain length increases from IBA to pentanol, the
optimal salinity decreases as shown in Figure 7-6. If the role of
salinity is considered as to promote the interfacial rigidity and
increase the packing ratio v/a 1 (decrease the natural radius) of
o c
w/o microemulsions, the salinity required by pentanol microemulsions
should be lower than that by IBA microemulsions due to higher inter¬
facial rigidity and packing ratio of pentanol microemulsions.
Figure 7-7 shows that, as oil chain length increases, the
optimal salinity increases. This can be explained by the increasing
fluidity and the decreasing v/aQlc of the interface with increasing
oil chain length. Thus, higher salinity is required to make a rigid
interface and to decrease the natural radius.
Figure 7-8(a) shows that the optimal salinity decreases with
increasing alcohol concentration. At very high alcohol concentra¬
tions, the brine solubilization directly decreases upon the addition
of salt (optimal salinity = 0). The addition of alcohol increases
the polarity of the oil phase, which in turn will increase the sol¬
vent (both alcohol and oil molecules) penetration into the aliphatic
layer of the interface and thus decrease the optimal salinity due to
higher packing ratio v/aQlc. Huh (245) has also concluded similarly
that alcohol increases the solubility of surfactant in oil phase,
resulting in a better interaction between surfactant chains and oil
molecules.

189
Figure 7-7. Solubilization of brine as a function of salinity
in microemulsions consisting of 1 gm sodium
stearate, 8 ml 1-pentanol and 10 ml different
oils: octane (curve 1); dodecane (curve 2);
and hexadecane (curve 3). Only the first
isotropic region is plotted.

190
Pentanol(ml )
Figure 7-8. The brine solubilization in the microemulsions
containing various amounts of 1-pentanol.
(a) Solubilization of brine as a function of salinity
in the microemulsions consisting of 10 ml dodecane,
1 gm sodium stearate and different amounts of 1-pentanol:
4 ml (curve 1) ; 6 ml (curve 2) ; 8 ml (curve 3) ; 12 ml
(curve 4); and 16 ml (curve 5).
(b) Plot of the maximum solubilization of each curve
in (a) as a function of 1-pentanol concentration.

191
As plotted in Figure 7-8(b), the maximum brine solubilization at
optimal salinity for each curve reported in Figure 7-8(a) decreases
with increasing amount of alcohol. Each point in Figure 7-8(b)
represents the maximum solubilization achievable for a specific sys¬
tem in Figure 7-8(a). The result of Figure 7-8(b) implies that the
addition of both salt and alcohol (cosurfactant) to a microemulsion
is essential for achieving its largest possible solubilization.
Though the addition of alcohol can decrease the interfacial tension
and increase the total interfacial area, only small amount of alcohol
is desirable due to its negative influence on interfacial curvature
and elasticity for solubilization at high alcohol concentrations.
The optimal salinity serves to drive more alcohol to the interface,
thus providing more interfacial area for solubilization, and to main¬
tain an optimal interfacial rigidity and curvature in order to minim¬
ize the interdroplet interaction. It is interesting to point out
that all the above-reported effects of alcohol and oil on the optimal
salinity in single-phase w/o microemulsions are in agreement with
that in middle-phase microemulsions (245,437).
7.6 Conclusions
Both theoretical and experimental aspects of solubilization and
phase equilibria of oil-external microemulsions have been studied. A
simple phenomenological approach has been applied to the theory and
the interpretation of experimental results by considering two
phenomenological parameters, namely the spontaneous curvature and
elasticity of interfaces. All the experiments presented are designed
to elucidate how the molecular structure of various components of

192
microemulsions influences these two parameters and hence the solubil¬
ization and phase equilibria of microemulsions. The results can be
recapitulated as follows:
1. Oil can influence the property of an interface through a surfac¬
tant chain-oil interaction. Strong interaction leads to large
penetration of oil molecules into the surfactant chain layer
(solvation), thus increasing the rigidity and curvature (or
packing ratio v/a^l^) of the interface. Oil molecules with
small molecular volume or high polarity often produce strong
solvation effect on the interface.
2. Alcohol (cosurfactant) is essential to promote the the interfa¬
cial fluidity for the formation of microemulsions. Decreasing
alcohol chain length can increase the interfacial fluidity and
hence the attractive interdroplet interaction. On the other
hand, increasing alcohol chain length often increases the rigi¬
dity and curvature of the interface. For alcohols which can
promote interfacial fluidity, increasing alcohol partitioning at
the interface can increase the natural radius and fluidity of
the interface.
3. There exists an optimal salinity for a given w/o microemulsion
at which maximum brine solubilization occurs. The addition of
salt usually leads to tighter molecular packing at the interface
and a decrease in natural radius. For salinity lower than the
optimal salinity, these effects together with salting-out effect
tend to increase the solubilization. However, for salinity

193
higher than optimal salinity, it increases the interfacial rigi¬
dity and curvature, thus decreases the solubilization.
All the above principles constitute the basis of self-consistent
interpretation of the experimental results. The implications of this
study are important. Some fundamental guidelines for the formulation
of w/o microemulsions are proposed as follows:
1. The solubilization of a microemulsion is determined by the cur¬
vature and elasticity of the interface, as well as the total
interfacial area. At constant total interfacial area, the solu¬
bilization is proportional to the radius of droplets. At a
given surfactant concentration, the maximum solubilization effi¬
ciency of the system can be achieved by adjusting the interfa¬
cial curvature and elasticity to optimal values at which both
the bending stress and the attractive force of interfaces are
minimized. Hence, one can increase the solubilization of a
microemulsion with a rigid interface by increasing its natural
radius and fluidity of the interface. On the other hand, the
solubilization of a microemulsion with a fluid interface can be
increased by increasing its interfacial rigidity and decreasing
the natural radius.
2. The study of phase equilibria of microemulsions can serve as a
direct measure to assess the property of the interface and the
driving force for phase separation. Knowing the cause of insta¬
bility of microemulsion droplets is the first step in the formu¬
lation of microemulsions. One often observes a w/o

194
microemulsion with a highly curved and relatively rigid interfa¬
cial film in equilibrium with excess water at the solubilization
limit due to interfacial bending stress. The solubilization in
this microemulsion is limited by the natural radius of the
interface. On the other hand, an oil-external microemulsion can
coexist in equilibrium an excess oil phase containing a low den¬
sity of microemulsion droplets due to attractive interdroplet
interaction when the interface is highly fluid. In this case,
the solubilization is determined by the stability of the dro¬
plets. Sometimes a birefringent phase will occur if the inter¬
face is rigid with intermediate value of v/a 1 (close to 1).
o c
3. When phase separation of microemulsions is driven by the bending
stress of a rigid interface, increasing the natural radius or
fluidity of the interface by increasing oil chain length,
decreasing alcohol chain length, increasing alcohol/surfactant
ratio, or decreasing electrolyte concentration can increase the
solubilization. Among all the factors, alcohol chain length
appears to be the most decisive factor in determining the basic
property of the interfacial film.
4. In contrast, when phase separation of microemulsions is driven
by attractive interdroplet interaction, increasing interfacial
rigidity and decreasing the natural radius by decreasing oil
chain length, increasing alcohol chain length, decreasing
alcohol/surfactant ratio, or increasing electrolyte concentra¬
tion can stabilize the microemulsion droplets and increase the
solubilization.

195
5. The addition of alcohol (cosurfactant) can increase the total
interfacial area at low alcohol concentrations, thus increasing
the solubilization. But at high alcohol concentrations, the
droplet size decreases due to an increase in attractive inter¬
droplet interaction. Hence, only an optimal amount of alcohol
is desired to form microemulsions with maximum solubilization.
6. For a given w/o microemulsion, there exists an optimal salinity
at which maximum brine solubilization occurs. The maximum solu¬
bilization results from the counteracting effect of interfacial
bending stress of rigid interfaces and attractive interdroplet
interaction of fluid interfaces. The optimal salinity decreases
with increasing alcohol chain length and concentration, but
increases with increasing oil chain length.
7. Addition of an optimal amount of alcohol and salinity together
with the effect of chain length compatibility may lead to the
largest possible solubilization of a given w/o microemulsion.

CHAPTER 8
REACTION KINETICS AS A PROBE FOR
THE DYNAMIC STRUCTURE OF MICROEMULSIONS
8.1 Introduction
In previous two chapters, the properties of surfactant interfa¬
cial films (i.e., interfacial curvature and elasticity) have been
shown to influence both equilibrium (e.g., structure, solubilization)
and dynamic (e.g., formation of transient dimers) properties of
microemulsions. When a microemulsion is used as a chemical reaction
medium, the structure and dynamics of the microemulsion can affect
the mechanism and rate of the chemical reaction. On the other hand,
being sensitive to the dynamic structure of microemulsions, reaction
kinetics can also be used as a probe for the dynamic structure of
microemulsions.
Silver chloride precipitation is one of the most extensively
investigated reactions due to its importance to the photographic
industry (438-447). Aqueous suspension of silver halide in the pres¬
ence of protective colloid such as gelatin, surfactants are usually
called emulsions in photography. Turbidity measurement is used to
monitor the precipitation process. It is well known that turbidity
increases rapidly upon mixing silver nitrate with alkali halide solu¬
tions in the absence of protective colloids. Matijevic and Ottewill
(438-439) have attributed such a turbidity enhancement to the fast
196

197
coagulation and precipitation of silver halide sols. They investi¬
gated the effect of cationic detergents on the stability of
negatively-charged silver halide sols. Periodic "sensitized" coagula¬
tion and stabilization regions of the sols were observed upon
increasing detergent concentration. When detergent concentration
approached its critical micellization concentration, the detergent
acted as protective colloids and the coagulation rate of silver
halide sols was drastically reduced. In another set of studies on
the stability of positively-charged silver halide sols in the pres¬
ence of anionic surface active agents, Ottewill and Watanabe (440)
have shown that the stability of the sols decreased initially (fast
coagulation) and then increased again up to a limiting value due to
the adsorption of surface active molecules on the sols. A theory has
also been proposed to account for these experimental findings (439-
440).
In this preliminary study, the coagulation rate of silver
chloride sols in microemulsions has been investigated. The results
are intimately related to structural dynamics of the microemulsions.
8.2 Experimental
8.2.1 Materials and Methods
The sodium dodecyl sulfate (SDS) was of purity higher than 99%
from BDH. AgNO^, NaCl were ACS certified grade from Fisher
Scientific Company. Isopropyl alcohol (IPA) and benzene were of 99%
purity from Fisher Scientific Company. All chemicals were used as
received without further purification.

198
The viscosity was measured by Cannon-Fenske viscometer (#100).
Light scattering was monitored by Duophotometer Model 5200 (Wood Mfg.
Co.). The ultrasonic absorption was measured using Matee Pulse Modu¬
lator and Receiver (model 6600). Pressure Jump studies were performed
using DIA-LOG system with conductivity detection. The stopped-flow
experiments were carried out using Durrum Model D-115 system. The
ultracentrifuge study was carried out using a Beckman Model E analyt¬
ical ultracentrifuge with Schlieren optics. All the measurements were
carried out at 25° C.
8.2.2 Preparation of AgCl Sols
Two stock aqueous solutions of 5 mM AgNO^ and of 5 mM NaCl were
first prepared separately. Microemulsions were then prepared by mix¬
ing specific amounts of SDS, IPA, benzene and either one of the two
aqueous stock solutions so that two identical microemulsion samples
at a desired composition were formed except that one contained AgNO^
and the other contained NaCl. Using stopped-flow apparatus, the AgCl
sols formed upon a rapid injection of these two identical microemul¬
sion samples into a mixing chamber. The turbidity development through
the coagulation of the AgCl sols was then followed by transmittance
measurement.
8.2.3 Coagulation Rate Measurement
As shown by Matijevic and Ottewill (438), the turbidity T
resulting from the formation of solid phase in a solution can be
defined by the relation:

199
I = I e
TP
[8.1]
where I and I are the intensity of the incident and transmitted
o J
radiation respectively, and p is the optical path length of the cell
employed. For small particles (r < X/20 ) in the absence of con¬
sumptive light absorption, T is related to the number of particles
Np per unit path length, and their individual volume by Rayleigh
equation, viz.,
T = A N V2 [8.2]
P P
here A is an optical constant given by:
A = 24 TT3n4X"4 (n2 - n2 )2 (n2 + 2n2)"2 [8.3]
o o o
where n is the refractive index of the solvent, n is the refractive
o 7
index of the particles. X is the wavelength of the light used (in
vacuo).
For a coagulation process, the change of turbidity with time is
given by the equation,
T = A N V2 ( 1 + kt ) [8.4]
P P
where k is a rate constant. When the particles are small enough to
obey Rayleigh's equation [8.2], a linear relationship between turbi¬
dity and time is obtained (438).
For all the samples studied, the use of initial coagulation rate
(the first 2 seconds in a overall process longer than 100 seconds)

200
does provide a linear (or pseudo-linear) rate constant k for compara¬
tive purpose. Moreover, the use of initial coagulation rate for com¬
parison is justified in view of the fact that the concentration pro¬
duct of AgCl in all the samples studied is considerably greater than
the solubility product (1.765 x 10 ^ at 25° C) and hence, upon mix¬
ing, the nuclei of AgCl are formed spontaneously without significant
induction-time delay. However, due to the possible complications
involved in the multi-component systems, all the initial coagulation
rates are reported in terms of relative rate constants k , using
r rel
pure water as a reference,
krel = Si 7 kW t8'51
where k„ is the rate constant measured in microemulsions, and kTT in
M W
water. Assuming the changes of A and in various microemulsion
media are negligible, and is proportional to the volume fraction
of water in a microemulsion sample, the following equation is used to
obtain a relative rate constant for a microemulsion sample,
k . = [(N (dln(I/I )/dt) ] / [N (dln(1/1 )/dt)]TT [8.6]
rel p o M p o w
8.3 Results and Discussions
8.3.1 Coagulation of Hydrophobic AgCl Sols
The experimental line along which all the measurements were made
in a microemulsion phase is shown in Figure 8-1. The phase diagram
of IPA/SDS/benzene/water system was obtained by Clausse (448). The
mass ratio of SDS/IPA is 0.5 in the phase diagram, and the mass ratio

MASS FRACTION OF WATER
Figure 8-1. The change of the relative coagulation rate constant K of
AgCl sols as a function of water content along the experimental
line AB in the phase diagram of SDS-IPA-benzene-water microemulsions.
201

202
of benzene/SDS is 0.33 for all the samples studied. The structure of
the microemulsions employed as reaction media may differ markedly
from alcohol-rich (point A) to water-rich (point B) region in the
phase diagram. Figure 8-1 shows the variation of relative rate con¬
stants as a function of mass fraction of water in microemulsions.
The enhancement in the coagulation rate at specific water mass frac¬
tions of 0.56 and 0.855 is quite striking. The highest peak at 0.855
water mass fraction is very narrow and intense. This narrow peak
does not seem to be related to the onset of micelle formation because
the surfactant concentration is about 15 times higher than the criti¬
cal micelle concentration. The broadness of the peak at 0.56 water
mass fraction is indicative of a certain structural property that may
persist in a wide range of alcohol-rich region of the microemulsion
phase.
It is of interest to note the resemblance of the data to that
reported by Friberg et al. (449). They have investigated the rate of
hydrolysis of p-nitrophenol laurate in a microemulsion system con¬
sisting of cetyltrimethyl ammonium bromide, butanol and water. Two
pronounced and broad peaks of reaction rate were observed. The
enhancement has been ascribed to conventional micellar catalysis
effect in which micellar surface charge density plays a dominant
role. However, this seems unlikely to be the reasons for the
enhancement observed in this study in view of the narrowness of the
peak at 0.855 as compared to that reported by Friberg et al. (449).
The reaction kinetics by which the turbidity increases will
first be discussed. As shown by Ottewill and Watanabe (440) in the
case of sol formation
a complex series of
consecutive and

203
simultaneous reactions occur. These can be schematically presented
as follows (440):
-> nuclei
x,
small crystals (primary particles)
coagula
large crystals
They involve nucleus formation (step A), crystal growth (step B) and
coagulation (step C and D) of primary particles. The turbidity
development observed in this study may not be related to reaction A
because the nucleus formation is usually very fast at reasonably
high supersaturation and therefore will terminate before any experi¬
mental observation is recorded. Another supporting evidence is that
the nuclei of AgCl sols may only consist of about five ions as
reported by Klein et al. (450). The change in opalescence resulted
from such small nuclei is probably not optically detectable. Thus,
the reactions observed in this experiment would appear mostly to be
B, C and D.
In many classical coagulation studies using performed sols
(439-440) with the addition of coagulatory agent (such as surface
active molecules), a simplified analysis is feasible in which the
growth of nuclei to primary particles is very rapid, and hence the
reaction observed is predominantly the coagulation of the primary
particles, i.e. the reaction D. This reaction has been found to

204
depend strongly on the the surface potential of the sol particles.
The adsorption of surface active agents on sols may modify the sur¬
face potential and consequently alter the coagulation rate. In this
study, a different approach has been taken by monitoring the forma¬
tion of hydrophobic sols in statu nascendi (438) using microemulsions
as reaction media. The analysis thus appears complicated due to the
possible influences of the microemulsions on nucleation and crystal
growth. Some of these influences are even not clear at present. It
can be stated, however, that the surface potential of the sols and
the nature of the media are primarily the controlling factors in the
crystal growth and coagulation process.
8.3.2 Physico-Chemical Properties of the Microemulsions
In order to delineate the correlation between the coagulation
rate and the dynamic structure of microemulsions, a number of meas¬
urements have been performed to determine various physico-chemical
properties of the microemulsion phase along the experimental line.
Figure 8-2 shows the conductance of the microemulsions. A maximum
conductance around 0.56 water mass fraction corresponds to the peak
of the coagulation rate in Figure 8-1. The increasing conductance
with addition of water in the alcohol-rich region may be attributed
to the increasing ionization of SDS molecules. The decreasing con¬
ductance beyond the maximum is presumably due to the structural tran¬
sition from w/o to o/w and dilution effect by additional water.
Figure 8-3 represents the plot of viscosity versus water con¬
tent. It shows that as the amount of water decreases, the viscosity
increases up to a water mass fraction of 0.56, and with further

CONDUCTANCE(S
205
Figure 8-2. The change of specific conductivity as a
function of water mass fraction along the
line AB (Figure 8-1) of SDS-IPA-benzene-water
microemulsion system.

VISCOSITY (cp)
206
Figure 8-3. The change of viscosity as a function of water
mass fraction along the line AB (Figure 8-1) of
the SDS-IPA-benzene-water microemulsion system

207
reduction in water content there is no significant change in viscos¬
ity. The decrease in viscosity in the region of water mass fraction
greater than 0.56 is consistent with the dilution effect shown in the
electrical conductivity measurement (Figure 8-2).
In order to further understand the association structure of the
surfactant in the solution, the light scattering of the samples have
been measured as shown in Figure 8-4. Interestingly, the results
exhibit two peaks corresponding to the peaks of coagulation rate in
Figure 8-1. The broad peak in Figure 8-4 also bears a strong resem¬
blance to the results reported by Friberg et al. (449). It can be
stated from these results that starting from the alcohol-rich region,
the addition of water in microemulsions results in a certain associa¬
tion structure which enhances the light scattering from the solution.
Similar conclusions have also been drawn by Sjoblom and Friberg (451)
for water/pentanol/potassium oleate/oil microemulsion system. The
association structure of the surfactant in the alcohol-rich region
may resemble water-in-oil (w/o) microemulsions (451).
It should be noted that concentration fluctuations can also
increase the light scattering intensity by orders of magnitude near
the vicinity of a critical point. It is well established from recent
studies (241,258,452-454) that a critical-like behavior has been
observed near the percolation threshold in w/o microemulsion systems
where strong concentration fluctuations occur due to the long range
attractive force between the microemulsion droplets. However, there
exist no experimental data to distinguish between the micellar growth
and concentration fluctuation mechanism to explain the observed light
scattering data. The light scattering peak around 0.855 water mass

208
Figure 8-4. Light scattering intensity Iqq/Iq as a function
of water mass fraction along’theUline AB (Figure
8-1) of the SDS-IPA-benzene-water microemulsion
system

209
fraction is extremely narrow. A broad and smooth transition zone is
usually observed in the transition of spherical to rod (cylindrical)
shape of micelles using viscosity, light scattering and magnetic
field measurement (455-456). It is likely, therefore, that this sharp
peak may not be related to a sphere-rod structural transition of
micelles, but a concentration fluctuation which will be discussed in
more detail later.
Besides the light scattering data, the ultracentrifugation
results further confirm the existence of surfactant aggregates in
this system (Figure 8-5). For the samples with 0.855 water mass frac¬
tion or greater, no sedimentation peak was observed. It is expected
that normal SDS micelles with solubilized benzene may exist in this
region. The absence of sedimentation peaks is probably due to the
electric repulsion force between the micelles. This is consistent
with an observation that no sedimentation peak was observed in a pure
0.5 M SDS aqueous solution containing normal micellar aggregates.
For solutions with water fractions 0.8 down to 0.25, sedimentation
peaks were observed and the sedimentation coefficients reported in
Figure 8-5 were calculated from the velocity of the sedimenting peaks
(457-459). The change of sedimentation coefficients with rotor speed
in Figure 8-5 was unexpected, suggesting that the aggregates are
rather compressible and sensitive to centrifugal force. The increase
in sedimentation coefficients as the amount of water decreases does
not necessarily indicate growth of the aggregates. It may also be
attributed to the decreasing buoyancy of the solvent due to the con¬
tinuous addition of IPA into the solutions. Further attempts to
determine the particle size from the sedimentation coefficient are

210
O
LU
(/>
UJ
O
LL
LU
O
O
¡s
z
111
5
O
UJ
Figure 8-5. Sedimentation coefficients at various rotor
speeds as a function of water mass fraction
along the line AB (Figure 8-1) of the SDS-
IPA-benzene-water microemulsion system

211
thus thwarted due to the variation in the composition of the continu¬
ous phase. For solutions with water fractions of less than 0.25, no
sedimentation peak was observed.
Figure 8-6 represents various Schlieren patterns of the samples
at different compositions. The Schlieren peak appears upward if the
refractive index increment is positive, dn/dc>0; and downward if
negative, dn/dc<0, where n denotes the local refractive index of the
solution and c is the local solute concentration in the solution
(457-459). Figure 8-6(a) shows an upward meniscus boundary for the
.^sample of 0.853 water mass fraction (lower curve) and a downward peak
for the sample of 0.8 water mass fraction (upper curve). It is evi¬
dent that an inversion of the refractive index increment dn/dc occurs
in the region of water fraction between 0.85 and 0.8, indicating a
change in the solution properties. Such inversion is also accom¬
panied with the onset of the sedimentation of aggregates. Figure
8-6(b) indicates the sedimentation peaks observed at 0.567 (lower
curve) and 0.35 (upper curve) water fraction. Both peaks are down¬
ward. It is noteworthy that there exists another upward peak near the
bottom of the cell for both samples. This peak appears to float up
against the centrifugal field during the course of centrifugation.
The reason for this floating peak is not clear at the present. How¬
ever, it can not be attributed to the floatation of the aggregates
because a floatation peak usually appears downward (with negative
refractive index increment). It is proposed that this peak may be
related to the compressibility of the system for which the aggregates
under compression tend to relax back through a back-diffusion of the
solutes against the centrifugal field (460). Figure 8-6(c) represents

212
Figure 8-6. Ultracentrifuge Schlieren patterns of the micro-
emulsions along the line AB in Figure 8-1. Note
that the left hand side is the meniscus, and the
right hand side corresponds to the bottom of the
cell, (a) Upper curve corresponds to the sample
at 0.8 water mass fraction, lower curve to the
sample at 0.853 water mass fraction. 1651 seconds
elapsed after reaching the speed of 33,350 rpm;
(b) Upper curve corresponds to the sample at 0.35
water mass fraction, and lower curve to the sample
at 0.567 water mass fraction. 2595 sec elapsed
after reaching the speed of 42,040 rpm; (c) Upper
curve corresponds to the sample at 0.24 water
mass fraction, and lower curve to the sample at
0.908 water mass fraction. 1377 sec elapsed after
reaching the speed of 20,410 rpm.

213
the Schlieren patterns of the sample at 0.24 water fraction. No sed¬
imentation peak was observed and the meniscus boundary was inverted
from downward to upward again, indicating the occurrence of a struc¬
tural transition (phase separation occurs at composition A, Figure
8-1).
The ultrasonic absorption (Figure 8-7) of the solutions at 5 MHz
also corroborates the picture that emerges from the ultracentrifuga¬
tion results. The maximum absorption observed at 0.56 water fraction
corresponds to the peak of light scattering and the coagulation rate
of AgCl, and is indicative of the ease of structural perturbation by
ultrasonic pressure. The possible processes of ultrasonic relaxation
in surfactant solutions include: (1) the exchange of alcohols between
mixed micelles and the surrounding solution; (2) the exchange of sur¬
factants between micelles and the surrounding solution; (3) ion
association-dissociation equilibrium of the electrolytes; and (4)
concentration fluctuations of the solution. Zana et al. (453) have
investigated the ultrasonic absorption behavior in many w/o
microemulsion systems. It was found that the high ultrasonic absorp¬
tion could only be detected if large concentration fluctuations
occurred in the system. Hence, the first and second process can be
excluded. The third process is also unlikely to be the cause for the
observed maximum absorption in view of the results reported by Fri-
81
berg et al. (449) that a continuous increase of Br line width
81
occurs as the water content decreases. The broadening of Br line
width indicates the increasing strength in the counter-ion binding.
The ultrasonic absorption, if any, will then appear as a monotonic
function, instead of a maximum. Hence only concentration

ABSORPTION COEFF. CM
Figure 8-7. Ultrasonic absorption as a function of water mass fraction along
the line AB (Figure 8-1) of the SDS-IPA-benzene-water microemulsion
system
214

215
fluctuations as the probable cause of the absorption maximum will be
considered.
The concentration fluctuation in the investigated system can
possibly further be subdivided into a solute (surfactant aggregates)
concentration fluctuation and a solvent concentration fluctuation.
The solute concentration fluctuation is similar to that of critical-
like behavior observed in many w/o microemulsion systems (453), while
the solvent concentration fluctuation may result from the mixed sol¬
vent of IPA and water. It has been reported (461) that a maximum
ultrasonic absorption occurs at 0.84 mass fraction of water (mole
fraction of IPA is 0.057) in a IPA + water mixture. A shift in the
composition of this maximum absorption may occur upon addition of
other additives. Therefore, the solvent concentration fluctuation as
a possible cause of the maximum absorption cannot be ruled out.
A pressure-jump relaxation study on the system has also been
carried out in an attempt to directly probe the microstructure of the
system. In the water-rich region (water mass fraction greater than
0.75) of the microemulsion system, the relaxation spectra resemble to
those of normal micelles except that there exists an additional slow
relaxation process (referred as Ts^ow^* The value of as plotted
in Figure 8-8 is smaller than that of pure SDS micelles (800 mil¬
liseconds to 5 seconds) in the same SDS concentration range of 100 to
200 mM as reported by Kahlweit (83). This may be attributed to the
presence of a short-chain alcohol as discussed in chapter 4. The
amplitude of Ts^ow process in the water-rich region is very small
and hence the resolution of relaxation time T , is poor. Within
slow r
the range of experimental accuracy, Ts^ow was found to be indepen¬
dent of microemulsion composition.

RELAXATION TIME T9(SEC)
216
WATER MASS FRACTION
Figure 8-8. The change of the slow relaxation time T£ as
a function of water mass fraction along the line
AB in Figure 8-1

217
As the amount of IPA and SDS continuously increases in the solu¬
tion, the relaxation spectra seem to undergo a smooth transition.
The amplitude of T g^ow process is gradually increasing, while the
amplitude of t ^ process is diminishing. Only the T ^ values for the
water rich region are reported in Figure 8-8 due to the poor resolu¬
tion of T 2 with increasing alcohol concentration. However, the
value of T ^ appears to increase with alcohol concentration and
approach a plateau value around 0.8 sec at alcohol-rich corner. It
should be noted that TS^QW is probably not related to micelle relax¬
ation. A 10 mM potassium chloride in mixed IPA/water solvent has
been found to give rise to a similar relaxation signal (consisting
of three relaxation processes) and relaxation times to that of the
sample at 0.24 water mass fraction. Hence, the slow processes may be
attributed to the compressibility of the mixed solvent. It is obvi¬
ous that the signal resulting from the relaxation of normal micelles
dominates the relaxation spectra in the water-rich region and hence
the amplitude of T . process is small. But as the alcohol concen-
r slow r
tration increases, the normal micellar aggregates disappear gradually
and a structural transition takes place. The relaxation spectra will
then be overshadowed by the relaxation of the mixed solvent of
IPA/water. The mechanisms corresponding to this mixed solvent relax¬
ation are not established at the present.
Piecing together all the experimental data thus far presented,
the following sketch can be attained for the structural properties of
the microemulsion system. The sedimentation studies confirm the
existence of association structures of surfactants in the alcohol-
rich region. Based upon the light scattering data (Figure 8-4) it can

218
be stated that the addition of water induces an association of the
surfactants starting at about 0.4 water mass fraction. Between 0.56
to 0.7 water fraction, the surfactant aggregates and concentration
fluctuations may coexist in the system. The association structures
over the alcohol-rich region should resemble the inverted micelles or
w/o microemulsions. Beyond 0.7 water mass fraction, a structural
transition from inverted to normal micellar structures occurs and
hence the light scattering decreases. If the normal micellar struc¬
ture persistently exists over the alcohol-rich region, it would have
been expected that the value of decreases continuously instead of
increases according to the relaxation study of high concentration SDS
solution reported by Kahlweit (83).
The structural studies reported by Bellocq et al. (267,462) on a
microemulsion system composed of SDS, butanol, water and toluene can
be referred to support the types of association structures mentioned
above. Three subregions consisting of different microstructures in a
single microemulsion phase region have been identified using quasie¬
lastic light scattering. In view of the striking resemblance of the
phase diagram in Figure 8-1 to that reported by Bellocq et al.
(267,462), similar conclusions can also be drawn for the system in
this study. The formation of inverted structures is responsible for
the light scattering enhancement starting at 0.4 water fraction as
shown in Figure 8-4. It is noteworthy that the light scattering
intensity remains constant upon further increasing the water fraction
from 0.56 to 0.7. This suggests that the additional water does not
induce further growth of inverted micelles, but partition in the con¬
tinuous medium. This not only explains the decrease of the

219
conductance beyond the 0.56 water mass fraction, but also indicates
that large micellar structures are not formed in the IPA/water mixed
solvent. Hence, the concentration fluctuation of aggregates may play
an important role in light scattering. This can also explain the
fact that the association structures in the alcohol-rich region are
not rigid. They are fragile and easy to perturb as may be concluded
from the ultracentrifuge study.
The structural transition from inverted to normal micellar
structure occurs around 0.8 to 0.7 water mass fraction. It is obvi¬
ously a progressive transition. The microstructure in this region is
not yet well established. The ultracentrifuge study has shown that
the inverted micellar structure may exist persistently down to 0.8
water mass fraction. But quasielastic light scattering measurements
have detected the trace of normal micellar structure as low as 0.7
water mass fraction in some other systems (462).
Lastly, it may be pointed out that the head group of the ionic
surfactant have to be hydrated by a minimum amount of water in order
to dissolve in a low polarity solvent (e.g., short-chain alcohols).
In the oil-rich corner of a microemulsion phase diagram, micelliza-
tion occurs as long as the minimum water required to hydrate the
ionic head group is added (462). Hence the minimum water/surfactant
molar ratio required for such hydration can be determined by light
scattering measurement. The ratio has been found to be about 10 for
sulfate surfactants in toluene and 8 for carboxylate surfactants in
dodecane (462). But in the alcohol-rich corner, the micellization
does not necessarily occur upon the addition of the necessary minimum
amount of water. Therefore light scattering measurement cannot be

220
used to determine the minimum water required. Instead, a titration
method has been developed for this purpose. Starting from the
surfactant-rich region of the phase diagram in Figure 8-1, the added
water is expected to partition in both surfactant phase (hydration)
and continuous medium (IPA). Then the total number of water molecules
N added to the solution is:
w
N = NS + N3 [8.7]
WWW
where Ns denotes the number of water molecules that hydrate the sur-
w
factant head groups, and is the number of water molecules parti¬
tioning in the IPA. Assuming h is the minimum number of water
molecules per surfactant molecule required for hydration, one can
write:
NS = hN [8.8]
w s
where the N is the total number of surfactant molecules in the solu-
s
tion. Nacan also be written as
w
Na = kN [8.9]
w a
where k is the water molecules per IPA molecule, and is the total
number of alcohol molecules in the solution. Combining equations
[8.7], [8.8] and [8.9] gives:
N /N = k(N /N ) + h
w s as
[8.10]

221
A clear microemulsion sample of 0.22 water mass fraction near
the phase boundary was taken as the starting point in the titration.
The sample was first titrated with IPA till the sample just became
turbid, then titrated with water till the sample became clear again.
Repeating these procedures many times and plotting the ratio of N /Ng
versus N^/N^, a straight line as shown in Figure 8-9 was obtained.
The slope yields the constant k and the intercept on y-axis
corresponds to the minimum number of water molecules per surfactant
molecule required for dissolution. It was concluded that minimum 8
water molecules are needed to hydrate each sulfate group for dissolu¬
tion of SDS into IPA. It should be noted that this titration method
can only be used in the miscibility range of short-chain alcohols
with water.
8.3.3 Interrelationship between the Reaction Kinetics
and the Dynamic Structure of Microemulsions
The correlation between the reaction kinetics and the dynamic
structure of the microemulsions is analyzed as the following. The
coagulation rate of AgCl sols depends on the surface charge of the
sols. High surface charge density prevents the collision of the prel¬
iminary particles of AgCl crystals, and consequently results in slow
coagulation rate and small precipitates. The enhancement of coagula¬
tion in Figure 8-1 from water mass fraction of 0.22 to 0.65
corresponds to the region where the nature of the continuous phase of
the microemulsions is dominated by alcohol (low polarity). The asso¬
ciation structures in this region are probably inverted micelles.
Hence, the enhancement of coagulation between at 0.56 water fraction
is associated with the low polarity of the solvent and inverted
micelles.

222
N./N,
Figure 8-9. Minimum number of water molecules, h, per surfactant
molecule needed to hydrate the sulfate group for
the dissolution of SDS into IPA.

223
It has been shown that the stability of colloidal suspensions
can also be influenced by a pure alcohol-water mixture, without the
addition of any surface active agent. In a study of the flocculation
of polystyrene emulsions in ethanol-water mixtures (463), the concen¬
tration of sodium chloride required to produce rapid flocculation
increases with increasing ethanol concentration up to 0.09 molar
fraction; beyond this composition, the concentration of sodium
chloride required for flocculation decreases rapidly. It will be very
informative, therefore, to compare the coagulation rate obtained in
microemulsion media to that in pure IPA + water mixture. The results
can be used to further delineate the role of inverted micellar struc¬
ture on the enhancement of coagulation.
The reasons for the sharp peak of coagulation rate at 0.855
water fraction can be explained as follows. The sharp increase in
the coagulation rate is presumably due to the fast mutual coagulation
of the normal micelles as a result of increasing concentration of
SDS. Many facts substantiate this conjecture. Using density and heat
capacity measurements, Roux et al. (268-269) have found that micellar
growth starts at about 0.8 water mass fraction along a dilution line
(by water), and at about 0.85 water mass fraction along the lower
deraixing line in a microemulsion system consisting of SDS, butanol,
water and toluene. The discrepancy in these two water fractions
appears to result from the micelles swollen by the solubilized
toluene along the lower demixing line. In view of the striking simi¬
larity of the phase diagram in Figure 8-1 to that reported by Roux et
al. (268-269), it is plausible to propose that micellar growth occurs
at 0.85 water mass fraction.

224
It may be pointed out that this micellar growth is probably not
the type of transition from spherical to cylindrical micelles as usu¬
ally observed in a concentrated surfactant solution (455-456). A
tighter molecular packing in micelles is generally expected as a
result of this transition. But in contrast, Roux et al. (268-269)
have found that a less structured micelle results after the micellar
growth. Two counter-acting forces may exist during the course of
this micellar growth: one force tends to increase the micellar size
due to the continuous increase of SDS concentration, while the other
tends to break down the micelles due to the increasing concentration
of alcohol (320-321,464). It is likely due to these counter-balancing
forces that the micellar growth in these microemulsion systems
differs from that of sphere to cylinder transition. The concentra¬
tion of SDS at 0.855 water mass fraction is about 150 mM. Recall
that a reversible coagulation-fragmentation of submicellar aggregates
occurs at about 180 mM SDS. It is proposed that a mutual coagulation
of micelles may occur due to the micellar growth at 0.855 water mass
fraction, taking into account that the presence of IPA may decrease
the surface charge density of the micelles. This can explain the
occurrence of the sharp light scattering peak at 0.855 water mass
fraction in Figure 8-4. This light scattering peak may be due to the
concentration fluctuation resulting from the mutual coagulation of
micelles.
The sharp enhancement in the coagulation of AgCl sols at 0.855
water mass fraction is the consequence of this mutual coagulation of
the micelles. It is likely that the primary particles of AgCl sols
are located in the micelles (439) or adsorbed at micelle surface.

225
The electrostatic repulsion force prohibits the coagulation of the
micelles, hence the growth of the particles is slow and the size of
the precipitate is small in the water-rich region as compared to
alcohol-rich region. Some preliminary data on the particle size
using scanning electron micrographs indeed confirm that the particles
precipitated in the water-rich region are much smaller than in the
alcohol-rich region. However, at the composition of micellar growth,
the rate of particle growth is enhanced through a fast coagulation of
the micelles. The sharpness of the coagulation peak suggests that
the micellar growth is probably limited to a very finite composition
range.
8.4 Conclusions
The coagulation of the hydrophobic AgCl sols has been investi¬
gated using microemulsions as reaction media. Both equilibrium and
dynamic studies have been carried out to delineate the microstruc¬
tures in the single microemulsion phase. The existence of different
microstructures in the single phase region has been established. For
the microemulsions with water mass fractions of 0.4 to 0.56, inverted
micellar structures are formed. The concentration fluctuation of
these inverted micelles may also play an important role in the region
from 0.4 up to 0.7 water mass fraction. For the microemulsions with
water fractions greater than 0.8, the existence of normal SDS
micelles have been indicated. It has been shown that the kinetics of
the chemical reaction is intimately correlated with the structures
and the nature of the microemulsion phase. Hence, the chemical reac¬
tion can serve as a useful approach for probing the dynamic structure

226
in microemulsions. Besides the reaction kinetics, the morphology of
the products from the chemical reaction is also influenced by the
microemulsion. This study is thus relevant to various technological
applications such as the manufacturing of photographic films (465),
catalysis (466) and fine powder technology.

CHAPTER 9
CONCLUSIONS AND RECOMMENDATIONS
9.1 Effects of Alcohols on the Dynamic Monomer-Micelle
Equilibrium and Conductance of Micellar Solutions
The effect of alcohols on two equilibrium parameters of micellar
solutions, namely the critical micellization concentration (CMC) and
the degree of counterion dissociation of micelles, have been investi¬
gated using electrical conductivity measurements. The change of CMC
reflects the shift of monomer-micelle equilibrium and the change of
thermodynamic stability of micelles. The addition of alcohols from
methanol to hexanol decreases the CMC of sodium dodecyl sulfate (SDS)
initially, but increases the CMC at higher alcohol concentrations. A
minimum CMC and maximum thermodynamic stability of micelles are
observed at about 2-3 alcohol/surfactant molar ratios in the micellar
phase, independent of alcohol chain length from butanol to hexanol.
It is concluded that the monomer-micelle equilibrium is sensitive to
the addition of alcohols. The decrease of CMC upon the addition of
alcohols is due to the mixing entropy resulting from the comicelliza-
tion of surfactants and alcohols, while the maximum stability of
micelles at the optimal 2-3 alcohol/surfactant ratios is attributed
to the closest geometric packing between surfactants and alcohols in
micelles.
227

228
The formation of surfactant and alcohol mixed micelles increases
the counterion dissociation of the micelles due to steric shielding
and dilution effects of alcohols on the micellar surface charges.
This tends to increase the conductance of a micellar solution when
the conductance is mainly contributed by the micelles at sufficiently
high surfactant concentrations. However, at low surfactant concen¬
trations, the formation of mixed micelles decreases the conductance
of the solution due to the decrease in surfactant monomer concentra¬
tion.
At sufficiently high alcohol concentrations or long alcohol
chain length, the solubilization of alcohols takes place in the
hydrophobic core of micelles, leading to the formation of "swollen"
micelles. Such a swollen micelle formation often results in recombi¬
nation of counterions to micelles due to closer packing of surfactant
head groups on less curved micellar surfaces. The formation of swol¬
len micelles also tend to attract more surfactant monomers into
micelles. It is suggested that the formation of swollen micelles is
the initial step of a structural transition from a simple micellar
solution to an o/w microemulsion. The change of electrical conduc¬
tance of the solution reveals this structural transition.
9.2 Effect of Alcohols, Oils and Polymers on the
Dynamic Properties of Micellar Solutions
The slow relaxation time of SDS micellar solution has been
investigated using the pressure-jump method with conductivity detec¬
tion. The addition of short-chain alcohols from methanol to butanol
labilizes the micelles by decreasing the slow relaxation time. It is
suggested that the mixed micelles of alcohols and surfactants may be

229
thermodynamically more stable than the pure micelles, but they are
kinetically more labile than the pure ones. The concept of "thermo¬
dynamic" stability and "kinetic" stability of micelles is thus dif¬
ferentiated. The labilizing effect of short-chain alcohols is
explained by a possible increase in micelle nucleus population and a
decrease in activation energy barrier for nucleus formation, hence
increasing the rate of micelle formation and dissolution. Propanol
exerts greater labilizing effect on micellization kinetics in the
reversible coagulation-fragmentation regime as compared to the step¬
wise association regime. The addition of polyvinylpyrrolidone (PVP)
polymer can decrease the slow relaxation time significantly by serv¬
ing as a nucleating site for micelle nuclei.
The micellization kinetics go through a transition from step¬
wise association to reversible coagulation-fragmentation process at
sufficiently high surfactant concentrations. However, such a transi¬
tion can also be induced at lower surfactant concentrations by adding
medium- and long-chain alcohols (i.e., pentanol and longer alcohols).
This is explained by the increasing counterion dissociation of the
mixed micelles. The formation of swollen micelles by alcohols and
oils slows down the micellization kinetics with a concomitant
decrease in the electrical conductance of the solution. It is sug¬
gested that the relaxation study together with electrical conductance
measurements can be used to probe the solubilization site of addi¬
tives in micelles and the structural transition from simple micelles
to swollen micelles and o/w microemulsions.

230
9.3 Effects of Spontaneous Curvature and Interfacial Elasticity on
the Solubilization and Phase Equilibria of W/O Microemulsions
The solubilization and phase equilibria of w/o microemulsions
have been shown to be dependent on two phenomenological parameters,
namely the spontaneous curvature and elasticity of the interfacial
films, when the interfacial tension is very low. The spontaneous
curvature of the interface is basically determined by the geometric
packing of surfactant and cosurfactant molecules at the interface,
whereas the interfacial elasticity is related to the energy required
to bend the interface. The droplet size and solubilization of
microemulsions is mainly determined by the radius of spontaneous cur¬
vature, and is further influenced by interfacial elasticity and
attractive interdroplet interactions. A w/o microemulsion with a
highly curved and relatively rigid interfacial film can exist in
equilibrium with excess water at the solubilization limit due to the
interfacial bending stress. Increasing the natural radius and
fluidity of the interface can increase the droplet size and hence the
solubilization in the microemulsion. On the other hand, a w/o
microemulsion with a highly fluid interfacial film can exist in
equilibrium with an excess oil phase containing low density of
microemulsion droplets due to attractive interdroplet interactions.
Increasing interfacial rigidity and decreasing the natural radius in
this case can increase water solubilization in the microemulsion by
retarding the phase separation process. Thus, a maximum water solu¬
bilization in a w/o microemulsion can be obtained by minimizing both
the interfacial bending stress of rigid interfaces and the attractive
interdroplet interaction of fluid interfaces at an optimal

231
interfacial curvature and elasticity. The study of phase equilibria
of microemulsions can serve as a simple method to evaluate the pro¬
perty of the interface and provide phenomenological guidance for the
formulation of microemulsions with maximum solubilization.
9.4 Effects of Oils, Alcohols and Salinity on the Solubilization
and Phase Equilibria of W/0 Microemulsions
The solubilization and phase equilibria of water-in-oil (w/o)
microemulsions have been studied to elucidate how the molecular
structure of various components of microemulsions and salinity influ¬
ence two interfacial parameters of the system, namely the spontaneous
curvature and interfacial elasticity of interfacial films. Most
solubilization and phase equilibria data presented are explained by
the change of these two parameters. It has been indicated that oil
can influence the property of an interfacial film through a solvation
effect. Oil molecules with small molecular volume or high aromati¬
city produce a strong solvation effect and consequently lead to a
greater penetration of oil molecules into the surfactant chain layer,
thus increasing the rigidity and curvature of the interface. The
addition of alcohol exhibits strong effects on the elasticity and
spontaneous curvature of an interface. Decreasing alcohol chain
length can increase the fluidity of the interfacial film and hence
increase the attractive interdroplet interaction. On the other hand,
increasing alcohol chain length often increases the rigidity and cur¬
vature of an interface. For alcohols which can promote the interfa¬
cial fluidity, increasing alcohol partitioning at the interface will
increase the fluidity and natural radius of the interface. The addi¬
tion of electrolytes results in tighter molecular packing at the

232
interface and a decrease in the natural radius and interfacial
fluidity. There often exists an optimal salinity for a given w/o
microemulsion system at which a maximum brine solubilization occurs.
The maximum solubilization has been interpreted as a result of coun¬
teracting effects of attractive interdroplet interaction of fluid
interfaces and the bending stress of rigid interfaces. Some
phenomenological guidelines for the formulation of microemulsions
have been proposed. It has been inferred that the addition of small
amount of alcohol at optimal salinity together with the condition of
chain length compatibility can result in the largest possible brine
solubilization in a given w/o microemulsion.
9.5 Reaction Kinetics as a Probe for the Dynamic
Structure of Microemulsions
The coagulation of hydrophobic AgCl sols has been investigated
by stopped-flow method employing microemulsions composed of SDS, IPA,
water and benzene as reaction media. Two enhancement peaks of the
coagulation rate have been observed. In order to correlate the
enhancement with the dynamic structures of reaction media, the
physico-chemical properties of the microemulsions have been measured
using various techniques including conductance, viscosity, light
scattering, ultracentrifuge, ultrasonic absorption and pressure-jump
relaxation. Subregions consisting of different microstructures
within a clear single microemulsion phase have thus been delineated.
Accordingly, the broad enhancement peak of the coagulation at 0.56
water mass fraction is associated with inverted surfactant aggregates
in the alcohol-rich solvent. The sharp enhancement peak at 0.855
water mass fraction has been attributed to the fast coagulation of

233
normal micelles leading to micellar growth. It has been found by a
titration method that a minimum of eight water molecules per surfac¬
tant molecule are required to hydrate the sulfate group for the dis¬
solution of SDS into IPA. Being sensitive to the dynamic structure
of micellar and microemulsion systems, it is suggested that reaction
kinetics can be used as a probe for the dynamic structure of
microemulsions.
9.6 Recommendations for Further Investigations
A study of the change of surfactant monomer activity of micellar
solutions with addition of alcohols using specific ion electrode
(348) is suggested. The study will provide direct experimental evi¬
dence about the change of surfactant monomer concentration and degree
of counterion dissociation of micelles with alcohol concentration
proposed in chapter 3. It would also be informative to study the
change of micellar aggregation number with alcohol concentration
using fluorescence method (341). An increase in the aggregation
number of micelles is expected upon the formation of swollen
micelles.
One fundamental question that arises from the dynamic study of
micellar solutions is how the micellization kinetics and the kinetic
stability of micelles influence the properties and the performance of
a micellar solution. With an understanding of the dynamics of micel¬
lar solutions, we are now in a position to alter the mechanism of
micellization kinetics and change the life-time of micelles by orders
of magnitude. It will be interesting to study how these changes
affect the processes occurring in the micellar solutions, such as the

234
rate of solubilization, fluorescence quenching kinetics, photochemi¬
cal and some other chemical reactions of which the time scales are
comparable to or faster than that of micelle formation and dissolu¬
tion. For comparative purposes, it is worthwhile to study some sys¬
tems reported in chapters 4 and 5 using the temperature-jump method
to delineate how different perturbation methods influence the
results. The use of the temperature-jump method also permits the
study of effect of salinity on the dynamic properties of micellar
solutions.
The establishment of microscopic correlation between the curva¬
ture elasticity and the natural radius of surfactant interfacial
films is fundamentally important for the understanding of solubiliza¬
tion and phase equilibria of microemulsions. Experimental techniques
such as the electron spin-label method (393) and fluorescence method
(236,397-398) can be used to measure the order parameter and elasti¬
city of interfacial films, and scattering methods (263,276-281) may
allow the determination of interfacial curvature and interdroplet
interfactions for the systems presented in chapter 7. The microemul¬
sion structure in the second isotropic region and the birefrigent
phase reported in chapter 7 also deserve further study.

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BIOGRAPHICAL SKETCH
Reger Yi-Ming Leung was born on August 26, 1955, in Taiwan,
Republic of China. He received his B.S. degree in chemical engineer¬
ing from Chung-Yuan University, Taiwan, in 1977. On completion of his
military service in the Chinese Army, he joined the graduate program
in the Chemical Engineering Department, University of Florida, in
August, 1980, and received his M.S. degree in chemical engineering in
August, 1981. He continued his work toward the Ph.D. degree while
working as a research assistant.
262

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
\Siv . c ,/- 0 . sk.- i -
Dinesh 0. Shah, Chairman
Professor of Chemical Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
£2
John P. O'Connell
Professor of Chemical Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
/q/jCAci (A U â–  tUjhh
Gerald B. Westermann-Clark
Associate Professor of Chemical
Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
Dale W. Kirmse
Associate Professor of Chemical
Engineering

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
Brij M. Moudgil
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.
j1 W. Chun
rProfessor of Biochemistry and
Molecular Biology
This dissertation was submitted to the Graduate Faculty of the
College of Engineering and to the Graduate School and was accepted as
partial fulfillment of the requirements for the degree of Doctor of
Philosophy.
May, 1987
0.-
Dean, College of Engineering
Dean, Graduate School

UNIVERSITY OF FLORIDA
3 1262 08554 1604



UNIVERSITY OF FLORIDA
3 1262 08554 1604