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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ix, 262 leaves : ill. ; 28 cm.
Leung, Roger Yi-Ming, 1955-
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Micelles   ( lcsh )
Emulsions   ( lcsh )
Liquid-liquis equilibrium   ( lcsh )
Chemical Engineering thesis Ph. D
Dissertations, Academic -- Chemical Engineering -- UF
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non-fiction   ( marcgt )


Thesis (Ph. D.)--University of Florida, 1987.
Bibliography: leaves 235-261.
Statement of Responsibility:
by Roger Yi-Ming Leung.
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My parents and my wife


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 coopcra-

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.


ACKNOWLEDGEMENTS ................................................ iii

ABSTRACT ........................................................ viii


1 INTRODUCTION ........................................... 1
1.1 Surfactant Molecules .............................. 1
1.2 Surfactant-Containing Aggregates .................. 1
1.3 Equilibrium Aspects of Surfactant-Containing ...... 3
1.4 Dynamic Aspects of Surfactant-Containing .......... 6
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

2.1 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
2.3.1 Geometric Packing Considerations in ........ 36
Amphiphile Aggregation
2.3.2 Non-Globular 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
2.4.1 Solubilization and Structure of ............ 48
2.4.2 Phase Equilibria of Microemulsions ......... 54

2.4.3 Phase Behavior of Winsor-Type .............. 56
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

3.1 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

4.1 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 ........... 111
4.3.4 Micelle Nucleus Formation as the ........... 113
Rate-Limiting Step: Evidence
from Polymer Additives
4.4 Conclusion ........................................ 121

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

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

6.1 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/O Microemulsions ..... 150
6.2.4 Equilibrium Droplet Size and Solubilization 151
of W/O Microemulsions
6.3 Discussions ....................................... 157

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
7.5.2 Effects of Alcohol and Oil ................. 188
on Optimal Salinity
7.6 Conclusions ....................................... 191

8.1 Introduction ..................................... 196
8.2 Experimental ...................................... 197
8.2.1 Materials and Methods ...................... 197
8.2.2 Preparation of AgC1 Sols ...... ............. 198
8.2.3 Coagulation Rate Measurement .............. 198

8.3 Results and Discussions .......................... 200
8.3.1 Coagulation of Hydrophobic AgC1 Sols ....... 200
8.3.2 Physico-Chemical Properties of the ......... 204
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/O Microemulsions
9.4 Effects of Oils, Alcohols and Salinity on the ..... 231
Solubilization and Phase Equilibria of W/O
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

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



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


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-


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



1.1 Surfactant Molecules

Surfactant molecules have a unique feature of possessing both a

polar hydrophilicc) 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 micelless" 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

___ -7 --- -- I -


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.

or detergents, are used as the dispersing agent, the single phase

region may consist of microdomains of the dispersed phase and complex

association structures of surfactant molecules. Such a single phase

region is often referred as a "microemulsion" (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).


/ Iliqld crystalline

'1* -


nilcroenimils on


normal nmcelle

---- ---


erted ilcelle


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.

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


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.

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, micelless 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.

+ +
+ +

+ + +

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.

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

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 T 1 in the order of

microseconds. The slow process is related to the micelle formation

and dissolution with a slow relaxation time T 2 in the order of mil-

liseconds. Figure 1-4 schematically illustrates these two relaxation

processes. At low surfactant concentrations, micelles are formed

through a step-wise association process as described by equation


Al + Al k A2

A, + A2 A3


Al + A3 k A [1.1]


Al + A-1 An


S- 7


Fast n-1
1 microsecondss)

K- / K" K"


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.

while the overall reaction is


n A A [1.2]
1 n

where Al denotes a surfactant monomer and An 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 1 and T 2 were formulated by Aniansson et al. (65) through an anal-

ogy to a heat conduction process as follows:

I/ T = k-/ a2 + (k /n)a [1.3]

1/ = n2A' R (1 + 2a/n)'1 [1.4]
2 1

a = (C A )/A1 [1.5]

where k is the dissociation rate constant of one surfactant monomer

from a micelle; a 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; A1

is the free monomer concentration in the bulk piase; C is the total

surfactant concentration; R is the rate-determining resistance of

micelle nucleus formation.




, 2o"

1 r


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

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/ T1 vs. a in equation [1.3], one can deter-
mine k /n from the slope and k / 2 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 TM is obtained from the fol-

lowing equation (113):

TM T 2na (1 + 2a/n)'1 [1.6]

When surfactant concentration is much greater than the CMC, TM is

approximately equal to n T2. 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


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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 (10-13 to 10-14 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. A.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

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):

I/ T22 8 ona(l + 02a/n)- (Ag/Ago)q [1.8]

where o 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
coagulation of submicellar aggregates; and q is a complex function of

the charge of counterions and of the surface potential of submicellar


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.

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.

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

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

added to the solutions, the study extends to mixed micelles and


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


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 T2, 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


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-



2.1 Introduction

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

(C4 to C7). 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

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

(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

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

(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):

dY = -E F. d r iRT d(In ai) [2.1]

where Y is the interfacial tension, ri is the surface excess of com-

ponent i (amount of component i adsorbed per unit area), Ii is the

chemical potential of component i, and ai is the activity of the

solute i. Equation [2.1] basically dictates that the increase of

surfactant activity ai 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

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
102 dyne/cm, stable microemulsions can be obtained. In other ther-

modynamic models (152-159), lower interfacial tensions in the order

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

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/RO, 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 kbT, where

kb is the Boltzmann constant, the interface is subject to a bending

instability resulting from thermal fluctuations.

Curvature Elasticity

(Rigidity Constant)

Radius of Spontaneous Curvature

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.

Safran and Turkevich (163) have expressed the interfacial free

energy FI of microemulsion droplets in terms of both interfacial ten-

sion and bending energy for an uncharged interface:

F = n [ 4 rYR2 + 16 1K(1 R/Ro)2] [2.2]

where n is the number density of droplets, Y is the interfacial ten-

sion, R is the droplet radius and Ro 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

value of K has been found to be in the order of 101 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 kbT, oil, water and surfactant

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). Because 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) = AG1 +AG2 +AG3


where A G1 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 G2 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 G3 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:

( aAGM/ R)R=R* = 0 [2.4]

(2 AG /R2)R=R > 0 [2.5]

Equations [2.4] and [2.5] indicate that a negative, minimum

A GM(R ) 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
x 102 dyne/cm, a stable microemulsion can be formed. Figure 2-4

shows the individual contribution of the three terms 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



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 AGM.
Curve B shows a kinetically stable emulsion and
curve C an unstable emulsion.

A GM (cal-cm-3)



R (A)


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.

10-1 (dyne-cm-1)

A GM ( cal cm-3)

A G2
0,001 -

0 100
r PR (A)

-0,001 AGi A .-

Figure 2-4. The contribution of AG AG and AG3 to the free
energy of microemulsion formation

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 [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-lipophilic balance (HLB) for the

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-


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.

2.3.1 Geometric Packing Considerations in Amphiphile 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/aolc, where v is the volume

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) ,ill favor the
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

interface requires v/a 1 = 1 which leads to the formation of

lamellar structure. Figure 2-5 schematically depicts the above


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/a 1 < 1 [2.7]

Equation [2.7] predicts the formation of (1) normal micelles for

v/aoc < 1/3; (2) o/w droplets for 1/3 < v/a 1 < 1; and (3) w/o

(inverted) droplets for v/aolc > 1. It should be pointed out that

the increase in packing ratio v/aol corresponds to an increase in

o/w droplet size, but to a decrease in w/o droplet size. The boun-

dary at v/a 1 = 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/aolc > 1, while a high HLB value in the range of 9-20 favoring

o/w emulsions corresponds to v/aoc < 1. Further, taking the

geometric packing term into account in a thermodynamic model can

serve as a simple approach to establish a unified thermodynamic

to c

0 4--
C o


S=o o
C 0

4~ 4-

0) 0

C 4

-.. -4.

-- I-- .0
o o



0 n4

^-H o

(Dl co
0 .4

(U 4J


L 0

r r

p rl





2; 0
U 0

H 2U
co H

2 H


.d4 i

M 0

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 ao or 1c (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/aol with increasing

temperature (176). This can be explained partly by the decrease in

water hydration of the head group (decreasing a ) at elevated tem-

perature (184). One thus expects a growth of normal micelles formed

by nonionic surfactants with increasing temperature due to increasing

v/a 1 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 o = 1 and thus a zero curvature.

2.3.2 Non-Globular 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-

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 ao and/or an increasing

hydrocarbon volume v according to the geometric model. But this con-

straint does not apply to o/w droplets because ao increases as the

droplet size diminishes.

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 prior 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


/W W/o


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.

Butonol/SDS = 2



Figure 2-6. Continued. (b) An experimentally determined nonglobular
domain (dotted area) in water/toluene/butanol/SDS
microemulsion system.

taking a cubic lattice model instead of the Voronoi tesselation. It

was shown (165) that a persistence length k can be defined as the

characteristic length of the water-oil interface. The value of C k

increases exponentially with increasing curvature elasticity K of the

interface. An isotropic microemulsion phase can exist when K and

consequently E k 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

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 1c and head group ao should satisfy the relation,

v/aol = 1, as the elementary design characteristic for a simple

three-component, namely oil, water and surfactant, microemulsion

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-

stant and fast exchange (characteristic time on the order of 10
109 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

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.

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 (L2)

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 L1 and L2

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)). folar 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


b Pentanol/SDS = 2


d Sodiun Coorylate

ler Denol

Water Deconal

Figure 2-7. Schematic ternary (or pseudoternary) phase diagrams of
various microemulsion systems. L represents a normal
micelle or o/w microemulsion region. L 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


Sodlum Caprylote


Butonol/SDS = 2


f Nonlonic Surfactont

31il Water

Figure 2-7. Continued.

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.

From a simple geometric calculation, it can be shown that the

total solubilization volume V in a microemulsion is equal to:

V = AtR/3 [2.8]

where A is the total interfacial area and R is the radius of dro-

plets. At 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 ao) 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

ao 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

(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:

F T =C-P

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

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 (L2) and o/w (L ) 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-





Increasing Salinity

Winsor TyDe I III II

b E



d S


S' 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.
YnM represents the interfacial tension between the
microemulsion and excess oil phases, and y is
the interfacial tension between the microemulsion
and excess water phases.


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


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

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/a 1 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 macroscopicc" 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

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 microemulsion

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.

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]

where V, Va and Vo are the volume of surfactant, added alcohol, and

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

Butanal + wter (cm3)
SOcm3 water 100 cm3 butanol c
3 ., 97 /
5 95 ..

10 I

,50,- s- 1B5
Toluene (cm3)

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.

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

(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.

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 -103 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


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, Vo/V and V /Vs) increases in both lower- and upper-phase

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

high,: 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

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 YWM below S* and YOM above S* (see Figure 2-

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).

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,

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).


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.


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 cetyltrimethylammonium

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 (Xo), where

X 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

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 200C 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 200C 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.

20 C
0 mM PENTANOL o 0o
20 mM PENTANOL / ,7"
100 nM PENTANOL o / ,

I\ '


I R~'

0 1 2 3 5 6 7 8 9 10 11 12

Figure 3-1. Specific conductivity of SDS solutions as a
function of SDS concentration with addition
of pentanol





0 0.1 0,2 0.3


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

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, Co is the monomer concen-

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]
g o 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 ) [3.3]
go o oo

The specific conductivity of a micellar solution is equal to:

K = X+C + X-C + X C [3.4]
sp og oo mm

where Ksp is the specific conductivity (S/cm) of the solution, X0,

A and X are the equivalent conductivity of the counterion, sur-
o m
factant monomer anion and micelle respectively. Substituting equa-

tions [3.1] and [3.3] into [3.4], one obtains:

K ( + X )C + ( c A + X /n)(C C ) [3.5]
sp o oo 0 m o

Equation [3.5] can be simply written as:

K = X C + X'(C C ) [3.6]
sp oo m o

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 A is equal to the slope of the conduc-

tance curve below the CMC and X' corresponds to the slope above the

CMC shown in Figure 3-1. The ratio of these two slopes gives:

A / X =( aX + A /n)/( X + X ) [3.7]
m o 0 m o 0

Since the mobility of a counterion is expected to be greater than a

surfactant monomer anion or a micelle, by assuming a X > A I/n and
X0 > Xo, equation [3.7] reduces to:

A' / A = a [3.8]
m o

Figure 3-3 reports the counterion dissociation of SDS micelles as a

function of pentanol concentration. The addition of pentanol to the

20 40 60 80 100

Figure 3-3. The change of degree of counterion disso-
ciation of SDS micelles with pentanol

micellar solution increases the counterion dissociation of micelles.

The C 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


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

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.

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





0 100 200 300
Alkonol Conc. ntroton (mM)

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


10 mM SDS 20C

0.50 0

o o. \
U 0.4

0 0

0 0.45

U 0 AD a

S0.4 0




S 5 10 15 20 25 30 35
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.

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

interfacial 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


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.

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' [3.9]
sp o m o o o m m

To observe no change in the conductance, dKsp = 0 and it is necessary
to have:

( X' )dC + C d( A I') + Cd a' = 0 [3.10]
S m o o o m 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, Co = 8.5 mM, o = 53.8 and X'm

22.3. Upon addition of 10 mM pentanol, Co = 7.7 mM, Xo = 54.8 and

X = 23.5. Then C = 22.4 mM is obtained from equation [3.10] at
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.


30 mM SDS 25 nM SDS
0.90 2


0,85. 0
22 MS 0 20 10 60 80

S20 mM SDS


r 3- Te 14 fM SDS



10 mM SDS

0.50 -


0 20 40 60 80 100 120 140 160 1

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.

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

a b

c d



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

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 the 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

0.40 -N

0,35 Cs03.H .90H
| 0,35

0.30- C H,OH

0,5'4 ,l CHOII

Clo H21 OH CII 130H

0 0.1 0.2 0.3 O.l" 0.5 0.6 0.7 0.8 0.9

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.



u <
40 0



o -'-4

.0 4)

.0 -1 40

00- (


) 4 "-
40 40

u uu
4) 0 Z

C3 s-

O 0
c X
4-1 0
C u

0 44

0 10 0 "

0 N a,- 0
o 0 0 0D

40 F-
'0 4N

00 0

Table 3-2 Initial Slopes of Conductance Increase in Figure 3-9

Fraction of Alcohol
Slopea Partitioning in the Sl9pee
(10 S/cm M) Micellar Phase (10" S/cm M)

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.

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