Surfactant aggregates and their stability in relation to foaming and emulsification

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Surfactant aggregates and their stability in relation to foaming and emulsification
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SURFACTANT AGGREGATES AND THEIR STABILITY IN RELATION TO
FOAMING AND EMULSIFICATION













By

SAMIR PANDEY


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

UNIVERSITY OF FLORIDA


2004














ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to Professor Dinesh 0. Shah,

chairman of my supervisory committee, for allowing me to be a part of the Center for

Surface Science and Engineering (CSSE), and for his kind guidance, motivation and

encouragement during my Ph.D. program. I would also like to thank other supervisory

committee members, Professors Richard B. Dickinson, Chang-Won Park and Hassan El-

Shall, for their valuable time and suggestions. Thanks go also to Professor Brij M.

Moudgil for his numerous inputs in my research and Dr. Yakov Rabinovich at the

Particle Engineering Research Center (PERC) for Atomic Force Microscopy

Experiments.

CSSE, a wonderful place with a number of fulltime and visiting scholars in

different areas of expertise, is the ideal place for a perfect brain massage. I thoroughly

enjoyed my learning experience here. I wish to thank all my colleagues for their help and

mentorship: Dr. Byron Palla, Dr. Dibakar Dhara, Dr. Rahul Bagwe, Dr. Juan-Carlos

Lopez-Montilla, Dr. James R. Kanicky, Dr. Tapan Jain, Dr. Manoj Varshney, Daniel

Carter, Dushyant Shekhawat and Monica James. Dr. Amar Shah is gratefully

acknowledged for several informal brainstorming sessions, one of which has finally

developed into the antifoaming chapter in this thesis and for proofreading of the

dissertation. Special mention goes to Dr. Tapan Jain and Dr. Rahul Bagwe for their

critique of my chapters and to Dushyant for help with referencing.









PERC is acknowledged for allowing me to use its research facilities and

acknowledgements are due to Gary Scheiffele and Gill Brubaker for teaching me how to

use them.

My heartfelt appreciation goes to my parents. Without their consistent emotional

support, I simply could not have come this far. Finally, I want to thank my wife, Aditi,

for her love and encouragement, not to mention the proofreading and editing of this

dissertation.














TABLE OF CONTENTS

page

ACKNOWLEDGMENTS................................................................................................... ii

L IST O F FIG U R E S ........................................................................................................... vii

A B ST R A C T ...................................................................................................................... xii

CHAPTER

1 INTRODUCTION................................................................................................... I

1.1 Role of Surfactant in Reducing Surface Tension..................................... 3
1.2 M icellization ............................................................................................ 4
1.3 Dynamic Properties of Surfactant Solution.............................................. 5
1.4 Meausurement of Micellar Relaxation Time ........................................... 8
1.4.1 Pressure-Jump with Conductivity Detection................................ 8
1.4.2 Stopped-Flow Method.................................................................. 9
1.5 Factors Influencing Micellar Relaxation Time...................................... 10
1.5.1 Chain Length of Surfactant ........................................................ 10
1.5.2 Electrolyte Content................................................................... 11
1.5.3 Chain Length of the Cosurfactant .............................................. 13
1.5.4 Chain Length of the Solubilized Oil .......................................... 15
1.5.5 Effect of Polymers...................................................................... 16
1.6 Importance of Micellar Relaxation Time on Technological Processes..... 17
1.7 Role of Surfactant in Stabilizing Interfaces........................................... 20

2 EFFECT OF COUNTERIONS ON SURFACE AND FOAMING
PROPERTIES OF DODECYL SULFATE ....................................................... 22

2.1 Introduction ............................................................................................ 22
2.2 Experimental Procedure ......................................................................... 24
2.3 Results and Discussions ......................................................................... 26
2.3.1 Foamability Measurements ........................................................ 28
2.3.2 Foam Stability Measurements.................................................... 32
2.4 C onclusion............................................................... ............................ 38









3 EFFECT OF IONIC/NON-IONIC MIXED SURFACTANTS ON
PROPERTIES AT GAS/LIQUID, LIQUID/LIQUID AND
SOLID/LIQUID INTERFACE .......................................................................... 40

3.1 Introduction ............................................................................................ 40
3.2 Experimental Method............................................................................. 41
3.2.1 M aterials..................................................................................... 4 1
3.2.2 Surface Tension Determination.................................................. 41
3.2.3 Thin Film Stability ..................................................................... 41
3.2.4 Spectrophotometry ..................................................................... 42
3.2.5 Detergency of Orange OT Dye Adsorbed on Cotton Fabric......... 43
3.2.6 Contact Angle Measurements .................................................... 43
3.3 Results and Discussion........................................................................... 43
3.4 C onclusions............................................................................................ 59

4 ANTIFOAMING ACTION OF ESSENTIAL OILS ......................................... 61

4.1 Introduction ............................................................................................ 6 1
4.2 Experimental Section ............................................................................. 62
4.2.1 M aterials....................................... ........................................... 62
4.2.2 Antifoam Efficiency................................................................... 63
4.2.3 Surface Tension Measurements for Calculation of Spreading
Pressure ............................................................................... ...... 63
4.2.4 Qualitative Spreading Pressure Measurement ........................... 64
4.2.5 Droplet Entry Barrier Measurement........................................... 64
4.3 Results and Discussion........................................................................... 64
4.4 C onclusions............................................................................................ 78

5 EFFECT OF COSURFACTANT CHAIN LENGTH ON
PROPERTIES AT THE LIQUID-LIQUID INTERFACE ................................ 80

5.1 Introduction ............................................................................................ 80
5.2 Experimental Method............................................................................. 81
5.2.1 M aterials..................................................................................... 81
5.2.2 Preparation of Emulsion............................................................. 82
5.2.3 Interfacial Viscosity Measurements........................................... 82
5.2.4 Atomic Force Microscope.......................................................... 82
5.2.5 Preparation of Silver Nanoparticles ........................................... 83
5.3 Results and Discussion........................................................................... 84
5.3.1 Emulsions................................................................................... 84
5.3.2 Microemulsion ........................................................................... 91
5.4 C onclusions............................................................................................ 97

6 SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK............. 99









APPENDIX

A GIBBS ADSORPTION EQUATION AND AREA PER
SURFACTANT MOLECULE DETERMINATION....................................... 104

B CALCULATION OF THE CHARACTERISTIC DIFFUSION TIME........... 107

C RUBINGH MODEL FOR THEORETICAL CMC ESTIMATION
FOR MIXED SURFACTANT SYSTEMS...................................................... 109

R E FE R EN C E S ............................................................................................................. ... 111

BIOG RA PH ICA L SKETCH ....................................................................................... 117














LIST OF FIGURES


Figure page

1-1 Mechanism for the two relaxation times, Tl and T2, for a surfactant
solution above cm c............................................................................................... 5

1-2 Distribution curve for the concentration of surfactant aggregate
against their aggregation num ber.........................................................................7...

1-3 Absorbance spectra of Eosin Y in water and 2 mM Triton X-100
solution (Eosin Y concentration: 0.019 mM )..................................................... 10

1-4 Plot of log (1/12) vs. the number m of carbon atoms of the
hydrophobic tail for several sodium alkyl sulfates at 25 oC at cmc ................... 11

1-5 Plot of log (1/12) vs. the concentration of added NaCl for STS with
Atot = 2.3 x 10-3 M at various temperatures........................................................ 12

1-6 Effect of tetraalkylammonium chloride (TC,AC: n = 1, 2, 3 and 4)
on the m icellar relaxation tim e........................................................................... 12

1-7 Variation of micellar relaxation time of 10 mM SDS as a function
of alkanol concentration ..................................................................................... 13

1-8 Effect of the chain length of cosurfactant on micellar relaxation
time: (a) of 25 mM SDS in presence of 5 mol% linear alcohols and
(b) of 100 mM SDS in presence of 5 mM cationic surfactant ........................... 14

1-9 Effect of solution pH on the micellar relaxation time of 100 mM
SDS as a function of added sodium laurate using stopped-flow
m ethod ................................................................................................................... 15

1-10 Plots of log (1/T2) vs. the concentration of various oils added to 0.3
M Hexadecyl pyridinium chloride + 0.2 M 1-pentanol mixed
micellar solutions at 25 "C. The curves for toluene, cyclohexane,
butylbenzene, hexane, heptane, nonane, dodecane and tetradecane
are shown from top to bottom ............................................................................ 16

1-11 Effect of polymers on relaxation time of 200 mM SDS: (a) anionic
polymer (b) cationic polymer (c) non-ionic polymer and (d) pure
SDS topmost (maximum at 200 mM); addition of polyethylene








glycol, poly N-isopropylacrylamide and methyl cellulose (0.01 %
w/v) shift the maximum to 150 mM .................................................................. 17

1-12 Schematic representation of adsorption of surfactant onto the
newly created air/water interface due to disintegration of micelles
during foam generation ...................................................................................... 18

1-13 Schematic diagram for the adsorption of surfactant monomers from
the bulk to the oil/water interface during emulsification ................................... 19

1-14 Liquid/liquid and solid/liquid interfacial phenomena exhibiting
minima and maxima at 200 mM SDS concentration .........................................20

2-1 Surface tension as a function of counter-ion of surfactant, dodecyl
sulfate, at 50 m M concentration......................................................................... 27

2-2 Effect of counter-ions on molecular packing of dodecyl sulfate at
the air/water interface. Area per molecule (Am): Am L+ > Am a+ >
A im s+ > A in .......................... ......... ............................ ................................. 28

2-3 Foamability (by shaking method) as a function of counter-ion of
surfactant, dodecyl sulfate, at 50 mM concentration .........................................29

2-4 Dynamic surface tension as a function of counter-ion of surfactant,
dodecyl sulfate, at 50 mM concentration........................................................... 31

2-5 Foam stability of surfactant, dodecyl sulfate, with different
counter-ions below critical micellar concentration (1 mM)............................... 33

2-6 Foam stability of surfactant, dodecyl sulfate, with different
counter-ions above critical micelle concentration (25 mM) .............................. 35

2-7 Foam stability of surfactant, dodecyl sulfate with different counter-
ions above critical micelle concentration (50 mM)............................................ 36

2-8 Dimensionless surface viscosity measurements as a function of
counter-ion of surfactant, dodecyl sulfate, at 50 mM concentration.................. 37

2-9 Schematic diagram showing effect of counter-ions on packing of
surfactant molecule at air/water interface .......................................................... 38

3-1 Variation in surface tension of SDS/Tween 80 mixed surfactant
system as a function of SDS/Tween 80 molar ratio at a total
surfactant concentration of 50 m M ................................................................... 44

3-2 Effect of SDS/Tween 80 molar ratio on the volume of the foam
generated by shaking method at a total surfactant concentration of
50 m M .......................................................................................................... 45








3-3 Effect of SDS/Tween 80 molar ratio on the stability of the thin film
at a total surfactant concentration of 50 mM ..................................................... 46

3-4 Variation of foam volume with SDS surfactant concentration ..........................49

3-5 Comparison between conductivity behavior of a pure SDS system
with that of SDS/Tween 80 mixture at various molar ratios.............................. 50

3-6 Variation of cloud point temperature as a function of SDS/Tween
80 m olar ratio ..................................................................................................... 5 1

3-7 Variation of surface tension against logarithm of surfactant
concentration for: (a) SDS/Tween 80 molar ratio = 3, (b)
SDS/Tween 80 molar ratio = 1, (c) SDS/Tween 80 molar ratio =
0.33 and (d) pure Tw een 80. ............................................................................... 53

3-8 Determination of cmc for mixed surfactant system using the dye
micellization method (Merrocyanine 540 conc. = 0.019 mM).
Absorbance ratios for peaks corresponding to dye in micelle and
dye in water are plotted above for: (a) pure SDS, (b) SDS/Tween
80 mole ratio = 4, (c) SDS/Tween 80 mole ratio = 1.5, (d)
SDS/Tween 80 mole ratio = 0.67, (e) SDS/Tween 80 mole ratio =
0.25 and (f) pure Tw een 80................................................................................ 54

3-9 Effect of SDS/Tween 80 molar ratio on the removal of dye orange
OT from cotton fabric at a total surfactant concentration of 50 mM................. 55

3-10 Emulsion droplet size distribution in: (a) SDS/hexadecane system
and (b) Tween 80/hexadecane system at a surfactant concentration
of 50 mM (hexadecane vol % = 15). Emulsions are produced by
vigorous stirring for 5 m in. ................................................................................. 56

3-11 Effect of SDS/Tween 80 molar ratio on the contact angle at a
PMMA surface at a total surfactant concentration of 50 mM (a)
and (b) schematic and definition of contact angle ............................................. 57

3-12 Effect of SDS/Tween 80 molar ratio on the solid liquid interfacial
energy at a total surfactant concentration of 50 mM.......................................... 58

4-1 Qualitative antifoaming behavior of various essential oils and
dodecane in a 5 mM 15 mL SDS surfactant solution (volume ratio
solution/antifoam = 30)...................................................................................... 65

4-2 Effect of volume fraction of essential oil in mixed antifoam with
dodecane on the foam volumes of a 5 mM 15 mL SDS surfactant
solution (volume ratio solution/antifoam = 30) ................................................. 66








4-3 Effect of 0.2 volume fraction of essential oil in dodecane on the
volume weighted emulsion droplet size in a 5 mM SDS solution..................... 68

4-4 Effect of essential oil volume fraction in dodecane on the diameter of
the spread oil. Five microliter of antifoam is deposited on a 5 mM
SD S m onolayer. .................................................................................................. 69

4-5 Spreading pressures of 0.2 volume fraction of various essential oils
in dodecane on a SDS monolayer (conc. 5 mM) ............................................... 70

4-6 Time required for merging of a 0.2 volume fraction essential oil in
dodecane droplet with the air/5mM SDS solution interface.............................. 72

4-7 Effect of surfactant concentration on antifoaming efficacy............................... 73

4-8 Spreading pressure of clove bud oil after being extracted with SDS
solutions of varying concentrations ................................................................... 75

4-9 Effect of essential oil volume fraction in dodecane on the foam
stability of a 200 mM sodium dodecyl sulfate solution (15 mL
surfactant + 0.5 mL antifoam): (a) Clove bud oil and (b) Cypress oil............... 77

5-1 Effect of co-surfactant chain length on emulsion droplet size at a
total surfactant concentration of 105 mM .......................................................... 84

5-2 Effect of co-surfactant chain length on bubble size at a total
surfactant concentration of 21 m M .................................................................... 85

5-3 Effect of co-surfactant chain length on creaming rate at a total
surfactant concentration of 105 mM .................................................................. 87

5-4 Effect of co-surfactant chain length on interfacial viscosity at a
hexadecane-water interface for a total surfactant concentration of
105 m M ........................................................................................................... 88

5-5 Schematic representation of a typical force-distance curve using
Atom ic Force M icroscope (AFM ) ..................................................................... 89

5-6 Effect of co-surfactant chain length on the maximum repulsive
force barrier on alum ina surface ........................................................................ 89

5-7 Schematic representation of a possible reason for interfacial
fluidity due to thermal motion in systems with mismatching chain
lengths .......................................................................................................... 90

5-8 Effect of co-surfactant chain length on interfacial tension at a
hexadecane-water interface for a total surfactant concentration of
105 m M ........................................................................................................... 9 1








5-9 Absorption spectra of silver nanoparticles synthesized in w/o
(SDS/Tween)/iso-pentanol/cyclohexane/water system...................................... 92

5-10 TEM pictures of silver nanoparticles formed in w/o
(SDS/Tween)/iso-pentanol/cyclohexane/water system and their
particle size distribution ..................................................................................... 94

5-11 Percolation behavior of various (SDS/Tween)/iso-pentanol /cyclo-
hexane/w ater system .......................................................................................... 96















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

SUFACTANT AGGREGATES AND THEIR STABILITY IN RELATION TO
FOAMING AND EMULSIFICATION

By

Samir Pandey

August 2004


Chair: Dinesh 0. Shah
Major Department: Chemical Engineering

Foams and emulsions are important industrial systems that require a varying degree

of control on the interfacial area (foamability, emulsion droplet size) and the stability

(foam stability, emulsion stability). Both foam and emulsion are not possible without

surfactant molecules, which significantly lower the energy requirements for creating new

interfacial area, simultaneously providing mechanisms to stabilize the interfaces. Above a

critical concentration of surfactant, aggregate structures known as micelles are present in

the surfactant solution. Micelles are dynamical in nature and keep breaking and

reforming. In this dissertation, it is shown that both the interfacial area (foam volume,

emulsion size) as well as the stability depends on the micellar stability, i.e., micellar

break up time. In particular, higher is the micellar stability, lower is the interfacial area

(smaller foam volume and bigger emulsion droplet size) and higher is the stability of a

foam or emulsion. An additional factor that contributes to the stability is the surface or








interfacial viscosity, which is a direct consequence of the cohesiveness of the adsorbed

film.

Counter-ions of the surfactant dodecyl sulfate, ionic/non-ionic surfactant mixtures

and the chain length of the cationic co-surfactant (CTAB) in a mixed system with

sodium dodecyl sulfate (SDS) are used as the variables to influence the micellar stability.

Specifically, it is shown that the foam stability can be increased to as much as 60 hrs

using Mg2+ as a counter-ion presumably because of a very high micellar stability as well

as due to a very cohesive adsorbed film. For the mixed ionic (SDS)/non-ionic (Tween 80)

system, it is shown that a voluminous foam-producing composition of pure SDS can

become entirely non-foaming because SDS molecules get trapped in mixed SDS/Tween

80 micelles of high stability. In the case of emulsions, the biggest droplet size and the

highest stability oil in water emulsions are obtained when the chain length of the

surfactant SDS and the co-surfactant, dodecyl trimethyl ammonium bromide, are

matching. This is explained through a chain length compatibility effect, which leads to

both a maximum in micellar stability as well as a tightly packed surfactant film of high

interfacial viscosity at the interface. In systems where the chain lengths are not matching,

the protruded part of chains above the neighboring molecules executes thermal motion,

which results in a poorly packed film. The stability of the oil in water emulsion is also

correlated with the strength of the surfactant film adsorbed at the solid liquid interface as

measured by Atomic Force Microscope. Finally, cosurfactant Tweens' chain length is

shown to affect the rate of species exchange between two water-in-oil droplets and the

higher the chain length of the cosurfactant, the lesser is the rate of exchange.














CHAPTER I
INTRODUCTION


Foams, dispersions involving a gas/liquid interface, are versatile systems that

have something to offer everybody. On one hand it fascinates the young minds, on the

other hand it inspires architects and challenges the very best of the intellectual minds. In

general, foam has applicability in areas that range from food products such as whipping

cream and ice cream (ice cream has as much as 200% by volume air dispersed in it), to

personal care products such as shampoos and shaving creams, to beverages such as beer

where foam is intentionally engineered into the solution only to add aesthetics, to

firefighting where it acts as a semiconductive blanket, to soil remediation to free the soil

from pollutants by trapping them in its high interfacial area, to enhanced oil recovery

where it is used to block the unwanted channels through which the compressed gas may

otherwise leak off, to mineral floatation onto laundry and dishwashing industries [Akers,

1975; Bikerman, 1970]. One of the very recent and a very important application of foam

emerged as a means of combating bioterror. Scientists at Sandia national laboratories

showed that only I out of 10 million of a virus as deadly as anthrax survives after they

have been trapped in the high interfacial area of suitably designed foam for a period of an

hour [Tadros and Thomas, 1999].

Emulsions, on the contrary, are dispersions involving a liquid/liquid interface.

They can be either water in oil type or oil in water type depending upon water to oil ratio,

emulsifier structure and concentration, temperature, addition of electrolyte and pressure









[Becher, 1985; Oh, 1993]. Like foams, emulsions too have applicability in a variety of

areas such as food products (mayonnaise, salad dressings), personal care (skin creams,

hand and body lotions), reactors for polymerization as well as reactors for reactions

occurring at interfaces, water purification, drug delivery systems, lubricant fluids for

metal cutting, media for dispensing pesticides and in asphaltic emulsions [Lissant, 1974].

These diverse applications of foams and emulsions impose stringent requirements

on the desirable properties of the system. While an ice cream manufacturer would prefer

a large interfacial area foam along with a high foam stability, a laundry detergent

manufacturer will detest the formation of any foam after the wash cycle as the presence

of the benign bubble indicates the "dirtiness" of the load to the customer. It is easy to

conceive that any randomly chosen detergent may not be able to provide an interfacial

area adequate enough for trapping the anthrax viruses from an infected area, to say the

least about providing the required "operating window" of an hour of foam stability. The

same is true about emulsions where most applications call for a maximum possible

interfacial area with the least energy input. And simultaneous is the issue to kinetically

stabilize or destabilize this high interfacial area so that for example a blue cheese salad

dressing has a different appeal because of its uniformly homogenous turbidity as against

zesty Italian salad dressing which is clearly a biphasic system with an altogether different

appeal even though most of the basic ingredients remain the same. Thus an understanding

of the fundamental molecular mechanisms controlling the behavior of the foam and

emulsion interfaciall area, stability) becomes imperative and to this end, I plan to devote

the subject matter of this dissertation.








Creating surfaces as in foaming or emulsification is always a costly enterprise as

the molecules on the surface are in an energetically unfavorable state as compared to

those in the bulk. This phenomenon, in which the surface molecules experience a net

inward force, is called surface tension and is partly responsible for absolutely no foaming

or emulsion formation with pure water, as the energy requirements are highest in this

case [or high surface tension]. Alongside, there are no mechanisms to stabilize the

air/water or the oil/water interface in pure water. The situation is different, however, in

the presence of surfactant molecules. Bubbles and emulsions are formed easily,

indicating (1) lower energy requirements for the process [or lower surface tension], and

(2) the adsorbed surfactant film providing mechanisms for stabilizing interfaces [Ball,

1999].


1.1 Role of Surfactant in Reducing Surface Tension

Surfactants or amphiphiles derive this particular uniqueness of lowering the

surface tension because of their structure, which has both a hydrophilic and a

hydrophobic part to it. The hydrophilic head group can be anionic, cationic, zwitterionic

(both positive and negative charge on the same molecule) or nonionic while the

hydrophobic part usually consists of linear, branched or unsaturated alkyl chains 8 to 20

carbons in length. The driving force for surfactants to adsorb at interfaces, i.e., their high

surface activity, is simply to minimize the free energy of the phase boundary and thus of

the whole system as well. When a surfactant molecule is placed in an aqueous solvent,

the polar head of such a molecule interacts favorably with the surrounding water

molecules. The hydrophobic tail portion, however, exists in a high-energy condition

caused by the tail's disruption of the surrounding hydrogen bonds connecting water









molecules. The free energy of the entire system can be minimized when the surfactant

molecule aligns itself at an interface with polar head facing towards the aqueous solution

and hydrophobic tail facing away from the water [Rosen, 1978].


1.2 Micellization

The surface tension of water (72 mN/m) can be lowered but only to a certain

extent (- 20 40 mN/m) by incremental additions of surfactant in solution. This critical

concentration of surfactant beyond which the surface tension of the solution does not

change much is known as the critical micelle concentration (cmc) and is an indicator of a

change over from a solution containing monomers to a state where surfactants are self-

assembled into clusters known as micelles. McBain [1913, 1920] was the first to observe

the unusual properties of the fatty acid solution but it was Hartley [1936] who proposed

the first concrete model of spherical micelles. Many physical properties of a surfactant

solution such as osmotic pressure, turbidity or light scattering, equivalent conductivity,

self-diffusion, magnetic resonance and solubilization of a water insoluble substance show

a dramatic change at cmc [Mukerjee, 1971; Shinoda, 1963]. Above cmc, the free

surfactant monomer concentration in the bulk solution remains constant because any

newly added surfactant immediately forms new micelles.

The cmc of surfactant depends moderately on the head group but strongly upon

the chemical structure of the hydrophobic region (chain length, branched chain, aromatic

groups and degree of unsaturation). In the case of ionic amphiphiles, it further depends on

the presence of counter-ions and their valencies as well as on the presence of any

cosolutes such as alcohols [Jonsson, 1998]. Thus, depending on the surfactant

concentration, a typical foam or emulsion forming surfactant solution can have both









micelles as well as monomers in the bulk solution, with the monomer concentration being

determined by the cmc value.


1.3 Dynamic Properties of Surfactant Solution

Even though a lower surface tension is a requirement for foaming/emulsification,

the rate of lowering of surface tension, i.e., the kinetics of adsorption of surfactant, is

even more important because sufficient amount of surfactant should reach the nascent

surface before destabilizing mechanisms take over. The kinetics of adsorption depends on

concentration of monomers (cmc), adsorption energy barrier, coefficient of diffusion and

the kinetics of micellization.






Fast relaxation time, microseconds


+ +



Slow relaxation time, milliseconds


Figure 1-1 Mechanism for the two relaxation times, Ti and T2, for a surfactant solution
above cmc.


Micelles are often drawn as static structures of spherical aggregates of oriented

surfactant molecules. However, micelles are in dynamic equilibrium with individual

surfactant molecules that are constantly being exchanged between the bulk and the

micelles. Additionally the micelles themselves are continuously disintegrating and

reassembling. There are two relaxation processes involved in micellar solutions. The first

one is the fast relaxation process referred to as T1 (on the order of microseconds), which









is associated with the fast exchange of monomers between micelles and surrounding bulk

phase. This process is considered as the collision between surfactant monomers and

micelles. The second relaxation time T2 (on the order milliseconds to minutes) is

attributed to micelle formation and dissolution process. Figure 1-1 shows the two

relaxation times T, and 12 associated with micellar solutions.

The two relaxation times can be used to calculate two important parameters of a

micellar solution (1) the residence time of the surfactant molecule in a micelle and (2) the

average lifetime or the stability of the micelles. The kinetics of these processes has been

evaluated by Aniansson and co-workers [1974, 1975, 1976]. The residence time of

surfactant monomer in micelles is related to Tcl and is equal to n/k -, where n is the mean

aggregation number and k is the rate constant of dissociation of a surfactant from a

micelle. 12 is related to the micellar lifetime and the average micellar lifetime is given by

the following expression:

-(1+ a)- (1.1)
T2 A, R n

na

l+--a
n

where TM is the average micellar lifetime, n is the aggregation number, a is the half-

width of the Gaussian distribution curve of micellar population and a = (Atot Ai)/Ai,

where Ato, is the total surfactant concentration and A, is the mean monomer

concentration, often approximated as the cmc and R is given by

R= (1.3)

s=s,+1 k;' A,








where s is the aggregation number of a particular aggregate, ks-' is the dissociation rate

constant of this aggregate and As is the equilibrium concentration of aggregates with an

aggregation number of s. Typical distribution curve of surfactant aggregates against

aggregation number is shown in Figure 1-2. For surfactant concentrations much greater

than cmc, c /n is of the order of one and a [= (A(to A1)/A,] is much larger than one.

Therefore, micellar lifetime is approximately equal to nT2 [Leung, 1986].




III


C(A.)



I



n
Aggregation number n


Figure 1-2 Distribution curve for the concentration of surfactant aggregate against
their aggregation number.

The theory described above was primarily developed for non-ionic surfactants and

it could successfully explain the relaxation times of ionic surfactant at small

concentrations too. However, the higher relaxation time with higher surfactant

concentration prediction of this theory could not explain the maximum observed for

certain ionic surfactants [Innoue, 1980; Lang, 1975; Lessner, 1981]. Kahlweit [1980,

1982] and Herrmann [1980] proposed that at low surfactant concentrations micelles are








stable with respect to coagulation due to the repulsive electrostatic forces but at high

surfactant concentrations, the counterion concentration increases which compresses the

electrical double layer and reduces charge repulsion, allowing the micelles to come closer

to each other so that the attractive dispersion forces (i.e. Van der Waals forces) lead to a

reversible fusion-fission coagulation according to

Ak +AA:>A, k+l=i, (1.4)

where k and I are classes of submicellar aggregates.


1.4 Measurement of Micellar Relaxation Time

The typical distribution curve of surfactant molecule in the solution as shown in

Figure 1-2 depends on thermodynamic variables such as temperature, concentration of

surfactant or pressure [Adair, 1974]. So, a change in any of the thermodynamic variables

will result in a change in the characteristics of the distribution curve such as monomer

concentration, number of micelles, micelle aggregation number and the half width of the

curve. Experimentally, the micellar kinetics can be probed by monitoring how fast the

micellar solution reaches a new equilibrium state after a sudden thermodynamic change is

exerted on the system [Muller, 1979] and is the basis of the experimental techniques of

pressure-jump with conductivity/pressure-jump with optical detection, temperature-jump

and stopped-flow methods.


1.4.1 Pressure-Jump with Conductivity Detection

As the name suggests, in this technique, the kinetics of a surfactant solution is

observed following a sudden change in the pressure. The technique relies on the fact that

the pressurization of the surfactant solution shifts the cmc to higher values, i.e., larger








number of monomers [Attwood, 1983; Kaneshina, 1983]. For ionic surfactants, that

translates into an increased conductivity. Then, as the pressure is suddenly released back

to atmospheric pressure, the surfactant solution is forced to go back to its original

condition of low cmc; i.e., few new micelles are formed and the electrical conductivity

decreases. The transition from high to low conductivity shows an exponential decay

profile indicative of a first order reaction [- dC/dt = k C; C is the conductivity] and the

micellar relaxation constant, 12, is calculated from the first order rate constant (12 = 1/k).

The disadvantage of this method is that only ionic surfactants at low surfactant

concentration can be studied by this method. At high concentration of ionic surfactants

the conductivity change with pressure is too small to be detected. Non-ionics, on the

other hand, cannot be studied because of their very small conductivities. For non-ionic

surfactants, temperature-jump or stopped flow method is used instead.


1.4.2 Stopped-Flow Method

In the stopped-flow method, the concentration of surfactant, i.e., the number of

micelles, is the thermodynamic variable of significance. This is a spectroscopic technique

and relies on suitably chosen dye molecules with an appreciable change of extinction

coefficient depending on the location (water phase vs micellar phase) of the dye molecule

[Hunter, 1987; Shinoda and Nakagawa, 1963]. Eosin Y, Merrocyanine 540, Rhodamine

and Sudan are a few such dyes that show a shift in the absorbance peak depending on

whether they are inside the oily micellar phase or the aqueous phase Figure 1-3 shows

typical spectra of Eosin Y in these two environments [Patist, 1999]. The perturbation is

brought about by rapidly mixing dye/surfactant solution with the dye/water solution

thereby suddenly decreasing the number of micelles forcing the dye contained within









those micelles to partition into the aqueous phase. The change in the absorbance is

followed at 542 nm where the two spectra are maximally apart.



Dye/water :\ Dye/surfactant solution
solution/ (2 mM TX-100)


./ \Due to micelle
S/ formation
S-a n \
-/'/ \ \




518 nm 542 nm
Wavelength (nm)

Figure 1-3 Absorbance spectra of Eosin Y in water and 2 mM Triton X-100 solution
(Eosin Y concentration: 0.019 mM).


1.5 Factors Influencing Micellar Relaxation Time


1.5.1 Chain Length of Surfactant

In the alkyl sulfate series, the measured relaxation time increases about four and a

half orders of magnitude as the chain length increases from decyl to hexadecyl [Jaycock,

1967]. The corresponding cmc decreases by two orders of magnitude and since A, is

equal to cmc, T2 should have decreased (equation 1.1). Also, as the chain length of the

surfactant increases, the aggregation number increases [Rosen, 1978] and that should

have decreased T2 too (equation 1.1). So increase in T2 must surely be because of an

overwhelming increase in R. Strong attractive forces between the hydrophobic tails,









which increase as a function of alkyl chain length have been suggested [Aniansson, 1976]

to be responsible for the increase in R and hence in T2 (Figure 1-4).


6











0
8 10 12 14 16 18

Number of carbon atoms (m)


Figure 1-4 Plot of log (1/T2) vs. the number m of carbon atoms of the hydrophobic tail
for several sodium alkyl sulfates at 25 "C at cmc.


1.5.2 Electrolyte Content

The addition of the electrolyte decreases the electrical repulsion between the

surfactant molecules in the micelles. Thus more surfactant molecules can be packed per

micelle; i.e., higher aggregation numbers and the resulting strong hydrophobic attractive

forces between the alkyl tails increases the T2. Figure 1-5 shows the change in 'U2 value of

sodium tetradecyl sulfate with salt concentration. In the case of surface-active electrolytes

such as tetraalkylammonium chloride salts alkyll tail 1 to 5 carbon chain in length), a

maximum in relaxation time is observed as the concentration of the electrolyte is

increased (Figure 1-6) and the higher the chain length, the smaller is the concentration

required to attain the maxima [Patist, 1998a]. The proposed mechanism is that at low

concentrations, electrolyte performs its usual role of decreasing the electrostatic repulsion









between surfactant molecules but at high enough concentration, the electrolyte with its

bulky head group starts to penetrate the interfacial region obstructing the close packing of

surfactant molecules, thereby decreasing the micellar relaxation time.

3 1 1


10 C .,C


Figure 1-5 Plot of log (1/1-2) vs. the concentration of added NaCI for STS with Atot =
2.3 x 10-3 M at various temperatures.


I

.2 _

5 I


10 20 30 40
TCrAC Concentration (mM)


Figure 1-6


Effect of tetraalkylammonium chloride (TCnAC: n = 1, 2, 3 and 4) on the
micellar relaxation time.


* 40 C


. 30 C



20 'C









100






C,





0.1
0 100 200 300 400 500 600

Concentration of 1-alkanol (mM)


Figure 1-7 Variation of micellar relaxation time of 10 mM SDS as a function of
alkanol concentration.


1.5.3 Chain Length of the Cosurfactant

Cosurfactants such as alcohol, both short chain and long chain as well as

oppositely charged surfactants of varying chain length, have been used to influence the

micellar relaxation time. It is reported (Figure 1-7) that alcohols shorter than pentanol

decrease the relaxation time while pentanol increases the relaxation time of SDS micelles

[Leung, 1986]. Oh [1991] found that increasing the carbon chain length further to

hexanol also increases the micellar relaxation time up to a critical concentration of

hexanol. The explanation is that short chain alcohols with three carbons or fewer mainly

partition in the palisade layer, where the microenvironment of alcohols is probably not

much different from the bulk water. Hence the exchange of alcohols between the bulk

phase and the micellar phase is extremely fast without any hindrance. Thus the measured

relaxation time is primarily from the micellization kinetics of the surfactant molecules









itself. However, as the chain length increases to four carbons or more, alcohols start

penetrating into the hydrocarbon core of micelles. The exchange of alcohol between the

micelles and the bulk solution will then be hindered and slowed down and the measured

relaxation times then partly reflect the rate of solubilization of alcohols in the micelles

[Hull et al., 1977; Leung, 1986].

While it may be expected that the addition of higher and higher chain length

alcohols (8-16 carbons) would increasingly stabilize SDS micelles, Patist [1998] made

the interesting observation that the alcohol chain length of 12, which is similar to the

alkyl chain length of the SDS molecule, stabilizes the micelle to the maximum extent

(Figure 1-8a) possibly because of a better packing of micelles in the chain length matched

case. The results appear to be of general importance and a similar maxima in micellar

relaxation time of SDS is observed for a cationic surfactant of 12 carbon chain length

[Patist, 1997]. Addition of a smaller chain (8 carbon) or longer chain length (12 carbon)

alcohol or cationic surfactant stabilizes the micelle to a smaller extent (Figure 1-8b).




25 mM SDS +
5maoI%COH 1.6

0.1 1.2 -
SN 0.8--
0o01 0.4 (b)
(a) o0
0001 ........................................ SDS SDS SDS SDS SDS
+ + + + +
CSOH CoOH C,2OH C.,OH CIsOH C+TAB C1oTAB C12TAB C+4TAB CeTAB
Long Chain Alcohol

Figure 1-8 Effect of the chain length of cosurfactant on micellar relaxation time: (a)
of 25 mM SDS in presence of 5 mol% linear alcohols and (b) of 100 mM
SDS in presence of 5 mM cationic surfactant










10
-pH60
--pH 7.0
"3" -0-PH 7 5
0 ---pH 8.0











0 1 2 3 4 5 6
Sodium laurate conc. (mM)


Figure 1-9 Effect of solution pH on the micellar relaxation time of 100 mM SDS as a
function of added sodium laurate using stopped-flow method.


Kanicky [2002] has shown that pH of the solution can change the relaxation time

of mixed micelles of SDS with fatty acids. The fatty acid molecules are undissociated at

pH values below pKa of the fatty acid and hence the partitioning of these somewhat

neutral molecules increases the micellar relaxation time but at pH values above pKa, fatty

acid molecules are also dissociated so there is no change in micellar relaxation time

(Figure 1-9).


1.5.4 Chain Length of the Solubilized Oil

In general, the solubilized oil tends to increase the micellar relaxation time with

the exception of toluene, which at low concentration destabilizes the micelles while at

high concentration stabilizes the micelles (Figure 1-10). It is suggested that toluene

behaves in a fashion similar to short chain alcohols at low concentrations and primarily

resides in the palisade layer of the micelles while at higher concentrations, the behavior is

like that of pentanol due to its partitioning in the core of the micelles [Lang, 1984].









3.00

2.50

2.00 -,




0.0--0-
1.5.5 1.50 Ef\ fect of Polers



0.50 v A-

0.00 -------
0.00 0.05 0.10 0.15 0.20 0.25 0.30

Oil (M)


Figure 1-10 Plots of log (1/-T2) vs. the concentration of various oils added to 0.3 M
Hexadecyl pyridinium chloride + 0.2 M 1-pentanol mixed micellar
solutions at 25 0C. The curves for toluene, cyclohexane, butylbenzene,
hexane, heptane, nonane, dodecane and tetradecane are shown from top to
bottom.


1.5.5 Effect of Polymers

The micellar relaxation time for SDS has been evaluated by Dhara [2001, 2001a]

in the presence of a number of anionic, cationic and non-ionic polymers (Figure 1-11).

The addition of polymers in general tends to destabilize the micelles and the extent of the

reduction depends primarily on the strength of surfactant polymer interaction The

polymers, which interact strongly with SDS such as polyethyleneglycol, polyacrylic acid

or polyethyleneimine lead to a greater reduction in the micellar relaxation time compared

to the non-interacting polymers such as dextran, polyacrylamide or carboxymethyl

cellulose sodium salt. Another interesting feature is the shift of SDS concentration at

which the maximum relaxation is observed. For pure SDS, the maximum is seen at 200

mM while in the presence of polymers, this maximum is shifted to 150 mM (Figure 1-







17



11 d) presumably because the large exclusion volume required for polymer chain reduces


the free volume of water available for micelles.


0.02 0.04 0.00 0.08
Concenlataon of polymew molarityy of
repeating unit) or (% w/v for CMCNa)


0.1


0 0.02 0.04 0.0 0.08 0.1
Concenration (% wfv) of polymer added


Figure 1-11


0 0.02 0.04 0.06 0.08 0.1 0o12
Concentration of polymers (% w/v) or
molarityy of repeating unit for PEI)
10






01



0 01



0,001
0 100 200 300 400 500 600 700
Concentration of SDS (mM)


Effect of polymers on relaxation time of 200 mm SDS: (a) anionic
polymer (b) cationic polymer (c) non-ionic polymer and (d) pure SDS
topmost (maximum at 200 mm); addition of polyethylene glycol, poly n-
isopropyl-acrylamide and methyl cellulose (0.01 % w/v) shift the
maximum to 150 mM


1.6 Importance of Micellar Relaxation Time on Technological Processes


The importance of micelle breakup in processes involving an increase in


interfacial area was first reported by Mijnlieff and Ditmarsch [1965]. A strong correlation


of T2 with various dynamic processes such as foamability, wetting time of textiles, bubble








volume, emulsion droplet size, and solubilization rate of benzene in micellar solutions

was found by Shah [1998] and Patist [2000] who reviewed the results recently. Figure 1-

12 shows schematically the importance of micelle breakup in foaming processes. When

air is blown through a surfactant solution, a substantial amount of new interfacial area is

created in the form of bubbles. The increased interfacial area has to be stabilized by an

adsorbed film of surfactant molecules. These molecules come from the bulk solution,

which contains monomers and (if above cmc) micelles. As monomers diffuse to the

newly created surface, the equilibrium condition between monomers and micelles is

disturbed, which forces existing micelles to break up to provide additional monomers to

the surface. Very stable micelles are not able to augment the flux necessary to stabilize

the newly created interface, and therefore foamability will be less.




Air



Surfactant 7 7777777777777777777777
solution Ani
jThin Liquid Fihn

t
Air
More stable micelles -- Less monomer flux -*
Lower foamability


Figure 1-12 Schematic representation of adsorption of surfactant onto the newly
created air/water interface due to disintegration of micelles during foam
generation.


In relation to foam stability, a maximum in single film stability was also found at

200 mM of sodium dodecyl sulfate, i.e., when the micelles were most stable [Patel,








1996]. An important factor influencing single film stability is the micellar structure inside

the thin liquid film, which has been investigated by Wasan and co-workers [Nikolov,

1989; Nikolov, 1989a]. The stratification of thin liquid films can be explained as a layer

by layer thinning of ordered structures of micelles inside the film. This structured

phenomenon is affected by micellar effective volume fraction, stability, interaction, and

polydispersity. Therefore, the results from this study indicate that very stable micelles

contribute to the stability of thin liquid films.














More stable micelles Less monomer flux -
Higher interfacial tension -- Larger droplet size


Figure 1-13 Schematic diagram for the adsorption of surfactant monomers from the
bulk to the oil/water interface during emulsification.

A picture similar to Figure 1-12 can be drawn for micelle breakup during

emulsification processes (Figure 1-13). When mechanical energy is applied to increase

the interfacial area between oil and water to produce oil droplets, the newly created

interface must be stabilized by the adsorption of monomers from the aqueous phase.

More stable micelles cause less monomer flux, which leads to a higher interfacial tension

at the oil/water interface resulting in large droplets [Oh, 1993]. Other phenomena at the










liquid/liquid and solid/liquid interface in SDS solutions are summarized in Figure 1-14

and details can be found in Oh's work [1992, 1993, 1993a].



Time to Reach Saturation
of SDS Solution by Benzene


rDetergency.
Removal of Orange OT dye


Solubilization Rate of
Benzene


Droplet Size in Emulsions

Wetting Time

200 mM

SDS CONCENTRATION


Figure 1-14 Liquid/liquid and solid/liquid interfacial phenomena exhibiting minima
and maxima at 200 mM SDS concentration.



1.7 Role of Surfactant in Stabilizing Interfaces

The role of surfactant in lamella stabilization is mainly either due to the electrical

double layer forces (in the case of ionic surfactants) or due to steric forces (in case of

polymers or water soluble proteins), both of which are repulsive in nature and act against

the destabilizing attractive Van der Walls interactions. Another way to increase the

lamella stability is by employing mechanisms, which oppose the drainage of water from

the thin film. The drainage of the water from the thin film is caused both by the gravity

and by a smaller pressure in the borders (Plateau border) as compared to the flatter parts

of lamella due to capillary effects. Forming a very cohesive molecular film at the

interface using surfactants, mixed surfactants, proteins, polymers, polysaccharides etc,









increases the surface viscosity leading to retardation in the drainage. In the case of mixed

surfactants, certain molecular ratios have been reported to be the most effective in

increasing the surface viscosity and hence the foam stability [Patist, 1999; Shah, 1978].

Similar retardation can also be affected by Marangoni effect. Here the draining

liquid drags surfactant with it, creating areas with high local surfactant concentration

further down the lamella. If the surfactant molecules from the bulk of the film are unable

to adsorb quickly onto the depleted areas, the surfactants from the high surfactant

concentration areas will move 'up' the lamella simultaneously dragging water along with

it, thus opposing the drainage. If the surfactant concentration is above cmc, additional

lamella stabilization mechanisms arise because of the ordering of the micelles in the foam

film, from which they flow out in a layer-by-layer manner further stabilizing the thin film

[Nikolov, 1989]. These authors further showed that instead of micelles, colloidal particles

could also stabilize the lamella because of the same mechanism.

Concentrated emulsions are in some sense analogous to foam films and similar

mechanisms are invoked to explain the stability of emulsions against coalescence. There

are two major differences, however: (i) the attractive Van der Walls forces are weaker

(because of a smaller Hamacker constant), and (ii) interfacial tension (or energy) is

smaller and thus the contributions from bending rigidity becomes significant near the

fusion boundary [Kabalnov, 1996]. The other major destabilization mechanisms in case

of dilute emulsions such as flocculation and coagulation are well captured by the DLVO

model of repulsive electrical double layer interactions and Van der Waals forces.














CHAPTER 2
EFFECT OF COUNTERIONS ON SURFACE AND FOAMING PROPERTIES OF
DODECYL SULFATE



2.1 Introduction

Foamability and foam stability are relevant properties for many industrial

applications such as mineral flotation, food processing, foam fractionation, processing of

textiles, personal care products, enhanced oil recovery and fire fighting. The surfactant

counter-ion strongly influences the critical micelle concentration (cmc), micellar

catalysis, micelle size and emulsion size as observed by various researchers [Kim et al.,

2001; Mukerjee, 1967; Oh et al., 1993]. The cmc in an aqueous solution is influenced by

the degree of binding of counter-ion to the micelle. For aqueous systems, the increased

binding of the counter-ions to surfactant causes a decrease in the cmc and an increase in

the aggregation number [Mukerjee, 1967]. The extent of binding of the counter-ion

increases with an increase in the polarizability and valence of counter-ions and decrease

with an increase in its hydrated radius [Rosen, 1978]. Thus, in aqueous solution, for the

anionic dodecyl sulfates, the cmc decreases [Mukerjee, 1967] in order Li+ > Na > Cs+.

The cmc of LiDS, NaDS, CsDS and Mg(DS)2 are reported by Mukerjee to be 8.92, 8.32,

6.09, 0.88 mM respectively at 25 C. At a given ionic strength, detergent concentration

and temperature, the micellar size is known to be in the following order Rh (CsDS) > Rh

(NaDS) > Rh (LiDS) [Missel et al., 1989]. Also, the ionized counter-ions perturb the local

ordering or structure of water molecules around the counter-ions [Israelachvilli, 1992].









Several water molecules are bound to counter-ions due to ion-dipole interaction between

counter-ions and water molecules. The counter-ion size follows the order of Li+ < Na <

Cs+ but the hydrated counterparts show reverse trend due to the greater hydration of a

lithium ion compared to sodium or cesium ion. The repulsive force between hydrated

ions appears to increase in the following order: Cs+-Cs+ < K+-K+ < Na+-Na+ < Li+-Li

(rather than the opposite as would be expected from the non-hydrated ion radii of the

ions) [Israelachvilli, 1992]. Therefore, the effectiveness of monovalent cations as

coagulants decreases according to the so-called lyotropic series [Hiemenz, 1977] viz. Cs+

> K+ > Na+ > Li+. The surface tension of surfactant solutions depends on the number of

surfactant molecules per unit area at the surface. For a given surfactant, the greater

concentration of surfactant molecules at the surface results in the lower surface tension.

Many physical factors are involved in control of foamablity and foam stability.

While foamability depends on surface tension (equilibrium or dynamic) and cmc, the

foam stability is determined by bulk and surface viscosities, the Marangoni effect,

disjoining pressure and hydrophobic interaction [Aveyard and Clint, 1996; Ivanov and

Kralchevsky, 1997; Malhotra and Wasan, 1988; Pugh, 1996].

Patist et al. [1998] have shown that by using long chain alcohols, the stability of

sodium dodecyl sulphate micelles can be tailored to control the dynamic surface tensions

achieved and hence the foamabilities. Chattopadhyay et al. [1992] reported that the

surface viscosity of a monolayer of surfactant, which is a good indicator of foam stability,

is sensitive to concentration and nature of salt. They observed increase in surface

viscosity with increase in NaCI concentration in contrast to a decrease in surface

viscosity with increase in NH4NO3 concentration. The present chapter investigates the









effect of counter-ions such as Li+, Na Cs+ and Mg+ of dodecyl sulfate on interfacial

properties in relation to foaming.


2.2 Experimental Procedure

Lithium Dodecyl Sulfate (99% purity) is purchased from Acros (Orlando, FL)

Sodium Dodecyl Sulfate (99% purity) from Sigma Chemical Company (St. Louis, MO)

and Magnesium Dodecyl Sulfate (98% purity) from Pfaltz and Bauer (Waterbury, CT).

Cesium Dodecyl Sulfate is prepared in our laboratory with the same procedure as shown

by Kim et al. [2001]. Chlorosulfonic acid (Aldrich, Milwaukee, WI, 553.5 mM) is added

to dodecanol drop by drop with vigorous mixing at 25 C under a nitrogen atmosphere.

The sulfation reaction is performed very slowly (40 minutes and cooled with ice) since

the sulfation process is highly exothermic. After, the sulfation process, nitrogen gas is

used to purge the reaction mixture to remove HCI produced during the reaction. Aqueous

CsOH solution (Aldrich, 50.0 wt.%) is added to the reaction mixture in a 1:1 molar ratio

to neutralize the acid. The CsDS is recrystallized three times with distilled water, keeping

the solution at 5 C.

Foamablity measurements are carried out by shaking method at 50 mM surfactant

concentration, where 10 mL of the surfactant solution is vigorously shaken 10 times by

hand in a 100 mL graduated cylinder and the volume of the foam is recorded immediately

after shaking. Each solution is tested at least five times and the reproducibility is about

5 mL.

For foam stability measurements, the solutions are foamed to the same height (13

cm) by slowly injecting air (100 cm3/min) through a fine capillary (diameter 1 mm) into

25 mL of surfactant solution contained in a quartz column (diameter 3.5 cm, height 80









cm). The time for collapse of the foam to half the initial height is then recorded.

Surfactant concentration of 1 mM is chosen for studies below cmc while concentrations

of 25 mM and 50 mM are chosen for studies above cmc (cmc z 1-8 mM depending on the

counter-ion). Appropriate caution (a long funnel that reached the bottom is used to pour

the solution) is taken in order to keep the walls dry and to avoid any initial foam

formation due to splashing.

The deep-channel surface viscometer [Mannheim and Schechte, 1970] is used to

measure the surface viscosity of each solution. Two concentric cylinders form the deep

channel of the viscometer. The walls of this channel are stationary, while the lower cup

moves at a constant angular velocity. To measure the centerline velocity of the air/water

(surfactant solution) interface, a small Teflon particle is placed on the surface and the

time for that particle to make one revolution is recorded as an average from five such

revolutions by visual observation. With this value, the surface viscosity can be

determined using the following equation [Chattopadhyay et al., 1992]:

[Lo D8Vb 1] (2.1)
tr xVe I

Where c is the surface viscosity, rI the bulk viscosity of the solution, yo the channel

width, Vb the plate rotational speed, V the centerline velocity of the air/water interface,

and D the ratio of depth to width of the liquid channel.

Equilibrium surface tensions are measured from freshly prepared solutions by the

Wilhelmy plate method [Osipow, 1977]. Before each measurement, the platinum plate is

cleaned by heating it to a red/orange color with a Bunsen burner.

Dynamic surface tension is measured by a similar method as reported in previous

studies [Patist et al., 1998]. Quantitative estimates of dynamic surface tension of









solutions are taken by forcing air into the solutions through a gauge 22, steel needle while

measuring the pressure changes as the bubbles are produced using a fast (1 ms)

transducer (Omega Instruments) in the maximum bubble pressure instrument [Bendure,

1971; Fainerman et al., 1994]. To show the importance of micellar break up in the

dynamic surface tension measurement a dimensionless parameter 0 is introduced [Patist

et al., 1998],

0 = (YD-Yeq)/(Yw-Yeq) (2.2)

where YD is the dynamic surface tension, Yeq is the equilibrium surface tension measured

by Wilhelmy plate method and yw is the surface tension of pure water at 25 C. The value

of 0 = 0 (or YD =eq) indicates that the surfactant adsorption under dynamic condition is

the same as that under equilibrium conditions and the micelles are labile as well as the

monomers are diffusing fast, whereas 0 = 1 (7YD Yw) indicates no surfactant is present at

the interface under the dynamic conditions existing during the bubbling process implying

either the presence of relatively stable micelles or monomers with high characteristic

diffusion time.


2.3 Results and Discussions

Figure 2-1 shows the equilibrium surface tension values for 50 mM surfactant

solutions having different counter-ions. Surface tension is related to the number of

surfactant molecules per unit area adsorbed at the air/water interface. The surface tension

decreases with increasing surface concentration [Rosen, 1978]. Figure 2-1 shows that the

surface tension decreases as follows: Lithium dodecyl sulfate (LiDS) > Sodium Dodecyl








Sulfate (NaDS) > Cesium dodecyl sulfate (CsDS) > Magnesium dodecyl sulfate

(Mg(DS)2).

45

40 -
z
35 -
0
30

o 25 -
I I1
20 2

15
LiDS NaDS CsDS Mg(DS)2

Figure 2-1 Surface tension as a function of counter-ion of surfactant, dodecyl sulfate,
at 50 mM concentration.

This can be explained by considering the molecular arrangement at the interface

shown in Figure 2-2. As one goes down the first group of the periodic table from Li+ to

Cs the ionic radius increases from Li (0.068 nm) to Na (0.095 nm) to Cs (0.169 nm)

while hydrated radius changes as follows: Li (0.38 nm), Na (0.36 nm), Cs (0.33 nm)

[Cotton and Wilkinson, 1983; Israelachvilli, 1992]. Thus the positive center (Li) in LiDS

is at a larger distance from the negative center (DS-) than in the case of CsDS, resulting in

higher coulombic repulsion between the adjacent sulfate ions in a LiDS monolayer than

in a CsDS monolayer. This leads to closer molecular packing and higher surface

concentration of surfactant molecules at the interface in the case of CsDS than for LiDS

adsorbed monolayer. In the case of Mg(DS)2, the larger separation of positive and

negative center [Mg +: ionic radii (0.066 nm), hydrated radii (0.428 nm)] is countered to










some extent because of the bivalency of the cation. This attractive force between a

divalent cation and two surfactant molecules probably partially dehydrates the Mg2+ ions

and hence causes the closest molecular packing at the interface. This is also evident from

area per molecule values obtained from Gibbs adsorption equation [Appendix A], in

Table I where DS-Mg++ occupies least area followed by DS-cs+, DS-Na and DSLi+.


Li


Figure 2-2


A,'"


(S


LiN1


CI


Cs


C


Effect of counter-ions on molecular packing of dodecyl sulfate at the
air/water interface. Area per molecule (Am): Am Li+ > ArnNa+ > AmCs+ >
mMg2+
Amn


2.3.1 Foamability Measurements

Foamablity has been studied using shaking method. In this method foam is

produced quickly by rapid shaking of cylinder causing a sudden expansion of interfacial

area. Assuming similar bubble size (or bubble size distribution), the foamability

experiments (Figure 2-3) yielded trends contrary to those as expected from equation (1)

[Adamson, 1990] relating work done in producing foam to interfacial area:


m


m









W= y. AA (2.3)

where W is the work done, y is the surface tension at the air/water interface and AA is the

change in interfacial area (i.e., the systems with smaller surface tensions instead of

producing larger interfacial areas, produce smaller interfacial area e.g. CsDS or

Mg(DS)2).

110

-o





0






LiDS NaDS CsDS Mg(DS)2


Figure 2-3 Foamability (by shaking method) as a function of counter-ion of
surfactant, dodecyl sulfate, at 50 mM concentration.

This can be explained on the basis of competitive time scales for interfacial area

expansion, the diffusional transport of surfactant monomers and ability of micelles to

break up in order to provide monomer flux necessary to stabilize the new air/water

interface as shown in Figure 1-12. Very stable micelles cannot break up fast enough to

augment the flux of monomers necessary to stabilize the new air/water interface resulting

in higher dynamic surface tension and hence low foamabilities. Simple calculations on

lines similar to Oh [Oh, 1992; Appendix B], show that for surfactant with first group

counter-ions (Li Na+ and Cs+), the characteristic diffusion time for surfactant monomers
counter-ions (Li+, Na+ and Cs+), the characteristic diffusion time for surfactant monomers









is of the order of 10-4 s, while for surfactant with Mg++ as counter-ion, it is of the order of

10-2 s because of its low cmc as well as least area per molecule at the air/water interface.

The characteristic diffusion time for micelles will approximately be an order of

magnitude larger because of the slower diffusion (coefficient of diffusion a Mw-"2) of the

aggregates. So, apparently the break up of micelles should be a rate-limiting step in the

supply of monomers to the new air/water interface for the case of LiDS, NaDS and

CsDS, while for Mg(DS)2, the resistance should largely come from the diffusion of

monomers. From the results in Figure 2-3, this further means that the low interfacial area

generated in case of CsDS is due to the resistance from micellar stability. For LiDS and

NaDS surfactant systems, the micelles appear to be relatively unstable, thus providing all

the monomer flux that is required to stabilize the interfacial area.


Table 1 Physical properties and dimensionless dynamic surface tension (0) of different
counter-ions of dodecyl sulfate

Ion Ionic Hydrated Area per 0 parameter
radius (A) radius (oA) molecule

Li+ 0.60 3.82 61 0.138

Na+ 0.95 3.58 51.5 0.131

Cs' 1.69 3.29 44.5 0.202

Mg+4 0.65 4.28 38.721 0.353


This is consistent with the dynamic surface tension measurements (Figure 2-4).

Here, at low flow rates and thus higher bubble life times (- 1 s) the order for the surface

tensions is similar to the order for equilibrium surface tensions with different counter-

ions. Apparently at these conditions, there is no resistance to monomer flux required to

stabilize the newly formed bubbles from either the micellar stability or from the diffusion









of the monomers. Thus, the bubbles are formed under equilibrium conditions. At higher

flow rates and thus smaller bubble life times (- 50 ms), while there is an increase in the

surface tension of each surfactant as compared to those at low flow rates, the change is

higher in magnitude for CsDS or Mg(DS)2 than for LiDS or NaDS suggesting a larger

resistance to monomer flux for CsDS and Mg(DS)2. As discussed before, this is probably

due to stable micelles in case of CsDS and larger characteristic diffusion time in case of

Mg(DS)2. Available literature data [Kahlweit, 1982] on relaxation times (t2) for NaDS

and CsDS systems at 50 mM surfactant concentration (NaDS: T12 = 1 ms, CsDS: 12 = 3000

ms) further confirms this interpretation.

50
5 2.5 cc/min
-45 E ~__ 12 cc/min

= 40 -
oI
S35 -

S30-

a 25

r20

15 -
LiDS NaDS CsDS Mg(DS)2

Figure 2-4 Dynamic surface tension as a function of counter-ion of surfactant,
dodecyl sulfate, at 50 mM concentration.

A more meaningful way to represent the dynamic surface tension might be using

the 0 parameter as it normalizes the dynamic surface activity (irrespective of the

mechanisms) with respect to the equilibrium surface activity (see equation 2.2). Table 1

shows the 0 parameter values at a bubble life time of 50 ms. The 0 parameter values are









lower and similar for LiDS and NaDS while they are higher for CsDS and Mg(DS)2

suggesting a higher dynamic surface activity for LiDS or NaDS than for CsDS or

Mg(DS)2 correlating well with the foamability behavior.


2.3.2 Foam Stability Measurements

Foam stability can be influenced by two independent factors: (1) molecular

packing in adsorbed surfactant film and hence, surface viscosity of the film at the

air/water interface [Jonsson et al., 1998] and (2) layering or structuring of micelles within

the bulk water in foam lamellae [Nikolov and Wasan, 1989; Nikolov and Wasan, 1992;

Wasan et al., 1992]. In order to delineate the effect of presence of micellar structuring,

the surfactant systems with different counter-ions are studied systematically below and

above the cmc concentration values.

Figure 2-5 shows the foam stability as a function of counter-ions at a surfactant

concentration of 1 mM. The concentration of 1 mM, which is either close to or below

cmc concentration, is chosen to ensure little or no presence of micelles in foam lamellae.

Here, one sees that the foam stabilities gradually increase from 10 minutes for LiDS to 30

minutes for Mg(DS)2. Foam stability differences are usually explained in terms of the

differences in the water drainage rate from the foam lamellae which in itself is influenced

by mechanisms like: (1) bulk viscosity, (2) fluidity of the film and (3) Marangoni

stabilization mechanism [Jonsson et al., 1998] because of the rapid adsorption of

surfactants. Mechanism (1) can be easily discounted because of the similar bulk

viscosities of all four solution containing different counter-ions. Mechanism (3) is also

not operative because then, a two orders of magnitude smaller adsorption for Mg(DS)2

would have resulted in a significantly more stable foam for Mg(DS)2 than for LiDS,









NaDS or CsDS, where a quicker monomer adsorption will oppose the Marangoni

stabilization mechanism, thus promoting the momentary stretch of foam lamella,

weakening it further. Thus one can conclude that it is the fluidity of the film that is

controlling the drainage rate at this concentration. Apparently, with an increase in

crowding of surfactant monomers at the interface, as one changes the counter-ion from

Li to Mg, the adsorbed film is becoming more rigid (discussed later in Figure 2-8),

thus retarding the flow of water.

35

30

25
E

20

15

o 10

5

0
LiDS NaDS CsDS Mg(DS)2

Figure 2-5 Foam stability of surfactant, dodecyl sulfate, with different counter-ions
below critical micellar concentration (1 mM).

Figure 2-6 shows the foam stability as a function of counter-ions at a surfactant

concentration of 25 mM. At this concentration, micelles are present in the foam film.

Here, it can be observed that foam stabilities are low for LiDS (20 min) and NaDS (60

min) surfactant system, while for CsDS and Mg(DS)2 surfactant system, the foam

stabilities are unusually high (900 min, 1200 min). This suggests an abrupt change in

micellar properties (either their number concentration or their stabilities) for CsDS and









Mg(DS)2 surfactant system in comparison to LiDS and NaDS surfactant system. Except

for widely studied NaDS system, the number concentrations of the micelles can't be

reliably determined because of the unavailability of the aggregation numbers for these

surfactant systems. Though one can calculate some bounds for micellar number

concentrations assuming the same aggregation number as that of NaDS. The aggregation

number for NaDS is reported to be 64 [Turro and Yekta, 1978]. For the surfactants at two

extremes (i.e., LiDS and Mg(DS)2), at 25 mM concentration, the number concentration of

micelles is 1.51 x 1020 and 2.26 x 1020 respectively. These number concentrations come

further closer, if one assumes that the packing in micelles will behave in a similar way to

as observed at the air/water interface (i.e., the aggregation number for LiDS should be

smaller than 64 and for Mg(DS)2, it should be greater than 64). This is not an

unreasonable assumption as such an analogy has been found to be true for mixed systems

of NaDS with cationic surfactants of varying chain lengths [Patist et al., 1997]. Thus the

number concentration of micelles is not very different for surfactant with different

counter-ions and the differences in the foam stabilities can be ascribed to the differences

in micellar stabilities. From the foamability results shown before (Figure 2-3), it is known

that the micelles of LiDS and NaDS are unstable in comparison to the micelles of CsDS.

Hence one can infer (1) the micellar stability of Mg(DS)2 is very high and (2) the micellar

stability predominantly determines the foam stability (i.e., higher the micellar stability,

higher is the foam stability).

These issues of structure imparting stability to colloids have been addressed by

Wasan and coworkers [Kralchevsky et al., 1990; Nikolov and Wasan, 1989; Nikolov et

al., 1990; Nikolov and Wasan, 1992; Wasan et al., 1992] where they have shown that









polystyrene latexes, suspensions of silica particles and micelles all tend to form ordered

or layered colloidal structures in the thin film. One of their results [Nikolov and Wasan,

1989] on step-wise film thinning at 30 mM and 100 mM NaDS concentration shows that

the time taken for removal of the penultimate micellar layer, before forming a stable film,

is highly surfactant concentration dependent (i.e., the time required is ~3 min at 30 mM

in comparison to -10 min at 100 mM). The micellar stabilities of NaDS [Oh et al., 1993]

(1 ms at 25 mM & 100 ms at 100 mM) correlate well with this draining behavior. This

observation suggests that it is not just sufficient to have micelles but the micelles must

have greater stability to cause enhanced foam stability.


1200 -


800



400


LiDS


Figure 2-6


NaDS


CsDS


Mg(DS)2


Foam stability of surfactant, dodecyl sulfate, with different counter-ions
above critical micelle concentration (25 mM).


Figure 2-7 shows the foam stability experiments at 50 mM surfactant

concentration. A comparison with Figure 2-6 shows that while the foam stabilities for

LiDS and NaDS do not change much; the foam stabilities increase considerably for CsDS









and Mg(DS)2. Thus, one can conclude that the foam stabilities can only be enhanced by

packing micelles of high stability in the foam lamellae.


LiDS


Figure 2-7


NaDS


CsDS


Mg(DS)2


Foam stability of surfactant, dodecyl sulfate with different counter-ions
above critical micelle concentration (50 mM).


In most of our foam stability experiments, lower drainage rates appear to be the

requirement for having high foam stabilities. However, the drainage rate measurements

(from shaking method) did not correlate well with the foam stabilities. For instance, at 50

mM surfactant concentration, LiDS was found to have a significantly lower drainage rate

as compared to NaDS, which is contrary to what is expected from the foam stabilities.

Presumably, it is not the bulk drainage but the small residual water drainage from the

foam lamellae that can explain the foam stabilities. This phenomenon is also seen in the

step-wise thinning of the films, where the majority of the micellar layers drain out

quickly. It is only towards the end that one observes the differences in the draining

behavior as a function of surfactant concentration [Nikolov and Wasan, 1989].










100
0




| 60

0 40


i 20-
0



LiDS NaDS CsDS Mg(DS)2


Figure 2-8 Dimensionless surface viscosity measurements as a function of counter-
ion of surfactant, dodecyl sulfate, at 50 mM concentration.

Another useful parameter that correlates well with the foam stabilities is the

surface viscosity. The increased stability of foam lamellae with surface viscosity has been

shown earlier by various researchers [Brown et al., 1953; Davies, 1963; Shah et al.,

1978]. Figure 2-8 shows the dimensionless surface viscosity measurements for 50 mM

surfactant concentration with different counter-ions. Here, one observes low surface

viscosities for LiDS and NaDS, reasonably high surface viscosities for CsDS and

ultrahigh surface viscosities for Mg(DS)2 in good correlation to the foam stabilities

shown in Figure 2-7. Apparently, the increased binding of counter-ions in CsDS and

Mg(DS)2 decreases the repulsion between adjacent surfactant head groups causing a more

tightly packed film of higher surface viscosity.

Summarily, in the case of cesium and magnesium dodecyl sulfates there is a good

contribution to foam stability from structuring effect of micelles while in case of lithium









and sodium dodecyl sulfate because of the faster micellar relaxation times, the structural

contribution is minimal. Moreover the closer packing of monomers at the air/water

interface (Figure 2-9) causes higher surface viscosities for cesium and magnesium

dodecyl sulfates, which also assists in slowing down the rate of drainage. It might be

interesting to study step-wise thinning of film with these surfactants having different

counter-ions. Possibly, one would observe that films with CsDS or Mg(DS)2 systems

would squeeze out the micellar layer lot slowly as compared to LiDS or NaDS systems.


Labile micelles (LiDS, NaDS) Stable micelles (CsDS, Mg(DS),)
Loosely packed adsorbed film Tightly packed adsorbed film













Figure 2-9 Schematic diagram showing effect of counter-ions on packing of
surfactant molecule at air/water interface.


2.4 Conclusion

1. Surface activity of dodecyl sulfate having different counter-ions is in the following

order Li < Na < Cs < Mg while the area per molecule is in the reverse order.

2. Foamability by shaking method is determined by micellar stability for LiDS, NaDS

and CsDS while for Mg(DS)2 it is determined by the diffusion of monomers to the

air/water interface due to the lower cmc for Mg(DS)2.









3. Foam stability can be controlled to a certain extent by the molecular packing in the

adsorbed film and to a significantly larger extent by the presence of stable micelles

in foam lamellae. Higher the micellar stability, the greater is the foam stability.

4. The stabilities of the micelles appear to be in the following order: LiDS ~NaDS <

CsDS << Mg(DS)2.

5. The surface viscosity is found to be in the order LiDS < NaDS < CsDS << Mg(DS)2

due to the increase in molecular packing at the air/water interface.

6. Dynamic surface tension at low bubble frequencies shows similar trend as

equilibrium surface tension whereas at higher bubble frequencies, the trend is LiDS

> NaDS > CsDS < Mg(DS)2.














CHAPTER 3
EFFECT OF IONIC/NON-IONIC MIXED SURFACTANTS ON PROPERTIES OF
GAS/LIQUID, LIQUID/LIQUID AND SOLID/LIQUID INTERFACE



3.1 Introduction

For most practical applications, pure surfactants are not used. At times, pure

surfactants are avoided primarily because of their high cost. A good example is the

inability of detergent manufacturers for using Dow Q2 5211, a silicon based surfactant,

even when it has a potential of saving millions of dollars worth of energy by reducing the

amount of residual water in the laundry after the wash cycle. At other times, surfactants

are intentionally mixed to achieve synergism in properties such as foaming, wetting,

dispersion stability [Jost et al., 1988; Rosen, 1978], dishwashing, detergency [Schwuger,

1984; Malmsten and Lindman, 1989], mineral floatation [Rybinski and Schwuger, 1986],

to make a detergent system tolerant to water hardness [Cox et al., 1985] and for lowering

the Krafft temperature [Scamehorn, 1992].

Recently, Palla and Shah [2002] showed the usefulness of synergism in mixed

SDS-Tween 80 surfactant system for stabilizing chemical mechanical polishing slurries

in harsh environment of high electrolyte content. The purpose of this chapter is to: (1)

develop a better understanding of the properties of SDS-Tween 80 mixed surfactant

system and (2) broadening of the scope of the synergism of mixed surfactant systems to

other processes such as foaming, emulsion droplet size, wettability and detergency.

Additionally, since non-ionic surfactants have low cmcs' and high micellar stabilites in








comparison to ionic surfactants [Patist, 1999], a gradual increase of the non-ionic

surfactant in the mixed system is expected to provide an opportunity to gradually reduce

the cmcs' while simultaneously increasing the micellar stability by shielding the

electrostatic repulsion between the ionic surfactant molecules in the micelle. In the

previous chapter, with the lowest cmc around 1 mM, it was difficult to deconvolute the

individual contributions to foaming from the monomer concentration and from the

micelles. For the mixed systems, compositions with low cmc/unstable micelles and low

cmc/stable micelles might be obtained in the range, which can help explain the

importance of micellar stability alone.


3.2 Experimental Method


3.2.1 Materials

Anionic surfactant, sodium dodecyl sulfate (99% purity), non-ionic surfactant,

Tween 80 and dye Sudan Orange are supplied by Sigma chemical Co. (St. Louis, MO).

Merocyanine 540 (C26H32N3NaO6S2, anionic) is supplied by Acros Organics (Fair Lawn,

NJ). Deionized, distilled water is used in all the experiments. All the experiments are

done at room temperature (22 0C).


3.2.2 Surface Tension Determination

Surface tension is measured using Wilhelmy plate technique and foaming is done

by shaking method. The details are similar to as described in previous chapter.


3.2.3 Thin Film Stability

A wire loop of 10 mm diameter is made at the end of a platinum wire, which is in

turn attached to a cork. Twenty-five milliliters of the desired surfactant solution is taken








in a 200 mL Erlenmeyer flask. The length of the wire is adjusted so that the loop is close

enough to the solution surface but not touching it. Ample time (- 1 hr) is provided for

water vapor to saturate the air within the flask. The wire loop is then filled with surfactant

solution by tilting the flask (- 45). Immediately after filling the loop, the flask is brought

back to its original position, leaving a thin vertical liquid film within the loop. The time

required to rupture the thin film is monitored using a stopwatch. The data reported are

average of four such measurements.


3.2.4 Spectrophotometry

Merocyanine 540 at concentration of 0.019 mM is used for the determination of

cmc using the dye method. Absorption spectra are taken using a Hewlett Packard UV-

VIS spectrophotometer (model 8453) with temperature control. The shift in the

wavelength between aqueous and micellar environment is about 22 nm and is significant

enough to detect the formation of micelle. A typical curve can be seen in Figure 1-3 of

chapter 1. For cmc determination, the absorbance ratio, which is defined as the ratio of

absorbance corresponding to dye inside micelles to absorbance at wavelength

corresponding to dye in water, is plotted against surfactant concentration. The region

below cmc is marked by a low absorbance ratio while the one above cmc is marked by a

high absorbance ratio. At high enough surfactant concentration, the absorbance ratio vs.

concentration curve becomes flat as most of the dye becomes solubilized in the micellar

core depleting the dye from the aqueous environment. The linear portion near the

inflection point is extrapolated to the point where absorbance matches that of dye in

absence of any surfactant and this concentration is defined as the cmc of the surfactant.








3.2.5 Detergency of Orange OT Dye Adsorbed on Cotton Fabric

A concentrated solution of dye Orange OT in ethanol (Abs 3.85) is prepared;

400 pL of this solution is applied to a 3" x 3" Hanes cotton fabric in two portions of 200

pL each. The alcohol was removed from the fabric by drying in an oven (- 60 oC/8 hrs).

Cotton pieces are then placed in 300 mL surfactant solution in a tergetometer (United

States Testing Co., Hoboken, NJ) operating at 100 rpm for 10 min. Wet cotton pieces

thus obtained are rinsed with water and then dried again. The absorbance of the dye on

the fabric before and after detergency with surfactant solutions is measured using a bench

top reflectometer at a wavelength of 500 nm and the difference is reported as a measure

of detergency.


3.2.6 Contact Angle Measurements

Contact angle of various SDS/Tween 80 mixed surfactant solutions were observed

on a PMMA surface using the low-power microscope of a contact angle goniometer.

Experiments are done in triplicate and the reported values are the means of the three

measurements. The error in measurement is about 3.


3.3 Results and Discussion

Figure 3-1 shows the equilibrium surface tension values for mixed SDS Tween

80 surfactant solutions at a total surfactant concentration of 50 mM. It can be observed

that the surface tension gradually increases with increasing Tween 80 content up to a

Tween 80/SDS molar ratio of 0.67 after which it attains a plateau. Surface tension is

related to the number of surfactant molecules per unit area adsorbed at the air-water

interface, i.e., higher the number of surfactant molecules per unit area, tighter is the






44


packing and lower is the surface tension and vice versa. Thus, the surface tension results

can be explained by a higher and higher partitioning of Tween 80 molecules at the air

water interface where Tween 80 molecule because of its comparatively larger head group

area (77 A 2/molecule) [Vogler, 1992] than a SDS molecule (51.5 A 2/molecule) [Table

1, chapter 2], results in the formation of a loosely packed mixed film of high equilibrium

surface tension.

40

39 -
E
38

= 37 -

36 -

4 35-

34 -

33 ..
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-1 Variation in surface tension of SDS/Tween 80 mixed surfactant system as
a function of SDS/Tween 80 molar ratio at a total surfactant concentration
of 50 mM.

However, the mixing of SDS and Tween 80 molecules in the adsorbed monolayer

at the air/solution interface is not ideal else one would have observed a linear behavior of

surface tension with Tween 80 content, shown by the broken line. It is interesting to note

that the surface tensions of 30/20 Tween 80/SDS and pure SDS exhibit surface tension in

a straight line indicating the mixture of these two films behave in an ideal manner. This

also suggests that the plateau region in the surface tension may not be due to an adsorbed









film of pure Tween 80, but rather from a Tween 80/SDS mixed film where the SDS

molecules are occupying the molecular cavities in the Tween 80 adsorbed film and hence

not contributing to surface tension, a concept originally proposed by Shah and Shulman

[1967].



100 -


80 -


60 -


o 40 -


20 -

0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0

Figure 3-2 Effect of SDS/Tween 80 molar ratio on the volume of the foam generated
by shaking method at a total surfactant concentration of 50 mM. (Note:
100 mL is the maximum limit of the foam that can be generated.)

The surface tension is often correlated with the amount of interfacial area that can

be generated in foaming, the pertinent relation being given by equation (2.3). But in

chapter 2, it was shown that foamability results correlate much better to the dynamic

surface tension. Here, I show again that equilibrium surface tension is irrelevant for

explanation of foaming by shaking method. Figure 3-2 shows the volume of the foam that

can be generated in a mixed surfactant system of SDS and Tween 80 at a total surfactant

concentration of 50 mM by shaking method. In shaking method foam is generated

quickly by rapid shaking of cylinders causing a sudden expansion of interfacial area. At









the same time, due to vigorous shaking, there is a destruction of foam too. Here, it is

observed that the foam volume essentially remains constant and high up to a critical

Tween 80/SDS molar ratio of ~ 1 beyond which it decreases significantly. This abrupt

change in foam volume behavior is unexpected from the predictions of equation (2.3)

because going from Tween 80/SDS molar ratio of 20/30 to 30/20, the surface tension

does not change abruptly (change of ~ 2 mN/m) while the corresponding foam volume

has reduced drastically.

The other possible mechanisms for this dramatic foam volume reduction beyond a

critical Tween 80/SDS molar ratio can be: (1) certain adsorbed surfactant mixed film

compositions due to their impact on film elasticity lead to an unstable foam lamella and

(2) an insufficient surfactant monomer flux to the newly created interfacial area, i.e., a

higher dynamic surface tension.

7000

6000 -

5000 -

0 4000 -

E 3000 -
1-
2000 -

1000 -

0 ----- I ---- --I-- 1 --I-- I--- r
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-3 Effect of SDS/Tween 80 molar ratio on the stability of the thin film at a
total surfactant concentration of 50 mM.








The role of the first mechanism can be examined by performing single film

stability measurements, i.e., if certain mixed surfactant compositions (high Tween 80

content) are resulting in low foam volumes, then thin films created under analogous

conditions should also be very unstable. Figure 3-3 shows the lifetimes of thin films

formed using a platinum wire loop at various Tween 80/SDS molar ratios at a total

surfactant concentration of 50 mM. It is seen that as the amount of Tween 80 in the

system increases, the stability of the thin film formed under equilibrium conditions

increases accordingly. In fact, thin films with a high Tween 80 content (above a

SDS/Tween 80 molar ratio of 1) are about an order of magnitude more stable than thin

films with high SDS content. This contrasting behavior of thin film stability to the

foamability observations, suggests that one can exclude foam destabilization owing to a

high concentration of Tween 80, in the adsorbed film at the interface.

One may still argue that above mentioned films are prepared under equilibrium

conditions and hence, are not truly representative of what happens during the dynamic

conditions of foaming. To counter this, foam was produced by shaking method at a

SDS/Tween 80 molar ratio = 1; right at the critical point beyond which a further addition

of Tween 80 drastically reduces the foam, and to this foam, ImL 50 mM Tween 80

solution is supplied from the top of the foam column. Absolutely no destruction of the

foam is observed leading to a further substantiation of the hypothesis that adsorbed film

compositions with high Tween 80 content are not the determining factor for a dramatic

reduction of the foam.

An insufficient surfactant monomer flux (mechanism 2) then appears to be the

controlling mechanism. For the two extreme cases of pure SDS and pure Tween 80,








simple estimates regarding the amount of surfactant necessary to produce a cylinder full

of foam (100 mL) can be made. Assuming a foam column with an average bubble size of

1 mm (radius) and the interfaces totally saturated with surfactant monomers, an SDS

foam column would require ~ 10-' mM surfactant concentration and a Tween 80 foam

column would require ~ 6.69 x 10-2 mM surfactant concentration. The necessary

concentrations for the mixed systems should lie in between these two limits. The

theoretical limit can also be verified experimentally, at least for the SDS system because

of its stronger foaming behavior. Figure 3-4 shows the volume of the foam generated by

shaking method as a function of decreasing SDS surfactant concentration. The foam

volume remains constant at 100 mL up to a SDS surfactant concentration of 4 mM and

then it suddenly drops to low values of 10 mL suggesting 4 mM is the experimental limit

for producing 100 mL of foam. This number is 40 times higher than the maximum

theoretical limit but given the crudeness of the assumptions involved, it can be said to be

reasonably close. Interestingly, 2 mM is the concentration of the SDS surfactant

predicted by a more rigorous Ward and Tordai model [Ward and Tordai, 1946] (equation

3.1) for stabilizing an interface created on a time scale as fast as 0.01 s in absence of

stirring or an energy barrier to adsorption for a surfactant.


n=2 t cN (3.1)
Vr 1000
2 2
where n is the number of molecules/cm D is the bulk diffusion constant (cm /s), c is the

bulk concentration (moles/liter), N is the Avogadro number and t is time (s). Assuming D

aX (Mw )-1 [Eastoe et. al., 1996], a similar concentration prediction for Tween 80 system


would be about 1.8 mM.









But even more importantly, since adsorbed film compositions are not affecting

foaming ability, the experimental limit of 4 mM implies that in mixtures of SDS and

Tween 80, a free SDS surfactant concentration greater than 4 mM should suffice to

produce full foam column. This condition (SDS conc. > 4 mM) is satisfied by all the

surfactant mixtures in Figure 3-2 except the pure Tween 80 system, once again,

predicting a foam volume of 100 mL for all systems except Tween 80. But this is not

what is actually observed from the foamability results where a low foam volume is

observed for systems exceeding a Tween 80/SDS molar ratio of 1. This suggests that at

high molar ratios of Tween 80 in the mixture, the partitioning of SDS molecules in mixed

SDS/Tween 80 micelles of high stability (large relaxation time) is probably making SDS

molecules unavailable for the foaming process.



100 -


3 80


S60 -
0
E 40 -
0

20 -

0 4 mM|
100 10 1 0.1
SDS Concentration (mM)


Figure 3-4 Variation of foam volume with SDS surfactant concentration. (Note: 100
mL is the maximum amount of foam that can be generated).






50


1600

1400- SDS + Tween 80
S.... o SDS
1200 .. .

1000 ..

800

600 -
U 0
400 -

200 -
0 -0
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-5 Comparison between conductivity behavior of a pure SDS system with
that of SDS/Tween 80 mixture at various molar ratios.


Electrical conductivity is a useful parameter for ionic surfactants. Surfactant

monomers because of their high mobility contribute considerably to the electrical

conductivity in comparison to the micelles with low mobility. And the break in electrical

conductivity measurement against the surfactant concentration is routinely used to

determine the cmc. Now, if in systems with high Tween 80 content, SDS monomers are

really partitioning into the mixed micelles, then conductivity values lower than pure SDS

should be expected. Indeed, this is exactly what is observed from results in Figure 3-5.

For SDS rich compositions, the conductivity curve of the mixed SDS/Tween 80 lies

above than that for the pure SDS system presumably because the penetration of Tween 80

molecules in the mixed micelle shields the electrostatic repulsion between the sulfate

head groups of SDS molecules. This leads to a higher degree of ionization of mixed

micelles, i.e., larger number of counter-ions and hence higher conductivities, than that of

pure SDS micelles where such a electrostatic shielding is only possible by binding more









and more counter-ions to the micelle resulting in lower conductivities. But for Tween 80

rich mixed surfactant compositions, a crossover is observed and the conductivity curve

lies below the pure SDS curve apparently due to the partitioning of SDS monomers into

the mixed micelles.

105

o 100 --

U 95 -




S85

U 80


75
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-6 Variation of cloud point temperature as a function of SDS/Tween 80
molar ratio. (Note: 100 C is the maximum temperature that can be
measured).


The partitioning of SDS monomers in the mixed micelle can also be ascertained

by measuring the cloud point temperature of the mixed systems. Just like conductivity is

a signature for ionic surfactants, cloud point; the temperature to which a non-ionic

surfactant solution can be heated before it starts to strongly scatter light and becomes

cloudy in appearance, is a characteristic feature for non-ionic surfactants. The phase

separation occurs because of a sharp increase in the aggregation number of the micelles

and the decrease in inter micellar repulsions originating from decreased hydration of

oxyethylene oxygen's in the polyoxyethylene hydrophilic group with increase in

temperature [Tiddy, 1980]. Figure 3-6 shows the cloud point temperature of various








SDS/Tween 80 mixed systems at a total surfactant concentration of 50 mM. It can be

seen that for pure Tween 80 system, the cloud point is 80 C whereas for all other mixed

systems, the cloud point is higher than 100 C. It is to be noted here that cloud point

values greater than 100 C cannot be reliably determined. But from the increase of cloud

point of Tween 80 on addition of SDS, it can be inferred once again that SDS monomers

are partitioning into mixed micelles whereby they impart charge to the micelles and the

electrostatic repulsion thus produced keeps the micelles apart at high temperatures.

Now that the SDS partitioning into SDS/Tween 80 mixed micelles for systems

with high Tween 80 content from the discussion of the above two paragraphs, the only

reason why such a trapped SDS is not generating large foam volumes could be because of

the high stability of the mixed micelles. Additionally, the free monomer concentration

can be quite small as well. Nothing can be said about SDS rich systems [SDS/Tween 80

molar ratio > 1] at this point and the high foam volumes observed could be a consequence

of: (1) a large free monomer concentration sufficient enough to produce a full foam

column irrespective of the micellar stability and (2) an insufficient and small free

monomer concentration aided by sufficient monomers from unstable micelles. To

delineate the contribution originating from surfactant monomers, their concentration (i.e.,

cmc) is measured next.

Surface tension method was attempted first to determine the cmcs' of the various

mixed systems and the surface tension versus log C plots for four mixed compositions are

shown in Figure 3-7. Sharp breaks indicative of micellization are not observed. For

SDS/Tween 80 molar ratio = 1, 3, the profiles are more or less linearly decaying while for

the other two compositions, the decay profile is highly non-linear with a possible break








around 0.01 mM concentration. This kind of behavior has been observed mainly for non-

ionic surfactants by others as well [Alexandridis et al., 1994; Wanka et al., 1990] and has

been attributed to broad molar weight distribution both in the hydrophobic part as well as

in the degree of ethoxylation. Surface tension method does not look like a tool sensitive

enough to determine the cmc's for SDS rich mixed systems.




(a) (b)







(c) (d)



Figure 3-7 Variation of surface tension against logarithm of surfactant concentration
for: (a) SDS/Tween 80 molar ratio = 3, (b) SDS/Tween 80 molar ratio = 1,
(c) SDS/Tween 80 molar ratio = 0.33 and (d) pure Tween 80.

Recently, Patist [1999], has measured the cmc's of non-ionic surfactants using the

solvatochromic shift property (shift in absorbance peak wavelength depending on the

environment) of dyes like Merocyanine 540 and Eosin Y, following a method earlier

developed by Shinoda and Nakagawa [1963] and Hunter [1987]. Figure 3-8 shows the

ratio of absorbance peaks for dye in micellar solution to dye in aqueous solution at

various SDS/Tween 80 molar ratios. The behavior is expected; for ionic surfactants,

which can be obtained in quite pure form, the absorbance ratio transition is sharp while

for non-ionic surfactants, the micellization is known to be much less sharp and the

absorbance ratio slowly increases over a range of surfactant concentration. Nonetheless,








the transition point can be identified for all the SDS/Tween 80 molar ratios and except

the pure SDS system; the cmc values for the all the ratios are quite small (- 0.01 0.03

mM). The cmc values thus found are also in excellent agreement with a theoretical cmc

prediction model developed by Holland and Rubingh [1983] using ideal solution theory.

The pertinent equation is:

_= Xi (3.2)
cmcn,,itre cmci

where xi is the composition of surfactant i with a cmci. The proof of the above expression

and theoretical cmcs' based on it are presented in appendix C.


















Figure 3-8 Determination of cmc for mixed surfactant system using the dye
micellization method (Merocyanine 540 conc. = 0.019 mM). Absorbance
ratios for peaks corresponding to dye in micelle and dye in water are
plotted above for: (a) pure SDS, (b) SDS/Tween 80 mole ratio = 4, (c)
SDS/Tween 80 mole ratio = 1.5, (d) SDS/Tween 80 mole ratio = 0.67, (e)
SDS/Tween 80 mole ratio = 0.25 and (f) pure Tween 80.

Except for pure SDS system with a cmc vales of 8 mM, the cmcs' for all other

mixed systems are considerable smaller than what would be required for producing full

foam column with either of the surfactants (Ward and Tordai prediction of 2 mM for SDS









and 1.8 mM for Tween 80). Thus, it can be concluded that except for pure SDS system,

there is a large resistance from monomers itself and if any foaming has to occur, the

resistance from the micelles will have to be smaller, i.e., micelles have to be very

unstable which is likely the case for mixed systems with SDS/Tween 80 molar ratio > 1.

And in systems where micelle stabilities are higher, lot less foam should be observed as

seen for systems with SDS/Tween 80 molar ratio < 1.

58

56 -

54 -
0
E 52-

S50-

48 -

46 -

44 -
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-9 Effect of SDS/Tween 80 molar ratio on the removal of dye orange OT
from cotton fabric at a total surfactant concentration of 50 mM.


As stable micelles because of their stronger hydrophobic core are known to be

more effective in removing dye molecules adsorbed on a cotton fabric [Oh and Shah,

1993a], it will be expected that for Tween 80/SDS molar ratio > 1, a large amount of dye

removal will be possible whereas for Tween 80/SDS molar ratio < 1, the dye removal

will be smaller. Figure 3-9 shows the removal of dye orange OT from Hanes cotton fabric

at various SDS/Tween 80 mixed compositions at a total surfactant concentration of 50

mM. It is observed that on increasing the amount of Tween 80 in the mixture, the % dye









removal first remains constant, then gradually increases up to a SDS/Tween 80 molar

ratio = 1 before finally leveling off. This indicates that the micelles are gradually

becoming more and more stable as the Tween 80 content of mixture is increasing and

beyond a Tween 80/SDS molar ratio =1, the micellar stability remains fairly constant in

accordance with our hypothesis.

16 18
14 (a) i6 (b)
12 14
0 12
t0
6
6
4 4


5 10 15 20 25 5 10 15 20 25 30
Emulsion Droplet Size (pm) Emulsion Droplet Size ( tm)



Figure 3-10 Emulsion droplet size distribution in: (a) SDS/hexadecane system and (b)
Tween 80/hexadecane system at a surfactant concentration of 50 mM
(hexadecane vol % = 15). Emulsions are produced by vigorous stirring for
5 min.


Next, the SDS and Tween 80 systems are evaluated at the liquid-liquid interface

by producing emulsions of 50 mM surfactant solution with hexadecane. High micellar

stabilities have been attributed as a likely reason for producing larger emulsion droplets

[Oh et al., 1993] because of their inability to provide monomers necessary to stabilize the

newly created interfacial area. Thus smaller droplets for pure SDS system and larger

droplets for Tween 80 system would be expected on the basis of unstable SDS micelles

and very stable Tween 80 micelles. This is not what is observed from Figure 3-10, which

shows the droplet size distribution of emulsions for the two cases; and Tween 80 system

rather than the SDS system, produces fine droplets. A possible reason for this

inconsistent behavior might be due to the fact that for pure SDS system, there are large









numbers of free monomers as well as unstable micelles, which can quickly provide

sufficient surfactant to stabilize all the emulsion droplets irrespective of their sizes. The

same might not be true for Tween 80 system where the micelles are very stable as well as

the free monomer concentration is small, which results in the stabilization of small

droplets only. The larger unprotected droplets coalesce fast and immediately get

fractionated due to buoyancy leaving behind a droplet size distribution weighted strongly

by small droplets.

44
42
40 ^LG
S38 G Ys
36
< 34
5 32
U 30
28 YSG = YSL + YLG COS 0
-26 (a)
24 (______b)
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-11 Effect of SDS/Tween 80 molar ratio on the contact angle at a PMMA
surface at a total surfactant concentration of 50 mM (a) and (b) schematic
and definition of contact angle.


Finally, the behavior of the mixed systems is analyzed at a solid-liquid interface

by measuring the contact angle at a PMMA surface to determine the wetting ability.

Results in Figure 3-11(a) show that SDS is an excellent wetting agent with a small

contact angle of about 250 but as soon as the non-ionic surfactant is incorporated into the

mixture, the wetting becomes poor and high contact angles of ~ 400 are observed. The

schematic diagram for a pure water droplet on a PMMA surface is shown along with the

equation defining contact angle in Figure 3-11(b). As surfactants are added to the water

phase, YSG remains unaffected but YSL and YLG are both reduced due to the adsorption of









the surfactants at respective interfaces to different extents depending on the nature of the

surfactant simultaneously lowering contact angle to different extents in comparison to

that of pure water. While the values of contact angle may depend on the complex

interplay of YSL and YLG, since YLG behavior is known from Figure 3-1, conclusions can be

drawn about the nature of the adsorbed film at the solid/water interface.

31.0

30.5 -

30.0 -

29.5 -

29.0 -

28.5 -

28.0 -

27.5 -

27.0 ..i..
0 10 20 30 40 50 Tween 80
SDS 50 40 30 20 10 0


Figure 3-12 Effect of SDS/Tween 80 molar ratio on the solid-liquid interfacial energy
at a total surfactant concentration of 50 mM.


The quantity Ay (Ay = YSG 'sL) has been calculated from the known values of

contact angle and YLG and is plotted in Figure 3-12. It is seen that Ay is almost similar for

SDS and Tween 80 while it is lower for mixed surfactant systems implying the mixed

surfactant compositions are poor in terms of reducing the solid-liquid interfacial energy

(ysL). For pure SDS system it is expected because the charged and cohesively packed

adsorbed molecular film on PMMA presents a very hydrophilic surface to water. A

plausible reason for ineffectiveness of the mixed systems can be due to a poor packing of









adsorbed mixed film due to steric effects from Tween 80 molecules (high area/molecule),

which leaves up exposed regions on PMMA. As the amount of Tween 80 in the mixture

increases and more and more SDS gets solubilized in the micelles; predominantly non-

ionic surfactants adsorb at the interface, once again presenting a hydrophilic surface to

water.


3.4 Conclusions

(1) SDS reduces the surface tension more than Tween 80 indicating it is more surface-

active than Tween 80. In mixed adsorbed films beyond a critical Tween 80/SDS

molar ratio, SDS molecules appear to occupy the molecular cavities present in

Tween 80 film.

(2) Equilibrium surface tension is not a good parameter to explain foamability as

achieved by shaking method.

(3) For cmc measurements of the mixed SDS/Tween 80 system, the dye method which

relies on absorbance peak shift in different environments is much more sensitive

than the widely used surface tension method.

(4) Non-ionic surfactant (Tween 80) interacts strongly with ionic surfactant (SDS) and

a small amount of non-ionic surfactant added to ionic surfactant can reduce the cmc

of the mixture considerably.

(5) When the monomer concentration in a surfactant solution is insufficient to produce

foam, it is the micellar stability that determines the amount of the foam that can be

produced. For unstable micelles [SDS/Tween 80 > 1], a large foam volume is

obtained while for stable micelles [SDS/Tween 80 < 1], small foam volumes are

seen.








(6) A strongly foam forming surfactant concentration of SDS (4 mM to 20 mM) may

become ineffective in presence of non-ionic surfactants such as Tween 80 because

of its partitioning into mixed micelles of very high stability.

(7) Strong correlation exists between the micellar stability and the detergency of a

model compound; in general higher the micellar stability, stronger the hydrophobic

core and higher is the dye removal.

(8) At a solid-liquid interface, the solid-liquid interfacial energy is reduced more by

pure ionic (SDS) or pure non-ionic surfactant (Tween 80) than by the mixed film. In

the case of SDS, the charged and cohesive adsorbed film presents a hydrophilic

surface to water while in the case of non-ionic surfactants; the hydrophilic ethylene

oxide groups are responsible for reducing the interfacial energy. The mixed

adsorbed films, however, are loosely packed due to the steric effect of a large

Tween 80 molecule, which leaves up exposed regions on PMMA and hence a high

interfacial energy.














CHAPTER 4
ANTIFOAMING ACTION OF ESSENTIAL OILS



4.1 Introduction

Foams, dispersions of air in water, have applications in a large number of areas

such as food products, personal care products, enhanced oil recovery, firefighting,

decontamination of soils and as shown recently, in combating bioterror. But foams are

not always desirable [Garret, 1993; Kroshwitz and Howe-Grant, 1993]. Plant operators

detest them because they can interfere with the continuous plant operations such as

pumping or distillation by seriously affecting the pumping machinery and the separation

efficiencies. The benign presence of a few soap bubbles after a washing cycle may

psychologically influence the consumers to significantly impact the sales of a particular

laundry detergent. The difficulty of flushing large amounts of "stable" foam after a

luxurious bubble bath is familiar to most users.

Consequently, a wide variety of antifoaming products have been developed that

range from paraffins, to alkanes, to hydrophobic particles, to synergistic combinations of

solid hydrophobic particles dispersed in an oil, to surfactant aggregates with tailored

stability. The mechanisms of antifoaming by these antifoaming agents can be

summarized as follows [Rosen, 1978]:

1. Removing surface-active materials from the air/water interface:

Surfactant molecules are removed from the surface by adsorption onto or dissolution

in the soil. Finely divided hydrophobic silica particles break the foam by adsorbing








surfactant molecules from the bubble surface and carrying them into solution. The

presence of certain types of soil in a surfactant solution shows decreased foaming due

to this mechanism.

2. Converting the surfactant film into a solid brittle film with no elasticity:

Calcium salts of long chain fatty acids break foams of SDS or sodium

dodecylbenzene sulfate by this mechanism.

3. Reducing the surface viscosity of the film:

Tributyl phosphate has a large cross sectional area per molecule at the air/water

interface. This reduces the cohesive forces between the surfactant molecules and

consequently reduces the surface viscosity, which leads to increased drainage of the

liquid film.

4. Replacing surfactant molecules with other molecules at the surface:

The surfactant molecules at the bubble surface are replaced by adding non-cohesive

molecules of limited solubility in the solution. The tertiary acetylenic glycols, ethyl

ethers and isoamyl alcohols break foam in this manner.

In this chapter, the focus is on the last of the mechanisms. In the earlier chapters,

it was shown that by having micelles of high stability, the foamability of a system could

be reduced considerably. The rationale behind this study is to evaluate the usefulness of

micellar stability against other competing mechanisms for antifoaming.


4.2 Experimental Section


4.2.1 Materials

Sodium dodecyl sulfate (99% purity) and dodecane are supplied by Sigma

chemical Co. (St. Louis, MO). Clove bud oil, cinnamon leaf, eucalyptus, cypress and








vetiver essential oils are from Aura Cacia Company and were purchased at a local store.

Hydrophobic silica (Cab-O-Sil TS 530) is obtained from Cabot Corporation. Deionized,

distilled water is used for all the solutions. All experiments are performed at room

temperature (22 OC).


4.2.2 Antifoam Efficiency

Antifoam efficacy measurements are done by a cylinder shake method. The

volume ratio of SDS surfactant solution to antifoaming oil was kept constant at 30. In a

typical antifoaming experiment, coarse emulsions of the antifoams are produced by

vigorous stirring of 60 mL SDS surfactant solution with 2 mL essential oils (pure or in

mixtures with dodecane). Foaming experiments are then performed using 15 mL of this

emulsion in a 100 mL graduated cylinder. The stoppered cylinder is shaken by hand 10

times and foam height is measured immediately after the cessation of foaming. Replicate

measurements of foaming are done and the results quoted are the average of at least three

repetitions.


4.2.3 Surface Tension Measurements for Calculation of Spreading Pressure

Surface tension is measured using Wilhelmy plate technique as described in

previous chapters. For spreading pressure determination, a 10 cm diameter petridish is

partially filled with 50 mL of surfactant solution. The Wilhelmy plate is then immersed in

the surfactant solution at one end of the petridish while 5 pL of the essential oil antifoam

is carefully placed at the solution surface at the other end. The distance between the

platinum blade and the spot of deposition of antifoam was kept 7 cm. The surface

tensions are measured before and after the application of antifoam and the difference is

taken to be the spreading pressure.









4.2.4 Qualitative Spreading Pressure Measurement

Qualitative measurements on spreading are performed by sprinkling hydrophobic

silica over air/surfactant solution interface (500 mL) in 30 cm diameter circular glass

tray. A small amount of essential oil antifoam (5 pL) is carefully placed on the surface at

the center of the tray. The spreading of the oil drags the hydrophobic silica particles along

with it in the radially outward direction. The maximum diameter to which the particles

are removed is simultaneously measured using a ruler and is taken to be a qualitative

estimate of the spreading pressure of the antifoam.


4.2.5 Droplet Entry Barrier Measurement

Emulsion droplets (essential oil/dodecane volume ratio = 0.2) were produced

beneath the air/surfactant solution (typically 5 mM) interface using a 25 PL Hamilton

syringe. The time required for merging of the single droplet to the interface is taken as a

measure of the droplet entry barrier. In one particular case, 0.5 mL of clove bud oil was

first extracted with 15 mL, 50 mM SDS solution for 60 s. The excess clove bud oil thus

obtained after centrifugation was used to make an emulsion with dodecane (volume ratio

of 0.2) for entry barrier measurement.


4.3 Results and Discussion

Figure 4-1 compares the effect of various essential oils on the antifoaming action

in foam created by shaking method, using surfactant, sodium dodecyl sulfate at a

concentration of 5 mM (below cmc), against a dodecane control. Here, it can be observed

that clove bud, cinnamon and vetiver oils (hereon abbreviated as class I essential oils) are

highly effective in suppressing the foam while there is almost no foam destruction in the









case of cypress or eucalyptus oils (hereon, termed class II essential oils). The control,

dodecane, is ineffective in foam reduction. Hence, class I oils must have some

characteristics to destroy the foam.





















Figure 4-1 Qualitative antifoaming behavior of various essential oils and dodecane in
a 5 mM 15 mL SDS surfactant solution (volume ratio solution/antifoam =
30).

Next, blends of essential oils in various volume ratios with dodecane are tested as

antifoaming agents to determine the minimum amount of essential oil required for

antifoaming. As some of the essential oils are denser than water, mixing with dodecane

also ensures reduced density variations across systems. Foaming results in Figure 4-2

indicate that as essential oil to dodecane volume ratio is gradually increased, the

antifoaming action of the class I essential oils sharply increases (i.e., the foam volume

decreases) and then attains a plateau. The transition occurs between a 0.1 0.25 volume

ratio of essential oil to dodecane and beyond the transition point a constant foam head (20

- 35 mL depending on the essential oil used) always persists irrespective of the antifoam









content in the blend. Class II essential oils, on the other hand appear to have no

significant effect on the antifoaming action at any volume ratio studied.


-+- Clove
100 -o- Cinnamon
-v- Eucalyptus
80 --v- Cypress
80 + Vetiver


60 -
0

o 40


20 -

0.0 0.2 0.4 0.6 0.8 1.0
Volume fraction of essential oil


Figure 4-2 Effect of volume fraction of essential oil in mixed antifoam with dodecane
on the foam volumes of a 5 mM 15 mL SDS surfactant solution (volume
ratio solution/antifoam = 30).


The differences in antifoaming action of these oils can originate primarily from

two mechanisms: (1) due to significant differences among droplet size; if oil droplets are

of very small size, i.e., hence high interfacial area, they would significantly deplete large

number of surfactant molecules from the foam forming solution in comparison to oil

droplets with larger droplet size, and (2) some of the oils may have a strong tendency to

spread thereby displacing the surfactant and covering up the interface with small non-

cohesive molecules. These unstable regions in the adsorbed film then destroy the foam.

The first hypothesis of significant differences in interfacial area among systems

can easily be verified by doing emulsion size measurements. Figure 4-3 shows the

volume averaged mean droplet size of emulsions produced by vigorously shaking








essential oil + dodecane mixtures (volume fraction = 0.2) with 5 mM SDS solutions

under conditions similar to those used in foaming. The emulsions droplet size appears to

correlate well with the antifoaming efficiency; higher the droplet size, smaller the total

interfacial area and lesser is the extent to which surfactant can be depleted from the

solution resulting in the higher the amount of the foam and lower antifoaming action. But

theoretical considerations discussed below show that this correlation is merely

coincidental and the amount of the surfactant that can be depleted by any of the

antifoaming system due to droplet interfacial area is a very small fraction of the total

amount of surfactant used. Considering a 15 mL, 5 mM SDS surfactant solution with 0.5

mL antifoam, the smallest spherical antifoam droplet size of 13 pm will correspond to

5.43 x 107 droplets with an interfacial area of 0.115 m2 while the largest spherical droplet

size of 28 upm will correspond to 5.43 x 106 droplets with an interfacial area of 0.053 m2.

Further assuming that all the available oil-water interfaces are totally saturated with SDS

surfactant with an area per molecule of 51.5 A2/molecule (table 2.1), the number of SDS

molecules required would be in the range of 1.02 x 1017 2.23 x 1017. The actual number

of SDS molecules at the interface can be expected to be at least an order of magnitude

smaller than these estimates due to the dynamic situation encountered during

emulsification. In any case, the upper bound is still two orders of magnitude smaller than

the actual number (4.5 x 1019) of SDS molecules in the foam forming solution. Thus there

is ample amount of SDS remaining in the system even after its adsorption at the oil water

interface. The upper bound for surfactant requirements for formation of a full foam

column of 100 mL (bubble size ~ 1 mm) under similar assumptions of a fully surfactant

saturated air-water interface comes to 5.82 x 1017 molecules (23,870 bubbles).









30

28

26

24
22
20

18

16

14
12 I .
Cinnamon Clove Cypress Eucalyptus Vetiver



Figure 4-3 Effect of 0.2 volume fraction of essential oil in dodecane on the volume
weighted emulsion droplet size in a 5 mM SDS solution.


Qualitative as well as quantitative spreading experiments are then performed to

elucidate differences in the spreading pressure of essential oils. Figure 4-4 shows the

qualitative measurements of spreading pressure, which is related to the diameter of the

spread oil, of essential oils at various volume fractions in dodecane. It can be seen that

the diameter of the spread oil increases as the volume fraction of essential oil in dodecane

is increased. The spreading pressure of class I essential oils remains higher than that of

class II essential oils. This correlates with the antifoaming behavior observed in Figures

4-1 and 4-2, i.e., the higher the spreading pressure, the higher is the antifoaming efficacy.

Such a correlation has also been observed by this laboratory for single surfactants,

for mixed pure surfactants as well as for mixed commercial surfactants by other

researchers [Jha et al., 2000] too. There are two discrepancies however: (1) as the

essential oil to dodecane volume ratio is increased, the spreading area also increases but

there is no concomitant enhancement in the antifoaming action beyond a certain essential









oil to dodecane volume ratio (see Figure 4-2), i.e., the foam column cannot be destroyed

completely and (2) Vetiver oil has the highest spreading area but its antifoaming action is

poorer than either clove bud oil or cinnamon oil with lower spreading areas.


10
I 10-





S6
0
c0




E
2-


I. ........ .. .




---^ ----V
Sy -- Clove
S/ ........ Cinnamon
o ---v--- Eucalyptus
S--.. ... Cypress
-* Vetiver

0.0 0.2 0.4 0.6 0
Voume fraction of essential oil in dodecane


Figure 4-4 Effect of essential oil volume fraction in dodecane on the diameter of the
spread oil. Five microliter of antifoam is deposited on a 5 mM SDS
monolayer.


The quantitative spreading results, the difference in air/5 mM SDS surfactant

solution surface tension before and after the spreading of essential oils are plotted in

Figure 4-5 at a 0.20 volume fraction of essential oil in dodecane, and shows a trend

similar to that from qualitative spreading experiments of Figure 4-4, i.e., higher spreading

pressures for class I essential oils than class II essential oils. The agreement between

quantitative and qualitative experiments also suggests the absence of artifacts of any

significant importance in the qualitative method due to a possible depletion of surfactant

molecules on to the hydrophobic silica particles. Interestingly, the spreading pressure

values for class I essential oils and in particular for Vetiver oil is quite similar to






70


spreading pressure values for spreading of Poly-dimethyl siloxane, the most prevalent

antifoam in practical use, over a number of different surfactant monolayers [Bergeron

and Langevin, 1996; Jha et al., 2000].

12

10
E



6 6

4

2


clove cinnamon cypress eucalyptus vetiver dodecane


Figure 4-5 Spreading pressures of 0.2 volume fraction of various essential oils in
dodecane on a SDS monolayer (conc. 5 mM).


The antifoam dosage independency of antifoaming efficacy for essential oil to

dodecane volume ratio greater than 0.2 in Figure 4-2 is certainly not limited by the

number of emulsion droplets because even in the extreme case of lowest interfacial area

emulsion in cypress/dodecane system (average size 28 pm) emulsion droplet size and a

high foam interfacial area (say 1 mm bubble size), approximately 2 x 102 emulsion

droplets are available to supply essential oil antifoam per foam bubble (see also

discussion of Figure 4.3). Other researchers [Arnaudov et al., 2001] explain this

indestructible foam head in terms of relative sizes of emulsion droplets and Gibbs Plateau

Border (GPB) and to the ease of the entry of the emulsion droplet to the air/surfactant

solution interface. Because of a low capillary pressure in the foam lamella, the emulsion

droplets are squeezed out from the thin film into the GPB. And, if the GPB cross-section








is much larger than the emulsion droplet size, then one would be able to see the foam

destruction only after sufficient drainage of liquid has occurred so that the narrowing of

GPB can cause a pressure sufficient enough for emulsion drop to enter the air/surfactant

solution interface. The GPB cross-section is not uniform along the length of the foam

column. Instead, the radius of curvature of the GPB decreases linearly as one goes up the

foam column and is given by the equation,

27
RGPB -= (4.1)
Ap gh

where RGPB is the radius of curvature of GPB, y is the surface tension, Ap is the density

difference between the gas and liquid phase and h is the height of the foam. By

geometrical arguments, it can be shown that the radius of the emulsion droplet (RDroplet)

that will fit in the GPB region will follow the relation [Basheva et al., 2001],


RDroplet = 8 RGPB 3 1 (4.2)


For the SDS system under consideration (5 mM, surface tension = 39.3 mN/m), the RGPB

is around 200 pm for h = 4 cm and is about 40 um for h = 20 cm and the corresponding

emulsion sizes are about 31 pm and 6 pm respectively. The maximum emulsion droplet

size in this study is around 28 pm (cypress/dodecane system), a size that is close to what

is needed for leaving at least 4 cm of undestroyed foam column. Thus, just because of the

relatively smaller sizes of emulsion droplets as compared to the size of GPB, a certain

height of foam is always expected in these essential oils, and the variation of the amount

of foam around this foam height should be directly due to the differences in the emulsion

droplet entry barrier.









Figure 4-6 shows the result obtained from an indirect method to differentiate entry

barriers. Small droplet of essential oils in 0.2 volume ratio with dodecane are produced

below the air/solution interface and the time required for complete coalescence of oil

drop with the interface is taken to be the indirect measure of entry barrier. Other

researchers [Hadjiiski et al., 2001] have used sensitive pressure transducers to get a direct

measure of the critical pressure required for drop entry. As is expected, droplets

containing cinnamon and clove bud oils penetrate the interface pretty rapidly, while

cypress and eucalyptus are comparatively slower in entering the interface. The behavior

of vetiver oil is quite surprising; it has an entry barrier that is comparable to class II

essential oils, and yet it is able to destroy foam much more efficiently than the class II

essential oils. Apparently, vetiver oil makes up for its high entry barrier with its unusually

high spreading potential so that only few entry incidences are required for antifoaming

action.

20



15 -



10



5 -



0
Clove Cinnamon Cypress Eucalyptus Vetiver


Figure 4-6 Time required for merging of a 0.2 volume fraction essential oil in
dodecane droplet with the air/5mM SDS solution interface.






73


It is important to note that the size of the droplets that were produced could not be

kept constant. Visual inspection showed class I essential oils formed smaller droplets in

comparison to class II essential oils. The drop size difference however, does not affect

our conclusion regarding a lower entry barrier for class I essential oil in comparison to

class II essential oils. Entry barrier is known to decrease with increasing drop sizes

[Hadjiiski et al., 2001], i.e., a larger drop of same species will have a smaller entry barrier

in comparison to a smaller drop. Thus, if we could have ideally produced a larger droplet

with class I essential oil to match the large drop size of the class II essential oil, then the

entry barrier for class I oil would have reduced even further, leading again to the same

conclusion.



100 -


3 80


60 -


40 -
S- Clove oil
0 .... Hydrophobic Silica
20 + Dodecane
Oo -"O ".. "- O ... ............... .. 0 ............ ............. .................. 0
0oo-.-------------,---. --- ------1---
0 50 100 150 200
SDS concentration (mM)

Figure 4-7 Effect of surfactant concentration on antifoaming efficacy. (Note: 100 mL
is the maximum amount of foam that can be formed).


To evaluate the true potential of essential oil antifoams in severe conditions, the

antifoaming experiments are performed at increasing SDS concentrations. At








concentrations above critical micelle concentration, other mechanisms such as essential

oil solubilization in micelle core, stabilization of foam lamella by layering of micelles etc

are expected to be important. Figure 4-7 shows such an antifoaming study with one of the

better antifoams (clove bud oil). The antifoaming results of 0.05 wt% hydrophobic silica

in dodecane mixture are also presented for comparison. It is seen that the antifoaming

efficacy of the clove bud oil sharply decreases and after a SDS concentration of about 40

mM, there is no antifoaming action at all while the hydrophobic silica dodecane mixture

is a perfectly efficient foam breaker over the whole range of SDS concentrations studied.

The mechanism of the antifoaming action by hydrophobic particles in presence of non-

polar oils has been described elsewhere [Aveyard et al., 1993; Zhang et al., 2003]. The

loss of antifoaming action with clove bud oil as a function of SDS concentration may

occur because of preferential fractionation of the active component of clove bud oil in the

micelles, as clove bud oil is not a single component system. The idea of active

component getting locked in micellar phase and hence not contributing to antifoaming

may sound counter-intuitive from the micellar kinetics perspective; micelles are dynamic

structures that keep breaking and reforming but a similar case was shown in previous

chapter where SDS molecules were tightly trapped in Tween 80 micelles. As a

consequence of such fractionation, a smaller amount of ineffective essential oil, possibly

with a high droplet entry barrier, is left in the foaming system. The smaller amount of the

essential oil left after solubilization should not be a limiting factor as much as its

ineffectiveness because: (1) trial experiments (results not shown) suggest that clove bud

oil solubilization limit in a 50 mM SDS concentration system is less than 0.2 mL; much

less than 0.5 mL used for antifoaming experiments and (2) Figure 4-2 shows that a clove









bud oil amount as small as 100 pL/15 mL of foaming solution is sufficient enough for a

strong antifoaming action.

9
-- Clove Oil





3-
6
5






0 20 40 60 80 100
SDS Concentration (mM)


Figure 4-8 Spreading pressure of clove bud oil after being extracted with SDS
solutions of varying concentrations.

The hypothesis of the ineffectiveness of the clove bud oil, if correct, should

manifest itself: (1) at the air/surfactant solution interface in terms of a smaller spreading

area and (2) in possibly an increased droplet entry barrier. Figure 4-8 shows the spreading

diameter obtained using 5 VL of clove bud oil that remains after 0.5 mL of oil is extracted

with 15 mL of varying concentrations of SDS solution. It is observed that the spreading

diameter steeply decreases first, up to a SDS concentration of about 25 mM, where after

the decline becomes much more gradual. This is in good correlation with antifoaming

performance in Figure 4-7. The droplet entry barrier experiments at a 0.2 volume ratio of

extracted clove bud oil to dodecane is also found to be considerably increased with

droplet interface merging times in excess of 30 s (compare with unextracted clove bud oil









+ dodecane at 0.2 volume ratio values of ~ 2 s). This indicates that the mass transfer

across interface of droplet promotes spreading and hence antifoaming process.

At high surfactant concentrations, in terms of spreading pressure and drop entry

barrier, both classes of essential oils might behave similarly, i.e., lower spreading and a

higher barrier, but it should be noted that the active antifoaming components (from class I

oils) would still be present in the system. Their locus might have changed due to the

sequestration in the micellar phase, but they should be able to influence the bubble

lifetime more dramatically than the class II essential oils due to spreading on foam

lamellae. Figure 4-9a and 4-9b shows the foam stability of a 200 mM SDS surfactant

solution at various volume ratios of essential oil (one from each class) and dodecane

mixtures. It can be seen that: (1) for essential oil to dodecane volume ratio less than 0.3,

clove bud oil system destroys foam faster in comparison to cypress oil and (2) for ratios

greater than 0.3, both systems have a similar and rapid foam destabilization potential.

Apart from the usual emulsion droplet entry mechanism in the GPB for foam

destabilization, one might conceive a possibility of bubble rupture in the thin film itself

due to the presence of micelles with active components solubilized in their core. While

emulsion droplets may be squeezed out of the foam lamella quickly, the presence of

micelles in the foam lamella for longer time scales is well documented [Nikolov and

Wasan, 1992; Patel et al., 1996]. In the subsequent process of micellar break-up, the

antifoam molecules may be released near the air/solution interface to be finally adsorbed

at the interface resulting in a local region covered by non-cohesive molecules with high

spreading pressure; a soft spot on the foam lamellae susceptible to rupture.




























0 20 40 60 80 100 120 140 160 180

Time (minutes)


0 ... 0 -- -0


S-- 0.2
.... ... ... .3
----- 0.4


.5


\0
S) 0 0- 0-




(b) V- V- V- o
V 7- "V = V- -y..,._V.. _.. _=^__v


0 50 100


Time (min)



Figure 4-9 Effect of essential oil volume fraction in dodecane on the foam stability of
a 200 mM sodium dodecyl sulfate solution (15 mL surfactant + 0.5 mL
antifoam): (a) Clove bud oil and (b) Cypress oil


Looking closely at the foam decay profile, one notices a step wise decay in foam

volume. Since the foam column is uniform across its entire height in terms of the


120


100


----0.2
........ ....... 0 .3
---7--- 0.4
---- ..0.5


120


100









presence of micelles in the lamella, the micelle-assisted rupture of thin film could have

randomly occurred anywhere along the foam column and not necessarily in a stepwise

manner starting from the top of the column. This observed inconsistency suggests that

micelles are definitely not the primary reason; they might just be acting in tandem with

the dominant emulsion droplet entry and subsequent spreading mechanism. The

possibility of emulsion droplets undergoing coalescence in the foam lamella forming

bigger droplets with small entry barrier should also be kept in mind.

Our mechanism for foam decay behavior is that as time progresses, the drainage

leads to GPB's with smaller and narrower cross-section, in the process applying more and

more pressure on trapped emulsion droplets forcing them to induce a rupture at a

air/solution interface which itself is constantly getting weakened simultaneously through

a supply of non-cohesive molecules from micelles, emulsions and molecularly

solubilized oil. The mechanical shock produced by the rupture of a bubble results in a

catastrophic effect that destroys several other bubbles. And above a certain amount of

essential oil (volume ratio = 0.3), the interfaces are so weak that one rupture event is able

to destroy a much larger volume of the foam.


4.4 Conclusions

(1) Clove bud, cinnamon and vetiver essential oils are found to be efficient antifoaming

agents for low surfactant concentration systems while cypress and eucalyptus oils are

ineffective.

(2) The antifoaming action is strongly correlated to the spreading pressures and the

droplet entry barrier; higher the spreading pressure and lower the entry barrier, higher

the antifoam efficacy. The spreading pressures of clove bud, cinnamon and vetiver in








mixtures with dodecane are comparable to the most commonly used PDMS based

antifoams for low SDS concentration systems.

(3) A certain amount of foam always remains in the cylinder irrespective of the antifoam

used. This is explained in terms of the relative sizes of Gibbs Plateau Border and the

emulsion droplet sizes trapped in them. Towards the lower part of the foam column

the plateau border cross-section is quite large and hence the droplets are not forced

into the surface for a drop entry event and subsequent rupture of the film.

(4) At high surfactant loadings (from 50 mM on to 200 mM SDS), the active antifoaming

component of clove oil partitions and gets trapped into the micellar phase, hence

becomes unavailable to be adsorbed at the interface to influence antifoaming by

spreading. And an emulsified oil that is ineffective both in terms of spreading and

entering the air/solution interface is left in the solution.

(5) Foam column destabilization with class I essential oils is much better than class II

essential oils below an essential oil to dodecane volume fraction of 0.3 due to smaller

entry barrier and higher spreading pressure. For volume fractions > 0.3, both the

classes destabilize the foam to an equivalent extent, possibly because of an interface

crowded by a high concentration of non-cohesive molecules which is susceptible to a

quick rupture from the emulsion droplet. The occurrence of steps in the foam stability

is explained by a drainage assisted droplet entry into the air/solution interface where

one entry event triggers an avalanche of ruptures.














CHAPTER 5
EFFECT OF COSURFACTANT CHAIN LENGTH ON PROPERTIES AT THE
LIQUID-LIQUID INTERFACE



5.1 Introduction

Emulsions are thermodynamically unstable, heterogeneous dispersions of one

liquid in another with droplet sizes typically exceeding 0.1 Pm and require small amounts

of surfactant. Emulsions can be either oil in water or water in oil type depending on water

to oil ratio, temperature, surfactant structure and concentration, order of mixing

components and presence of salt. They are of considerable interest to a variety of

industries such as paints, polishes, pesticides, metal cutting oils, food, textile processing,

cosmetics and polymerization and hence are an active area of research.

Microemulsions on the other hand are isotropically clear, thermodynamically

stable multi component systems of surfactant (and cosurfactant), oil and water and are

stabilized by a very soft surfactant film that can be spontaneously broken by the kT

thermal energy of the system [Degennes and Taupin, 1982]. The surfactant concentration

required for microemulsion formation is quite high in comparison to that for ordinary

macroemulsions. The nanosized droplets of water (or oil) in oil (or water) continuous

phase are not static in nature and keep coalescing and disintegrating in a manner similar

to micelles, leading to the exchange of reactants dissolved in the water core. The

collisions are 'sticky' in nature owing to a strong attractive interaction (100 times

stronger than the Van der Waals attraction) which is believed to be resulting from an








energetically favorable overlap of the tips of the surfactant tails as two neighboring

droplets approach each other [Caljie et al., 1977; Hamilton et al., 1990; Lemaire et al.,

1983; Roux et al., 1984].

The role of cosurfactant is of course very important. In microemulsions where the

length scales of droplet (- 10 nm) and surfactant (- 1 or 2 nm) are within an order of

magnitude, insertion of few molecules of cosurfactant can bring about dramatic change in

surfactant film properties such as solubilizate or reactants exchange [Bagwe and Khilar,

2000; Lisecki et al., 1995]. For emulsions however, the cosurfactant mainly influences

the interfacial rheological properties [Bertilla et al., 2004].

In this chapter, the focus is on understanding the role of alkyl chain length of the

cosurfactant on both microemulsion (water in oil) and emulsion (oil in water) properties.

The microemulsion property of interest here is the kinetics of solubilizate exchange and

has been studied via the rates of silver nanoparticle formation. For emulsion, the property

of interest is its stability. Another scope of this chapter is to expand upon Oh and Shah's

earlier work [1993b], connecting micellar stability and emulsion droplet size.

5.2 Experimental Method

5.2.1 Materials
Sodium dodecyl sulfate (SDS) of 99% purity, Sodium borohydrate of 98% purity,

hexadecane and silver nitrate were purchased from Sigma, St. Louis, USA. Octyl, decyl,

dodecyl, tetradecyl and hexadecyl trimethyl ammonium bromide (CnTAB: n = 8, 10, 12,

14, 16) were supplied by TCI America Inc. Non-ionic surfactants Tween 20, Tween 40

and Tween 60 were generous gifts from Uniqema, Delaware, USA. All of the chemicals

were used as received. De-ionized, distilled water was used for all the solution

preparations.








5.2.2 Preparation of Emulsion

Emulsions are produced by vigorous shaking of 2 mL of hexadecane with 10 mL

of 105 mM (100 mM SDS + 5 mM CnTAB) mixed surfactant solution on a vortex mixer

for 10 s. Immediately after mixing, a sample of emulsion was added to 125 mL of water

in the coulter counter particle sizer for drop size measurements. Emulsion stability

measurements were performed by observing the emulsion/water interface movement of a

60 vol% (6 mL hexadecane + 4 mL 105 mM surfactant solution) hexadecane in water

emulsion in a 10 mL graduated cylinder.


5.2.3 Interfacial Viscosity Measurements

Interfacial viscosity was measured in a deep channel surface viscometer. 30 mL

of hexadecane was slowly placed on top of 100 mL of 105 mM mixed SDS/CnTAB

solutions. A very small piece of Teflon particle was placed on top of hexadecane layer,

which was gently pushed by a sharp needle to make it sink in the hexadecane layer and

stop at the hexadecane/surfactant solution interface. This enables the placement of tracer

Teflon particle at the alkane/surfactant solution interface. The interfacial viscosity data

was then measured in the usual way by measuring the rotational speed of the particle

against the rotational speed of the plate.


5.2.4 Atomic Force Microscope

The atomic force measurements were made at a mixed SDS/CTAB concentration

of 1.1 mM (molar ratio SDS/CnTAB = 10) in the presence of 0.1 M sodium chloride

electrolyte on an alumina surface at pH 7.








5.2.5 Preparation of Silver Nanoparticles

Nanoparticles were produced by mixing water in oil microemulsion containing

silver nitrate with another compositionally equivalent water in oil microemulsion

containing sodium borohydride. Before making nanoparticles, all microemulsion systems

were observed between crossed polarized sheets oriented perpendicularly to each other

(with respect to the plane of polarization of light) for any possible phase separation. The

molar concentrations of silver nitrate and sodium borohydride in the microemulsions

were kept at 4.7 x 10-4 M and 2.05 x 104 M, respectively for both methods. The water-to-

surfactant molar ratio, R, was kept constant at 7.5 for all the studies.

Particle size and aggregation was followed over time through spectral changes, on

an ultraviolet-visible (UV-Vis) spectrophotometer (Hewlett-Packard, model 8453) in I

cm path-length quartz cuvets.

Transmission electron microscopy (TEM) measurements for measuring particle

sizes were done at Interdisciplinary center for biotechnology research (ICBR) at

University of Florida on a Hitachi at an operating voltage of 75 kV. For sample

preparation a drop of microemulsion containing nanoparticles was placed on the butvar

coated nickel grid placed onto a clean filter paper. Particle size distributions and number

averaged particle diameters were calculated manually using Scion Image software from

Scion Corporation.

Percolation studies were done by measuring conductivity changes on an Oakton

conductivity meter with a digital output, as a function of increasing water content in the

microemulsion systems.









110
Total surfactant concetra = 105 mM
100 [SDS]/[CnTAB] = 20
E
90


0
70
o
-C 60
E
50

40
SDS SDS SDS SDS SDS
+C8TAB +CI0TAB +C12TAB +Cj4TAB +C16TAB


Figure 5-1 Effect of co-surfactant chain length on emulsion droplet size at a total
surfactant concentration of 105 mM.

5.3 Results and Discussion

5.3.1 Emulsions
In order to assess the effect of alkyl chain length of cosurfactant on emulsion

properties, SDS and CnTAB (n = 8 16) in the molar ratio of 20/1 are used as mixed

surfactant systems. Figure 5-1 shows the volume average emulsion droplet size as a

function of the chain length of cationic co-surfactant at a total surfactant concentration of

105 mM. It can be seen that the average droplet size is maximum when the chain length

of surfactant and co-surfactant are equal. Analysis of an emulsion system for mechanisms

controlling droplet size is usually difficult because of the presence of a multimodal

droplet size distribution. However, since analogous mechanisms control the foaming and

emulsification behavior [Oh, 1992; Oh and Shah, 1993b], indirect insights regarding

emulsification mechanisms can be obtained from foaming experiments. In this regard,

bubbles can be produced in a controlled manner in a maximum bubble pressure setup for

measuring dynamic surface tension.









Figure 5-2 shows the bubble volumes as a function of the chain length of cationic

co-surfactant at a total surfactant concentration of 21 mM. This low concentration is

chosen because at high surfactant concentrations, deviations from equilibrium surface

tension are negligible at the bubble frequencies accessible with the current setup. Here

again, one observes that the bubbles with maximum volume are produced in a SDS +

CI2TAB system.

22
Total surfactant concentr = 21 mM
21 [SDS]/[CnTAB] = 20

S20 -
E
E
N
18

S 17 -

16

15
SDS SDS SDS SDS SDS
+C8TAB +CIoTAB +C12TAB +C14TAB +C16TAB


Figure 5-2 Effect of co-surfactant chain length on bubble size at a total surfactant
concentration of 21 mM.

The interfacial area generated on applying a work is related to the dynamic

interfacial/dynamic surface tension according to equation (2.3). Thus for the same

amount of work done, a system with higher dynamic interfacial/surface tension will result

in a smaller interfacial area, i.e., larger droplet/bubble sizes for a given amount of gas or

oil. The interfacial/surface tension is directly related to the number of monomers

adsorbed at the interface and is dependent on: (1) rate of monomer diffusion and (2)

stability of association structures. Since, all the mixed surfactant systems under study are








predominantly SDS systems, the rate of monomer diffusion should not be significantly

different. And for SDS monomers, the characteristic diffusion time has been shown in

chapter 2 to be on the order of 105 s indicating that there is no resistance from diffusion.

Thus, it is the micellar stability that appears to be controlling the monomer flux for these

systems, i.e., the higher is the micellar stability; the smaller is the monomer flux and the

higher is the dynamic interfacial/surface tension. Patist et al. [1997] have studied the

micellar relaxation time (12) for these systems and found a maximum for SDS + CI2TAB

system which further corroborates our hypothesis. Stable micelles such as those for SDS

+ CI2TAB cannot quickly supply the monomers resulting in higher dynamic

interfacial/surface tensions and hence lower interfacial area while relatively unstable

micelles for SDS + C8TAB or SDS + C16TAB can rapidly disintegrate resulting in lower

dynamic interfacial/surface tensions and higher interfacial area. The molecular

mechanism for the chain length compatibility effect has been reviewed in a recent review

article [Shiao et al., 1998].

Apart from the emulsion droplet size, another crucial parameter in emulsion

studies is its stability. An emulsion can become unstable either by coalescing or

flocculating. The primary emulsion instability mechanism for mixed SDS + CnTAB

systems apparently is because of flocculation as there was no significant difference in

coalescence for all the mixed systems as observed from studies under microscope (data

not reported). Systems with 16 vol% dispersed phase content (10 mL surfactant solution

+ 2 mL hexadecane) are difficult to study for any possible differences in emulsion

stability because of a diffuse separating interface. So emulsion stability experiments were

done at a higher dispersed phase content of 60 vol% (4 mL surfactant solution + 6 ml








hexadecane). Figure 5-3 shows the upward movement of emulsion-water interface as a

function of time for mixed surfactant systems at a total aqueous surfactant concentration

of 105 mM. It can be observed that flocculation is comparatively slower for mixed

systems of SDS + C12TAB and SDS + CI4TAB than SDS + CnTAB (n = 8, 10, 16).

2.2
Total surfactant concentration = 105 mM
S2.0 [SDS]/[CnTAB] = 20 ^,
1.8



1.2 / 0 SDS/C8TAB
C 1.0 ........ o .. ..... SDS/C10TAB
0.8 / ---v--- SDS/CI2TAB
v 0.6_.._.. SDS/C4TAB
S0.4- --- SDS/C1 6TAB
> 0.2 ....
0 1000 2000 3000 4000 5000 6000 7000
Time (sec)

Figure 5-3 Effect of co-surfactant chain length on creaming rate at a total surfactant
concentration of 105 mM.

Since concentrated emulsions are similar to foams, as a corollary, the above

results can also be interpreted as a slower water drainage from thin liquid lamella

separating two emulsion droplets for SDS + CI2TAB and SDS + CI4TAB systems and

vice versa. The drainage rate from thin liquid films in both emulsions and foams has been

shown to correlate well with the interfacial/surface viscosities. In general, the tighter is

the molecular packing at the interface, the higher is the interfacial/surface viscosity, the

lower is the drainage rate and higher is the stability of an emulsion or foam. Figure 5-4

shows the dimensionless interfacial viscosities for the mixed surfactant systems at a




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