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IN SURFACTANT SOLUTIONS: FROM MICELLES TO
MONICA A. JAMES-SMITH
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
Monica A. James-Smith
To my parents
who have been my #1 supporters since October 17, 1977.
I thank my Almighty Heavenly Father for allowing me to make it to this point and for
seeing me through every obstacle that arose. I am forever grateful to my husband, Rod, for all of
his support, love and encouragement. I sincerely thank my parents, Dan and Elaine James, for
always believing in me, for their constant prayers, and for always providing the right words
when the journey seemed difficult. I would like to thank Melanie, Dan, Chris, and Bruce for
knowing how to make me feel like I can accomplish anything. I owe a huge debt of gratitude to
my best friend, Brandi Chestang, who has been there to answer every phone call and has cheered
me on all my life. I am also greatly appreciative to all of my other friends, family, and loved
ones. I must also extend my sincerest appreciation to my in-laws who have taken me in as a
family member and provided tremendous support as I have pursued this degree.
I am forever grateful to Dr. Dinesh O. Shah for being a mentor, an advisor, and a
confidant, for providing me with the highest caliber of guidance and for always pushing me
towards greatness. I would like to extend sincerest thanks to Drs. Brij Moudgil, Manoj Varshney,
Yakov Rabinovich, Anuj Chauhan, Oscar Crisalle, Ranga Narayanan, Donn Dennis, and Richard
Dickenson for stimulating conversation and insightful comments and suggestions. I also would
like to thank all of my colleagues who have been of great assistance throughout me years here at
Last, but definitely not least, I would like to thank Pastor Kevin W. Thorpe and the Faith
Baptist Church family for showing me that my family is bigger than I think and for helping me to
make it through this journey.
TABLE OF CONTENTS
ACKNOWLEDGMENTS .............. ...............4.....
LIST OF TABLES ........._..__..... ._._ ...............8....
LIST OF FIGURES .............. ...............9.....
AB S TRAC T ............._. .......... ..............._ 12...
1 LITERATURE REVIEW ................. ...............14...............
1.1 Micelles............... ...............14
1.1.1 Introduction ................ .. ......... ..........1
1. 1.2 Dynami c Nature of Mi cellar Soluti ons ............_...... ..............1
1.2 M acroemulsions................ ............2
1.2.1 Emulsion Droplet Size............... ...............26..
1.2.2 Viscosity of Emulsions.............__ ..........__ .........__ ...........2
1.2.3 Determination of Emulsion Type (O/W or W/O) ................. ................. ......28
1.2.4 Emulsion Stability .............. ...............29....
1.2.4. 1 Coalescence ........................ ._ .. ..... .__ ............2
18.104.22.168 Charge stabilization: The electrical double layer............. ..._.........__ ...30
22.214.171.124 Phase inversion in emulsions .............. ...............31....
126.96.36.199 Emulsion creaming............... ...............3
1.2.5 Demulsification ............... ........ .. .............3
1.2.6 Surfactant Selection for Emulsification ....._ .....___ ..........._ ........3
1.2.7 Applications of Emulsions .............. ...............34....
1.3 Microemulsions .............. .... ...............35.
1.3.1 Formation of Microemulsions .............. ...............41..._.__ ...
1.3.2 Applications of Microemulsions .............. ...............42....
1.3.3 Nano-emulsions ............ ...... .___ ...............42...
2 A NOVEL METHOD TO ELUCIDATE THE PRESENCE OF SUB-MICELLAR
AGGREGATES IN SURFACTANT SOLUTIONS .............. ...............61....
2. 1 Introducti on ................. ...............61........... ...
2.2 Experimental Procedure............... ...............6
2.2. 1 M ateri al s ................ ...............63..............
2.2.2 Ultracentrifugation ...................... .. .... .. .. ..... .......6
2.2.3 Two-Phase Dye Transfer (Methylene Blue Complexation) and UV-Vis
Analy si s............... ...............64
2.2.4 Foamability ................. ...............64.......... ......
2.2.5 Fabric W getting ................. ...............65................
2.2.6 Dynamic Surface Tension .............. ...............65....
2.3 Results and Discussion .............. ...............65....
2.3.1 SDS Surfactant Solutions .............. ... .... ... .... ... ............6
2.3.2 Effect of Counter-lons on Sub-Micellar Aggregate Concentration .......................73
2.3.3 Importance of Sub-Micellar Aggregates in Technological Processes..................75
188.8.131.52 Foaming ............ ...... ._ __ ...............76....
184.108.40.206 Fabric wetting............... ...............77
220.127.116.11 Dynamic surface tension .............. ...............77....
2.4 Conclusions............... ..............7
3 DETERMINATION OF DRUG AND FATTY ACID BINDING CAPACITY TO
PLURONIC Fl127 IN MICROEMUL SINS FOR DETOXIFICATION. ................... ..........94
3 .1 Introducti on ................. ...............94........... ...
3.2 Experimental Procedure............... ...............9
3.2.1 Material s. .............. ..... ...............96.
3.2.2 Microemulsion Preparation .............. ...............96....
3.2.3 Turbidity Analysis ................ ...............96................
3.2.4 Dynamic Surface Tension .............. ...............97....
3.2.5 Foamability ................. ...............97.......... .....
3.2.6 Fabric W getting ................. ...............98.......... .....
3.2.7 Surface Tension ................. ...............98................
3.3 Results and Discussion ................. .. ........... ... .... .... ........9
3.3.1 Effect of Sodium Caprylate Concentration on Drug and Fatty Acid Binding to
Mi croemul si ons ............... .... .... ... .... ..... ... .. ...............98.
3.3.2 Determination of Free Fatty Acid by Dynamic Processes .............. ... ................101
3.3.3 Effect of Fatty Acid Chain Length on Drug and Fatty Acid Binging to
M i croem ul si ons ............... ... ........ ....... ..... ........ ... ...... .. ............ 0
3.3.4 Effect of the Number of Ethylene Oxide (EO) and Propylene Oxide (PO)
Groups of Pluronics on Fatty Acid and Drug Binding ................. ................. ... 105
3.4 Conclusions............... ..............10
4 A Novel Method to Quantify the Amount of Surfactant at the Oil/Water Interface and to
Determine Total Interfacial Area of Emulsions ................. .........____...... 117___ ...
4. 1 Introducti on .................. ...............117._____....
4.2 Experimental Procedure .....___................. ...............119 ....
4.3 Results and Discussion ................. ........_ ._ ........ ........ ..... ........ .........12
4.3.1 Effect of Surfactant Concentration on Partitioning to the Oil/Water Interface....121
4.3.2 Effect of Alkyl Sulfate Chain Length on Partitioning to the Oil/Water
Interface ........... .. .......................... ......................12
4.3.3 Effect of Oil Chain Length on SDS Partitioning to the Oil/Water Interface........ 128
4.3.4 Effect of Alcohol Chain Length on SDS Partitioning to the Oil/Water
Interface ............ ... .. ........................................ 1
4.3.5 Effect of Oil Phase Volume Fraction on SDS Partitioning to the Oil/Water
Interface .............. ...............13 1...
4.4 Conclusions.............. .............13
5 SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK ............... .... .........._.146
5.1 Micelles............... ...............14
5.1.1 Summary............... ...............146
5.1.2 Future W ork. ........._.. ..... ._ ._ ...............147..
5.2 M icroemulsions .............. ...............149....
5.2.1 Summary............... ...............149
5.2.2 Future W ork ................. ...............15. 1..............
5.3 M acroemul sions............... ............15
5.3.1 Summary............... ...............152
5.3.2 Future W ork ................. ...............155........... ...
A GIBBS ADSORPTION EQUATION AND AREA PER SURFACTANT MOLECULE
DETERMINATION ................. ...............157......... ......
B CALCULATION OF TOTAL INTERFACIAL AREA FROM FILTRATION
RE SULT S ................. ...............160......... ......
C CALCULATION OF TOTAL INTERFACIAL AREA FROM DROPLET SIZE
RE SULT S ................. ...............161......... ......
LIST OF REFERENCES ................. ...............162................
BIOGRAPHICAL SKETCH ................. ...............173......... ......
LIST OF TABLES
1-1: Summary of methods used to produce emulsions ................. ...............54........... .
1-2: Common tests for determining emulsion type (W/O or O/W)44....... ..............~~~~~~~~~~~~~.55
1-3: Types of breakdown processes occurring in emulsions ................. .........................56
1-4. Factors influencing the stability of emulsions .............. ...............57....
1-5. Parameters that affect phase inversion in emulsion and the effect they have. ................... 58
1-6. Commonly used physical methods of demulsification ................ .......... ...............59
1-7. A summary of HLB ranges and their application .....___._ .... ... .__ ......._._.......5
1-8. Microemulsions vs. Nano-emulsions. ......___ ........__ ...._ ............6
2-1. Dimensionless dynamic surface tension (6) of different counter-ions of dodecyl
sulfates (50 mM) at a bubble lifetime of 50 msec (from ref 33) ................. ................. .93
3-1. Effect of fatty acid chain length on maximum binding ................. .......... .............1 16
3-2. Effect of # of PO groups on maximum binding ................. ...............116............
3-3. Effect of # of EO groups on maximum binding ................. ...............116............
4-1. Effect of SDS concentration on total interfacial area 1% (v/v) hexadecane-in-water
em ulsions. ............. ...............144....
4-2. Area per molecule values at the hexadecane/water interface for alkyl sufates and
total interfacial area as a function of alkyl sulfate chain length .............. ....................144
4-3. Effect of oil chain length on total interfacial area (TIA) .............. ......................145
4-4. Effect of alcohol chain length on total interfacial area (TIA) ................. ............... ....145
LIST OF FIGURES
1-1. Schematic diagram of a surfactant molecule, micelle, and reverse micelle. .....................48
1-2. Properties of surfactant solutions showing abrupt change at the solution critical
micelle concentration (cmc) ................. ...............48........... ....
1-3. Schematic design of micellar solution. ................ .............. ......... ........ .....49
1-4. Schematic diagram of the four maj or micellar structures ................. .......................49
1-5. Mechanisms for the two characteristic relaxation times for a micelle............... ................50
1-6. Typical size distribution curve of aggregates in a micellar solution .............. .................50
1-7. Schematic of sodium counter-ion "cloud" around SDS spherical micelle. .......................51
1-8. Schematic diagram of the adsorption of surfactant monomers from the bulk to the
oil/water interface during emulsion formation .............. ...............51....
1-9. The emulsion droplet size in the hexadecane/SDS solution system .............. .................51
1-10. Schematic depiction of the Stern-Graham model of the electrical double layer. ..............52
1-11. Schematic diagram of an oil-in-water (O/W) microemulsion .............. .....................5
1-12. Thermodynamic explanation for behavior of macroemulsions and microemulsions ........53
2-1. Schematic diagram of the ultracentrifugation process ......____ ... ......_ ..............80
2-2. Size distribution curves of aggregates in a micellar solution .............. ....................8
2-3. Schematic diagrams of surfactant solutions, filtration of solutions, and plot of filtrate
concentration as a function of total surfactant concentration. ................ ............... .....82
2-4. Schematic representation of the two possible reaction paths for the formation of
m icelles. ............. ...............84.....
2-5. Filtration of SDS through 10,000 MWCO ultracentrifuge tubes. ...........__.................85
2-6. Tailoring of micellar stability. ...........__.....___ ........ ......... ...............85
2-7. Filtrate of SDS+C12TAB through 10,000 MWCO ultracentrifuge tubes .......................86
2-8. Filtration of SDS alone or SDS + C12X (X = OH or TAB) through 10,000 MWCO
ultracentrifuge tubes............... ...............87.
2-9. SDS concentration in the filtrate for 80:20 SDS:C12TAB systems after filtration
through 3,000 and 10,000 MWCO tubes, as compared to pure SDS solutions (50
mM ) ................. ...............88.................
2-10. Filtrate surfactant concentrations for 25 mM lithium dodecyl sulfate (LiDS), sodium
dodecyl sulfate (NaDS), and cesium dodecyl sulfate (CsDS) and 12.5 mM
magnesium dodecyl sulfate (Mg(DS)2). ............. ...............89.....
2-11. Schematic depiction of foam column ................ ............. ......... ........ .......90
2-12. Foamability of SDS micellar solution and SDS + C12X mixed micellar solutions ...........90
2-13. Wetting time of lin2 Strips of 50:50 cotton:polyester blend fabric............... .................9
2-14. Dynamic surface tension of solutions of 50 mM SDS and 50 mM SDS + 12.5 mM
C 12TAB .............. .. ...............92................
3-1. Amitriptyline Hydrochloride, MW = 313.9............... ...............109.
3-2. Titration of microemulsions with 0.2 M AMT ..........._ ..... ..__ ......__ .........0
3-3. Titration of mixed micelles and microemulsion systems. ................ ............ .........1 10
3-4. Schematic diagram of turbidity in various solutions ................. ......... .................11 1
3-5. Titration of microemulsion systems with AMT ([SC] = 25 -125 mM) ................... ........1 12
3-6. Properties of Pluronic F l27 microemulsion. ................ ...............113........... ..
3-7. Binding of SC and drug to Fl27. ................ ...............114........... .
3-8. Schematic depiction of microemulsion droplet. ........... ..... .__ ..........__......11
4-1. Schematic depiction of filtration of oil-in-water emulsion through nanoporous filter
membrane ...... ................. ...............134......
4-2. Effect of total SDS concentration on SDS concentration in the filtrate ................... .......13 5
4-3. Effect of SDS concentration on mean droplet diameter of 1% (v/v) hexadecane-in-
water emulsions. ............. ...............136....
4-4. Master diagram showing the changes in emulsion characteristics with increasing
SDS concentration.. ............ ...............137.....
4-5. Amount of alkyl sulfate surfactant that partitions to the interface as a function of
chain length ....._ ................. ........__ _..........13
4-6. Mean droplet size of hexadecane-in-water emulsions as a function of alkyl sulfate
chain length. Droplet size was determined by light scattering. ............. ....................13
4-7. Effect of oil chain length on the amount of SDS that partitions to the interface. ............140
4-8. Schematic depiction of emulsion droplets ................ ...............141.....__._...
4-9. Effect of alcohol chain length on the amount of SDS that partitions to the interface
and mean droplet size............... ...............142.
4-10. Effect of oil phase (hexadecane) volume fraction on partitioning of SDS to the
oil/water interface. ............ .............143......
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
MOLECULAR INTTERACTIONS INT SURFACTANT SOLUTIONS: FROM MICELLES TO
Monica A. James-Smith
Chair: Dinesh O. Shah
Co-chair: Brij M. Moudgil
Major Department: Chemical Engineering
To effectively use surfactants for various applications, one must have a basic
understanding of the molecular interactions occurring within surfactant solutions. Micelles,
microemulsions, and macroemulsions are three of the most commonly investigated surfactant
containing systems. For many years, these systems have been studied in light of their structure,
properties, and applications. In this dissertation, we investigate these systems using filtration
through nanoporous membranes to understand the role of monomers and sub-micellar aggregates
in controlling their properties.
It is commonly accepted that micellar solutions consist of three surfactant species;
adsorbed monomers at the air/water interface, monomers dispersed in the aqueous phase, and
micellar aggregates. Using filtration through nanoporous membranes, we found evidence
suggesting that another species, sub-micellar aggregates, exists, in significant concentration, in
micellar solutions. This is a potentially revolutionary finding that provides for a more accurate
picture of micellar solutions and yields insight to the mechanism by which micellar stability
affects dynamic processes.
The presence of monomers and sub-micellar aggregates in microemulsions is important in
determining the efficacy of microemulsions for drug binding. We used filtration through
nanoporous membranes and turbidity analysis to delineate the drug-fatty acid monomer
interactions. The results showed that one Pluronic F l27 molecule binds a maximum of eleven
sodium caprylate fatty acid molecules and twelve Amitriptyline drug molecules. We were also
able to determine that the initial driving force for Amitriptyline uptake is electrostatic attraction.
Filtration through nanoporous membranes is ideally suited to determine an unresolved
issue of surfactant portioning at the oil/water interface and in the bulk solution in
macroemulsions. We were able to approximate the total interfacial area (TIA) of the emulsions
and showed that this calculation produced values approximately two orders-of-magnitude larger
than the area calculated using mean droplet sized from light scattering analysis. Possible
explanations for this difference are discussed.
Surfactants, or "surface-active agents," are used to enhance the quality of products used in
every aspect of life; from food to cosmetics, from oil recovery to detergency, and even from
pharmaceutics to chemical mechanical polishing of silicon wafers. Given these many diverse
applications, it becomes critical to have a sufficient understanding of the molecular interactions
that occur within surfactant solutions in order to effectively manipulate them for a specific use.
Three invisible compartments in a surfactant solution are an adsorbed film, monomers, and
micelles made up of surfactant molecules.l Surfactant molecules are continually exchanging
among these three compartments, and hence surfactant solutions are in dynamic equilibrium. For
technological processes, such as foaming, emulsification, and wetting, surfactant molecules are
driven to the new surfaces and it becomes necessary for micelles to disintegrate and provide
monomers to these surfaces. In such rapid processes, micelle stability could become a rate-
The following is the detailed review of micelles, microemulsions, and macroemulsions.
A surfactant, or surface-active agent, is defined as a substance that adsorbs onto surfaces
or interfaces of solutions to lower the surface or interfacial tension of the system.3 The
magnitude of the lowering of the surface or interfacial tension depends on the surfactant
structure, concentration, and the physico-chemical conditions of the solution (e.g. pH, salt
concentration, temperature, pressure, etc.).3 Surfactants are typically amphiphatic species,
meaning that they are made up of a hydrophobic component, referred to as the "tail," and a
hydrophilic component, referred to as the "head" group (see Figure 1-1). When placed in
solution, surfactant molecules tend to orient in such a way as to minimize the interactions of the
hydrophobic "tail" with water in aqueous solutions, or to minimize the interactions of the
hydrophilic "head" with oil in organic solvents. This leads to adsorption of the surfactant
molecules onto surfaces or at interfaces, and above a certain concentration, known as the critical
micelle concentration or "cmc," surfactants form aggregates known as micelles. When placed
into aqueous solutions, surfactant molecules will form spherical aggregates at the cmc where the
hydrophobic tails are pointed inward and removed from interaction with water molecules by the
hydrophilic head groups as shown in Figure 1-1. When placed into organic solutions, surfactant
molecules will form reverse micelles with the hydrophobic tails pointed outward (see Figure 1-
When the critical micellar concentration, or cmc, is reached, many of the physical
properties of the surfactant solution in water show an abrupt change as shown in Figure 1-2.
Some of these properties include the surface tension, osmotic pressure, electrical conductivity,
and solubilization. The cmc is a measure of the free monomer concentration in surfactant
solutions at a given temperature, pressure, and composition. Mcbain4 fifSt investigated the
unusual behavior of fatty acid salts in dilute aqueous solution at the cmc in the 1910s and 1920s
and was followed by Hartley5, 6 in the 1930s. Other evidence for surfactant aggregation into
micelles was obtained from vapor pressure measurements and the solubility of organic molecules
in water. The formation of colloidal-sized clusters of individual surfactant molecules in solution
is known as micellization.
McBain first suggested that micelles are spherical in shape. However, the first concrete
model for spherical micelles is attributed to Hartley.6 It is currently accepted in the field that
above the cmc, a typical surfactant solution consists of three major species: 1) surfactant
molecules dispersed as monomers in the aqueous phase, 2) aggregates in the form of micelles,
and 3) adsorbed films at the air/liquid interface (see Figure 1-3). The surfactant is in dynamic
equilibrium between these states, implying that the rates of adsorption and desorption are equal.
Thus, at a given temperature, pressure, and concentration, the number of monomers adsorbed at
the air/water interface, the number of monomers in the bulk phase, and the number of micelles
present in solution is fixed under equilibrium conditions. The concentration of monomers and
micelles changes with equilibrium conditions such as pressure, temperature, or surfactant and
The micellization process is primarily an entropy-driven process.7" When surfactants are
dissolved in water, the hydrophobic tail disrupts the hydrogen-bonded structure of water thereby
increasing the free energy of the system. As previously mentioned, for this reason surfactant
molecules will concentrate at interfaces so that their hydrophobic tail groups are removed or
directed away from the water minimizing the free energy of the solution. The formation of
micelles is yet another means that the system will use to reduce its free energy. The orientation
of the hydrophobic tails in the interior of the micelle decreases their interaction with water
molecules. However, the surfactant molecules that are confined in the micelle conceivably
experience some loss of freedom. In addition, in the case of ionic surfactants, the molecules that
are present in the micellar aggregate may experience electrostatic repulsion from other similarly
charged surfactant molecules. These forces increase the free energy of the system and oppose
micellization. Hence, micelle formation depends on the force balance between the factors
favoring micellization (Van der Waals and hydrophobic forces) and those opposing it (kinetic
energy of the molecules, electrostatic repulsion, and chemical potential factor due to
concentration gradient for micelles and monomer).9
The number of surfactant molecules that make up a micelle is known as the aggregation
number. This number can be determined through the use of NMR self-diffusion coefficients,'o
small-angle neutron scattering,11, 12 freezing point and vapor pressure methods, osmotic pressure,
and fluorescent probes. The surfactant structure plays a significant role in the aggregation
number of the micelles formed by a give surfactant. The aggregation number tends to increase
with increasing chain length of the hydrophobic tail and decrease with increasing size (i.e., cross
sectional area) of the hydrophilic head group. The aggregation number is also affected by the
nature of the aqueous phase. For example, the addition of neutral electrolytes to ionic surfactant
solutions leads to an increase in the aggregation number. This is due to a phenomenon known as
"salting out" by which the effective amount of water that is available to solubilize the surfactant
molecules is decreased because it is used up to accommodate the ions (a more favorable
interaction). Increasing the temperature of an ionic surfactant solution typically leads to a small
decrease in aggregation number which can be owed to an increase in the size of the head group
as a result of thermal agitation as well as the increase in the kinetic energy of the surfactant and
solvent molecules. When the temperature is increased in nonionic surfactant solutions
(particularly of the polyoxyethylene variety), the aggregation number slowly increases until the
"cloud point" of the surfactant is reached. The cloud point is the temperature at which the
solution begins to exhibit turbidity due to the dehydration of the polyoxyethylene head groups.
Factors that increase the aggregation number tend to decrease the cmc. It is also important to
mention that the cmc values of nonionic surfactants are much lower than the cmc values for ionic
In the past couple of decades considerable interest has been generated in self-assembled
surfactant aggregates such as cylindrical, lamellar, and reverse micelles due to the ability of
surfactant association structure to mimic biological structures.13 Enzymes, for example, are
proteins that speed up (i.e., catalyze) chemical reactions by providing sites for a substrate to fit
into to form a reactive intermediate. The highly efficient and specific catalytic effect of enzymes
makes their investigation an interesting area of biomedical and detergent research (as enzymes
are often added to laundry detergents to improve performance).14, 15 Likewise, cell membranes
perform a variety of functions in cellular biochemical and physiological processes. Surfactant
structures can be used as model systems to mimic both enzymes and membranes. Micelles and
reversed micelles now play an increasingly important role in catalysis and separation processes
in engineering and environmental science and technology.16-18
A theory of micellar structure, based upon the geometry of various micellar shapes and the
space occupied by the hydrophilic and hydrophobic groups of the surfactant molecules, has been
developed by Israelachvili et all 9 In aqueous systems, for example, surfactants with bulky or
loosely packed head groups and long, thin hydrophobic tails tend to form spherical micelles,
while those with short, bulky tail groups and small, close-packed head groups tend to form
lamellar or cylindrical micelles. At concentrations slightly above the cmc, micelles are
considered to be of spherical shape.20 Changes in temperature, surfactant concentration, or
additives in the solution may change the size, shape, aggregation number, and stability of the
There are four maj or types of micelles: 1) spherical micelles which generally have an
aggregation number less than 100 molecules, 2) cylindrical or rod-like micelles, 3) lamellar
micelles, and 4) vesicles. These four structures are shown in Figure 1-4.
1.1.2 Dynamic Nature of Micellar Solutions
Although micelles are often shown as static structures in solution, they are in fact quite
dynamic in nature, constantly breaking and reforming in solution. As stated above, there is a
dynamic equilibrium between the surfactant monomers in the bulk, the surfactant molecules in
the micelle, and molecules that are adsorbed at the interface. There are two characteristic time
scales of relevance in micellar dynamics.21, 22 The first is a fast relaxation time, referred to as zl,
which is a measure of the time that it takes for one surfactant molecule to go in or to come out of
the micelle. The second is a slow relaxation time, referred to as z2, which is a measure of the
time that it takes for one micelle to completely disintegrate or to completely form. The fast
relaxation time is generally on the order of microseconds, whereas the slow relaxation time is on
the order of milliseconds to minutes. The rate of disintegration of a micelle is equal to the rate of
micellar formation. Thus in a micellar solution, if X number of micelles are broken down per
second, X number of new micelles are formed somewhere else in the solution during the same
time. If this requirement is not obeyed, then the number of micelles in the solution would change
as a function of time. The fast and slow relaxation processes are illustrated in Figure 1-5. The
lifetime of a micelle can be approximated to be equal to nz2 where n is the aggregation number of
the micelle23. Thus for a typical solution of sodium dodecyl sulfate (SDS) at 200 mM, the z2 is 7-
8 seconds and n is 65. Thus, nz2 is equal to 7.6 to 8.6 minutes. Therefore, when comparing the
lifetime of micelles (nz2) at 10 mM and 200 mM, it is 65 msecs to 8.6 minutes, respectively.
There are many methods that are used to measure micellar relaxation time. The stopped-
flow method24 inVOlves the rapid dilution of a micellar solution containing an oil soluble dye and
monitoring of the dye adsorbance as a function of time. The temperature-jump25 and pressure-
jump26 methods involve analysis of the electrical conductivity of the system after perturbation of
the temperature or pressure of the system. Ultrasonic absorption27 is yet another method to
measure the kinetics of micelles.
Micellar kinetics may be manipulated by changing physical properties of the solution, such
as temperature, pressure, and concentration, or by placing additives into the surfactant solution.
Leung and Shah28 have shown that the addition of short chain alcohols (methanol to pentanol)
leads to the destabilization of sodium dodecyl sulfate (SDS) micelles and subsequently leads to a
decrease in the z2 value. Patist and Shah29 30 later showed that the addition of longer chain
alcohols such as dodecanol or the addition of oppositely charged surfactants, such as
dodecyltrimethylammonium bromide (C12TAB) to SDS solutions leads to marked increases in
the z2 ValUeS, indicating enhanced stability of mixed micelles. A 25 mM SDS solution has a z2
value of 1 millisecond, but the addition of dodecanol or C12TAB increases the z2 value to 230
msec or 2000 msec, respectively.
During the 1970s, Aniansson and Wall discovered the existence of the two (fast and slow)
relaxation processes and developed a model of the kinetic process of micelle formation and
disintegration.21 The first maj or assumption of this model was that the free surfactant monomers
are assumed to be completely dissociated and the size distribution of the aggregates in a
surfactant solution is assumed to follow the behavior that is shown in Figure 1-6 where C(A,,)
denotes the total concentration of aggregates containing n monomers and is a function of
temperature, pressure, and total surfactant concentration.
The second maj or assumption Aniansson and Wall made was that the association and
dissociation of micelles is a stepwise process involving the entry and departure of one monomer
at a time from the micelle. Thus, there is a series of equilibriums,
A, + A,,_ z A,, n = 2,3,4,..., (1-1)
where An represents an aggregate containing n monomers, and kn' and kn- are the forward and
reverse rate constants for a given step, respectively. As a result, when the equilibrium of a
surfactant solution is perturbed (e.g., by temperature or pressure), the excess surfactant has to
move through regions of different aggregation numbers (Region II of Figure 1-6). According to
equation (1-1), this occurs in steps that are very small compared to the distance in aggregate
space traveled. Therefore, the process will have the characteristics of a flowing system, which is
important because it allows the kinetics of the abstract process of micelle aggregation to be
studied in terms of the more familiar phenomena of heat and material flow.
Initially, micellar kinetics was analyzed on the basis of a heat conductance problem. It was
later viewed in light of a mass transfer diffusion problem, based on a model of mass flow
through a tube having two wide ends that are connected by a narrow section. This model is
analogous to the two high concentration regions (Regions I and III) of Figure 1-6 being
connected by the low concentration region (Region II). In this mass transfer diffusion model, the
rate limiting step to equilibration between the two wide ends is considered to be the diffusion of
materials through the narrow section of the tube. Analogously, the low concentration region of
transient intermediate sub-micellar aggregate states (Region II) that surfactant monomers must
pass through (i.e., Az, A3, A4, An-1 in Equation (1-1)) when a micelle is formed (Region III), or
disintegrated (Region 1) into free monomers is the rate-limiting step in the formation or
disintegration of a micelle. Assuming the aggregation number n to be a continuous variable and
applying the above analogy to mass transfer, Aniansson and Wall derived the expression for the
fast relaxation process
1 k cr C cmc
= 1+- 0 itha (1-2)
Z1 CT ICHIC
where %~ is the half-width of the distribution curve of micellar sizes (assumed to be Gaussian,
Figure 1-6), k- is the stepwise dissociation rate constant, which is assumed to be independent of
n in the micellar region, C is the total surfactant concentration, and cmc is the critical micelle
concentration. Equation (1-2) predicts a linear relationship between 1/zl and the total surfactant
concentration, which is in agreement with pressure-jump and sound absorption experiments.31, 32
It is obvious that as the total surfactant concentration increases, the number of micelles increases
also, resulting in a decrease in intermicellar distance. Hence, the time required for a monomer to
collide with a micelle is shorter at higher surfactant concentration. The length of the surfactant' s
hydrocarbon tail affects the magnitude of zl (i.e., the shorter the chain length, the faster the
relaxation time). This is because the longer the chain length, the greater the Van der Waals
attractive forces will be between the chains of neighboring surfactant molecule. This will lead to
more stable, tighter packed micelles with increasing chain length.
Using the same analogy of diffusion through a variable-width tube as described above, an
expression for the slow relaxation time, z2, WAS derived and simplified to
1 n2 2 -
v2 cmc x R n
where R is a term which may be visualized as the resistance to flow through the narrow region
(i.e., Region II in Figure 1-6) connecting the monomers to the micelles and is given by
R = i[ (1-4)
n-n-, k A
where n is the aggregation number of some colloidal aggregate and An is the equilibrium
concentration of aggregates of order n. The dependence of 1/z2 upon ionic strength,
concentration, and temperature has been interpreted in terms of their effect on R. Interestingly,
the two relaxation times can be used to calculate two important parameters of a micellar solution:
1) the residence time of a surfactant monomer in a micelle and 2) the average lifetime or stability
of micelles33-36. The residence time of a surfactant monomer in a micelle is equal to aI F., where n
is the mean aggregation number and k- is the dissociation rate constant of a monomer from a
micelle. The average micellar lifetime T,, is given by 23
T,, = z, naz= n r, (1-5)
When the concentration of surfactant is much greater than cmc, the micellar lifetime is
approximately equal to n T2-
Although first derived for nonionic surfactants, the results of Aniansson and Wall's theory
on micellar aggregation kinetics were compared primarily with experiments on ionic systems,
simply because it was much easier to detect the relaxation times in ionic systems than in
nonionic systems. Even so, the agreement between theory and experiment was, in general,
satisfactory in the regime of low surfactant concentrations.37 At higher concentrations, however,
the theory did not match experimental results.38 As previously stated, equation (1-3) predicts that
T2 Should increase with increasing surfactant concentration. However, it has been reported that
for some ionic surfactant systemS T2 fifSt increases, passes through a maximum, and then
decreases again.39-41 This behavior of r2 in ionic micellar solutions is not predicted in the
Aniansson-Wall model. Kahlweit and coworkers, using their own temperature-jump and
pressure-jump results22, 42' 43 COncluded that in ionic surfactant systems at high concentration, the
reaction path for the formation of micelles must be different than that at low concentration.
Therefore, the following model was proposed explaining the occurrence of a maximum in T2-
Ionic micelles, including sub-micellar aggregates, can be considered charged particles. When
ionic surfactant molecules such as SDS are added to water, the surfactant molecules dissociate
into negatively charged dodecyl sulfate molecules and their positively charged sodium counter-
ions. These counter-ions are present in solution as a cloud surrounding the negatively charged
micelle (see Figure 1-7). It is believed that at low surfactant counter-ion concentration, the
micelles are stable with respect to coagulation due to repulsive electrostatic forces. Consequently
they can grow only by stepwise incorporation of monomers according to Equation (1-1) above.
As more and more surfactant is added into the system, the counter-ion concentration also
increases, which compresses the electrical double layer and reduces charge repulsion, allowing
the micelles to come closer to each other so that attractive dispersion forces (i.e., Van der Waals
forces) lead to a reversible fusion-fission coagulation according to
Ak + A, A, k +1=i (1-6)
Therefore, Kahlweit suggested that there are essentially two possible pathways for micelle
formation in ionic surfactant solutions: 1) formation as a result of step-wise incorporation of
monomers, and 2) formation as a result of sub-micellar aggregates coming together. The same
idea holds true for the dissociation of micelles by two possible pathways. His model proposed
that the first pathway was dominant for low surfactant concentrations, where electrostatic
repulsion would be high between droplets. The second pathway would become dominant for
higher surfactant concentrations, where the counter-ion concentration is high and repulsion is
It is a commonly known fact that oil and water do not mix. However, emulsifying agents,
typically surfactants can be added to a mixture of oil and water to promote the dispersion of one
phase in the other in the form of droplets. Over the years, emulsions have been defined in a
variety of ways. For the purpose of this dissertation, emulsions will be defined as
thermodynamically unstable, heterogeneous systems, consisting of at least one immiscible liquid
intimately dispersed in another in the form of droplets, whose diameters are generally in the
range of 1 100 Clm.44 There are two main types of emulsions: oil-in-water emulsions, in which
oil droplets are dispersed in a continuous water phase, or water-in-oil emulsions, in which water
droplets are dispersed in a continuous oil phase.
The most fundamental thermodynamic property of any interface is the interfacial free
energy, or interfacial tension. The interfacial free energy is the amount of work necessary to
create a given interface. The interfacial free energy per unit area is a measure of the interfacial
tension between two phases. A high value of interfacial tension implies that the two phases are
highly dissimilar in nature. There are many methods available to measure the interfacial tension
between two liquids including the Du Notty ring method, Wilhelmy plate method, drop-weight
or drop-volume method, pendant drop method, spinning drop method, and Sessile drop method.3
Emulsification involves the generation of a large total interfacial area. Considering that the
two phases in emulsions are not miscible, in order to generate this large interfacial area, the
interfacial tension must be lowered significantly according to the following equation:
W = y *A (1-7)
where W is the work done on an interface, y is the interfacial tension, and AA is the change in
interfacial area associated with the work W. According to Equation (1-7), when a constant
amount of work is applied to generate an interface, AA will be large if y is small, and thus the
interface will expand significantly to form smaller emulsion droplets.
As previously mentioned, the primary means by which the interfacial tension is lowered is
through the addition of emulsifying agents, usually surfactants. The surfactant molecule also
plays a second role in emulsions which is to stabilize the interface for a time against coalescence
with other droplets and concomitant phase separation.
A large number of methods have been developed to provide the energy needed to achieve
complete emulsification in a given system. These methods can be broken up into a number of
classes. Table 1-1 summarizes methods and apparatus used to produce emulsions, including a
characterization of each method.36, 44-46 Some of these methods are almost exclusively used in
laboratory settings (i.e., bench scale), such as 1, 4d, 8b, 9, 10 and 12. For large-scale production
of emulsions, methods 4b, 5, 7, and 8a are used. There are often times when two methods are
combined, such as 3 and 7 or 4 and 7.
1.2.1 Emulsion Droplet Size
Emulsions are classified as either water-in-oil (W/O) or oil-in-water (O/W)
depending on which phase is continuous and which is dispersed. The dispersed phase in
emulsions, whether oil or water, is usually composed of spherical droplets within the continuous
phase. These droplets may be nearly monodisperse in terms of droplet size; or they may have a
wide size distribution depending on several factors.44 In mOst cases, the wider the size
distribution, the less stable is the emulsion. In other words, emulsions with a more uniform size
distribution tend to remain stable for longer time while those with wide size distribution will
usually undergo Ostwald ripening, a phenomenon where larger droplets grow at the expense of
smaller droplets.45 In general, emulsions with a narrow size distribution and a small mean
droplet size tend to exhibit a greater emulsion stability, all other things being equal.44 The
change in the size distribution with time reflects the kinetics of coalescence in emulsions.
Depending on the surfactant that is used to stabilize the emulsion, emulsions can have lifetimes
ranging from hours to as long as a few years.44 In general, emulsions exhibiting a higher yield
stress tend to show higher emulsion stability and shelf life.44
Emulsion droplet size is also related to the method of preparation that is employed to
generate the emulsion. This is a result of the relationship between interfacial area and work that
is done on the system, according to Equation (1-7). As can be seen in this equation, if the
interfacial tension is constant with time, the change in interfacial area is directly proportional to
the amount of work that is put into the system. Some emulsion preparation methods provide
more energy (work) than others, and thereby lead to smaller droplets and higher interfacial area.
Micellar stability is another factor that affects the droplet size of emulsions.46 The ability
of the surfactant molecules to adsorb rapidly from the bulk to the droplet interface in an
emulsion determines the dynamic interfacial tension, which is related to the total interfacial area
and therefore the droplet size. In emulsion formation, as the large total interfacial area is being
generated by dispersion of oil phase, additional monomers have to be provided to this newly
created interface by disintegration of micelles. If the micelles are very stable, flux of surfactant
monomers to the interface of droplets will be low, resulting in a higher interfacial tension at the
droplet surface and a large droplet size will occur as predicted by Equation 1-7. The process of
monomer diffusion from the bulk to the oil/water interface is illustrated in Figure 1-8.
A considerable amount of work has been done on characterizing micellar stability in
surfactant solutions. It has been shown that more stable micelles provide less monomer flux to
the oil/water interface, which results in a higher interfacial tension and hence, a larger droplet
size.46 In Order for the droplet size in an emulsion to be small, a short-lived (or labile) micelle is
desired, since it will supply monomer to the oil/water interface with ease. This is illustrated in
for emulsions of hexadecane/sodium dodecyl sulfate (SDS) solutions in Figure 1-9.
The droplet size is largest for the most stable micelle, which is known to be at the 200 mM
concentration for SDS solutions.37, 41 This relationship between micelle stability and droplet size
has also been verified for cesium dodecyl sulfate solutions which forms the most stable micelle
when compared to micelles of sodium dodecyl sulfate and lithium dodecyl sulfate.47 Therefore,
knowledge of the surfactant micellar stability as a function of concentration is necessary to
predict the droplet.
1.2.2 Viscosity of Emulsions
The viscosity, or resistance to flow, of emulsions could be considered as one of their most
important properties. This is true from both a practical and a theoretical viewpoint. In a practical
sense, certain cosmetic or even food emulsions are only desirable at a specific viscosity (e.g.
lotions, milk, salad dressings, etc.). Manipulation of emulsion viscosity to achieve the desired
product specifications is not a trivial matter. From a theoretical perspective, the viscosity
measurements can be used to provide insightful information about the structure and possibly the
stability of an emulsion. The overall emulsion stability is affected by the following factors:44, 48
Viscosity of the external phase
Concentration (i.e., volume fraction) of the internal phase
Viscosity of the internal phase
Nature of the emulsifiers
Surface viscoelasticity of the interfacial film formed at the oil/water interface
Droplet size distribution
1.2.3 Determination of Emulsion Type (O/W or W/O)
Emulsions consist of a dispersed phase and a continuous phase. Most often the dispersed
phase is present as spherical droplets within the continuous phase. Most emulsions follow the
Bancroft49, 50 Tule for emulsions, which states that the phase in which the surfactant is most
soluble becomes the continuous phase in an emulsion. There are several methods commonly
used to determine which phase is continuous and which phase is dispersed. These methods
utilize the property differences between oil and water to determine phases. The methods are
listed in Table 1-2.
1.2.4 Emulsion Stability
As mentioned previously, one of the most important parameters in emulsifieation processes
is emulsion stability. For example, milk is a natural emulsion of the O/W type in the food
industry. If the stability of milk was only a week or two, the milk would have to be shaken
vigorously before pouring. However, in this case nature has provided us with a stable
emulsion. Another common example is shampoo, another emulsion. It would be inconvenient if
the shampoo were not a stable emulsion, since shaking would be necessary. There are also cases
where it is necessary to break down unwanted, naturally occurring stable emulsions. Such
examples are the W/O type emulsions which build up in oil storage tanks, or the O/W type
emulsions that arise in effluent waters.
So it becomes necessary to understand how to develop an emulsion system to be stable or
unstable, depending on the needs of the industry. To understand emulsion stability, it is
important be cognizant that there are Hyve types of breakdown processes which can occur in
emulsions. These are listed in Table 1-3, along with factors that influence that type of
When two oil drops approach each other, a thin film of the continuous water phase is
trapped between the drops. The behavior of the thin film determines the degree of stability of
the emulsion, and the rate of thinning of the film determines the time required for the two drops
to coalesce (i.e., coalescence rate). When the fi1m has thinned to a critical thickness, it ruptures,
and the two drops unite or coalesce to form one larger drop.52, 53 The rate of film thinning
depends on the surface viscosity of the surfactant film adsorbed at the oil/water interface. The film
may drain evenly or unevenly depending on the interfacial tension gradient due to adsorbed
The factors that influence the rate of film thinning between droplets therefore influence the
emulsion stability. A summary of all of the factors influencing coalescence of droplets is given in
18.104.22.168 Charge stabilization: The electrical double layer
Breakdown of an emulsion can occur due to electrostatic attraction between droplets in the
dispersed phase. The attraction can be induced if it is desirable to breakdown an emulsion, or
attraction can be eliminated if it is desirable to maintain a stable emulsion. To understand the
concept of charge stabilization, it is necessary to understand the nature of the electrical double
layer that surrounds the droplets and the factors that influence it. The electrical double layer
refers to the volume around the emulsion droplets that is influenced by the charge on the
droplet, if any. This volume can be broken down into two distinct regions, and this is illustrated
in Figure 1-10.
The electrical double layer is composed of two layers; the first is known as the Stern plane
and is characterized by a linear drop in electrical potential, and the second is referred to as the
shear plane and it is characterized by an exponential drop in potential. The presence of counter
ions on the surface effectively neutralizes some of the surface charge on the droplet. These
counter ions may be introduced in the system by the presence of an electrolyte.
The electrical double layer defines the region of influence of a droplet caused by the
surface charge. If the double layer thickness, also known as the Debye length, is large for a pair
of similarly charged droplets, the droplets will be electrostatically repelled. If an electrolyte is
added to the system to decrease the double layer thickness, the repulsion will decrease and may
be low enough that the droplets can get close enough to be attracted to one another. This
phenomenon would result in destabilization of the emulsion. Therefore, in an emulsion in which
the droplets are charged, any additive or parameter change which influences the electrical double
layer thickness will influence the stability of the emulsion.
22.214.171.124 Phase inversion in emulsions
Phase inversion can occur in emulsions due to a number of factors. For a given emulsifier
concentration, the viscosity of an emulsion gradually increases as the phase volume of the
dispersed phase is increased. However, at a certain critical volume fraction Oc, there is a sudden
decrease in viscosity, which corresponds to the point at which the emulsion inverts. Oc was found
to increase with increasing emulsifier concentration.56 The sudden decrease in viscosity is due to
the sudden reduction in dispersed phase volume fraction. Often Oc is in the range of 0.74, so that
upon inversion, the dispersed phase volume fraction reduces from 0.74 to 0.26, thus reducing the
viscosity significantly. Oc should theoretically be in the range of 0.74 for spheres of equal radii to
be at the maximum packing," but #,=0.99 was found for paraffin oil/aqueous surfactant
solutions"" and #,=0.25 was found for olive oil/water emulsions.59 The phase inversion of
emulsions can be brought about by several parameter changes, listed in Table 1-5.
126.96.36.199 Emulsion creaming
As described previously, "creaming" is a special case of emulsion instability that occurs
when there is no change in droplet size or size distribution, but buildup of an equilibrium
droplet concentration within the emulsion occurs. Creaming is not so much a breaking of an
emulsion as it is a separation into two emulsions, one of which is richer in the dispersed
phase, the other poorer, than the original emulsion.44 The more concentrated emulsion is
referred to as the "cream." The creaming phenomenon can result from an external force field,
such as gravitational, centrifugal, or electrostatic. In most cases creaming is undesirable, as in
pharmaceutical products and agricultural sprays where the product must first be shaken in order
to uniformly disperse the droplets again. In some cases, however, creaming is desirable,
such as in the separation of isoprene droplets from water in the rubber latex industry. Regardless, it
is important to understand the physical parameters that affect the creaming, or sedimentation
As discussed previously in this section, it is not always desirable to have a stable emulsion.
Often an emulsion is present in a system in which it is undesirable. One example is the presence
of aqueous emulsion droplets dispersed in crude oil. Crude oil is always associated with water or
brine in oil reservoirs and also contains natural emulsifying agents, such as resins and asphaltenes.
These emulsifying agents form a thick, viscous interfacial film around the water droplets,
resulting in a very stable emulsion. Therefore, demulsification is very important in the crude oil
industry. Many physical methods have been developed for demulsification, depending on the
industrial application. These methods are described in Table 1-6.
A wide variety of chemical additives for demulsification have been developed in recent
years. These additives are all relatively high molecular weight polymers capable of being
adsorbed at the O/W interface and displacing the film. The primary advantage of these additives
is that they can be added to the system even before emulsion formation, so that they act as
In the petroleum industry, demulsifiers have been considered for breaking the common
fuel oil emulsions. In this area, chemical demulsifiers that have been investigated include ultra-
high molecular weight polyoxiraneS60 and micellar solutions containing petroleum sulfonates,
electrolytes, and cosurfactants.61
It is evident that there are many methods for demulsification. The nature of the emulsion to
be separated is the key factor in determining which methods) is best for each particular
1.2.6 Surfactant Selection for Emulsification
Often the selection of surfactants in the preparation of either O/W or W/O emulsions is
made on an empirical basis. However, in 1949, Griffin62 introduced a semi-empirical scale
for selecting an appropriate surfactant or blend of surfactants. This scale, termed the hydrophile-
lipophile balance (HLB), is based on the relative percentage of hydrophilic to hydrophobic
groups in the surfactant molecules and ranges from 1 to 40. An HLB of 1 represents a surfactant
that is highly oil-soluble and an HLB of 40 represents a highly water-soluble surfactant.
Surfactants with a low HLB number normally form W/O emulsions, whereas those with a high
HLB number often form O/W emulsions.63 A summary of the HLB range required for various
purposes is given in Table 1-7.
The calculation of the HLB number for a given surfactant, as developed by Griffin,62 is quite
laborious and requires a number of trial and error procedures. Simplification methods were later
developed by Griffin62 that applied to certain surfactants. Davies63 developed a method for calculating
the HLB values of surfactants directly from their chemical formulas, using empirically
determined numbers. The HLB number can also be determined experimental through several
correlations that have been developed. These correlations relate the HLB number to such
parameters as the cloud point,64 water titration value for polyhydric alcohol esters,65 and the heat of
hydration of ethoxylated surfactants.66
Another method that may be used to select a surfactant suitable for forming an emulsion is by
using the phase inversion temperature (PIT) method. The phase inversion temperature (PIT) is the
temperature at which an emulsion experiences phase inversion, as described in a previous section. The
PIT of non-ionic emulsifiers has been shown to be influenced by the surfactant HLB number, so the PIT
can be used similarly to the HLB number in selecting an emulsifier.59 The primary distinction is that the
PIT is a characteristic property of the emulsion, not of the emulsifying agent.59 Due to this, the PIT
includes the effect of additives on the solvent, the effect of mixed emulsifiers or mixed oils, etc. In other
words, the HLB number is actually a function of all of these properties, but only the PIT completely
analyzes a given emulsion system. The PIT method is useful because the PIT is a measurable property
which is related to the HLB number. A summary of effects of PIT and droplet stability from different
investigations is given below:59
The size of emulsion droplets depends on the temperature and the HLB of emulsifiers
The droplets are less stable toward coalescence close to the PIT
Relatively stable O/W emulsions are obtained when the PIT of the system is some 20 to
650C higher than the storage temperature
A stable emulsion is obtained by rapid cooling after formation at the PIT
The optimum stability of an emulsion is relatively insensitive to changes of HLB value or
PIT of the emulsifier, but instability is very sensitive to the PIT of the system
The stability against coalescence increases markedly as the molar mass of the lipophilic or
hydrophilic groups increased
When the distribution of hydrophilic chains is broad, the cloud point is lower and PIT is
higher than when there is a narrow size distribution.
The PIT can be measured by the following methods: 1) direct visual assessment,67 2) conductivity
measurement,34,68, 69 3) Differential Thermal Analysis (DTA) or Differential Scanning
Calorimetry (DSC),70 and 4) viscosity measurement.71, 72 Both the HUB and PIT methods for
selecting an emulsifier in a system have been widely used and adapted to meet industry needs.
1.2.7 Applications of Emulsions
Emulsions are desirable for many different applications because they provide a system
having a large interfacial area. Historically, cosmetic emulsions are the oldest class of
manufactured emulsions.44 Emulsions are desirable for cosmetic applications because: 1) they
increase the rate and extent of penetration into the skin, 2) they open up the possibility of
applying both water- and oil-soluble ingredients simultaneously (e.g. deodorants), and 3) they
provide for efficient cleansing.
Emulsions are also widely used for pharmaceutical applications in the form of creams or
ointments and as drug delivery vehicles. They are also ideal for use as polishes (e.g. furniture
polishes, floor waxes, etc.) paints, and agricultural sprays. Many foods are manufactured in the
form of emulsions including mayonnaise, salad dressings, milk, and margarine. Another industry
where emulsion technology is important is the asphalt industry when the principal requirement is
the production of water-repellent surfaces. Emulsions are also used as polymerization vehicles to
aid in the production of high polymeric materials such as plastics, synthetic fibers, and synthetic
rubbers. These are just a few of the many applications of emulsions.
A microemulsion is a thermodynamically stable, isotropic dispersion of oil and water
containing domains of nanometer dimensions stabilized by an interfacial film of surface-active
agent(s).73 The term "microemulsion" originated from Jack H. Schulman and coworkers in
1959,74 although Hoar and Schulman originally described water-in-oil microemulsions, which
they referred to as transparent water-in-oil dispersions, in 1943.7 As implied above,
microemulsions may be of the oil-in-water (O/W) (see Figure 1-1 1) or the water-in-oil (W/O)
type depending on conditions of the system and system components.
According to Bancroft,49, 50, 76 phase volume ratios are less important in the determination
of the microemulsion type that will be formed (i.e., W/O or O/W) than the surfactant
characteristics (e.g. HLB). As previously mentioned, the Bancroft rule states that whichever
phase the surfactant has a greater affinity for will typically be the continuous phase.
The creation of a microemulsion entails the generation of a huge interfacial area, which,
according to the following equation," requires a significant lowering of the interfacial tension
(usually << 1 mN/m):7
W = y *A (1-7)
where W is the work performed, y is the surface or interfacial tension at the air/water or oil/water
interface and AA is the change in surface or interfacial area. This ultra-low interfacial tension in
spontaneously formed microemulsions is achieved by the incorporation of surfactant(s) (typically
a surfactant + a cosurfactant, especially when ionic surfactants are used).l Figure 1-12 shows the
thermodynamic explanation for the behavior of macro- and microemulsions. As can be
concluded from the graph, there is an optimum radius for microemulsion systems where the free
energy of dispersion becomes negative, thereby making the microemulsion stable and its
formation energetically favorable.79
Schulman and others first noticed microemulsion systems in 1943 when they observed that
the addition of a medium chain-length alcohol made a coarse macroemulsion that was stabilized
by an ionic surfactant become transparent.75 Even then, Hoar and Schulman recognized the
important role of a very low interfacial tension in causing spontaneous emulsification of the
added water in oil.75 They concluded that the role of the alcohol is as a stabilizer against the
repulsive electrostatic forces that the ionic surfactant head groups would experience.
Schulman and others used a variety of experimental techniques (e.g. X-ray diffraction,74
ultra-centrifugation, so light scattering,74 VISCOSimetry,s and nuclear magnetic resonance
(NMR)82, 83) to elucidate some of the characteristics of these microemulsion systems following
the groundbreaking work of Schulman and Hoar. These studies were instrumental in providing
them with information about the structure, size, and interfacial film behavior of microemulsions.
They were able to determine the size of the droplets and they found that the presence of the
alcohol within the system led to greater interfacial fluidity.
Later, in 1967, Prince74 prOposed a theory that the formation of microemulsions was due to
the negative interfacial tension that results from high surface pressure of the fi1m. Prince
explained this negative interfacial tension based on the depression of the interfacial tension
between the oil and water phase that occurs when surfactant is added. The principle behind this
theory is described by the series of equations that follow. The surface pressure of the film at the
air/water interface, naw, is defined as:17, 74
awIM = 70 T (1-8)
where yo is the surface tension of the pure surface (without surfactant) and ys is the surface
tension of the surface with surfactant. In the case where oil is the second phase (oil/water
system), the surface pressure of the surfactant film at the oil/water interface, now, can be defined
ow Yow o f/w s(1-9)
where (yo/w)o is the interfacial tension of the "pure" oil/water interface (i.e., in the absence of
surfactant) and (yotw)s is the interfacial tension of the oil/water interface in the presence of
surfactant film. Rearrangement of Equation (1-9) gives:
70w) =7/wo ow (1-10)
Based on Equation (1-9), for a surfactant film that can generate a very high surface
pressure (now), the interfacial tension of the surfactant film at the oil/water interface (yo/w)s
becomes negative. This is only a transient phenomenon because generation of a negative
interfacial tension leads to a negative free energy of formation of the emulsion, which is an
unstable situation. This is illustrated by the following equation:
AGronn = yAA TASconfig (_1
where AA is the increase in interfacial area, ASconfig iS the configurational entropy of the droplets
of the liquid that are formed and T is the absolute temperature.84 The negative interfacial tension
accounts for the spontaneous increase in interfacial area that occurs in the formation of
microemulsions. When a transient (unstable) negative interfacial tension is experienced, the
system will seek to stabilize by spontaneously generating new interfacial area, thereby raising the
interfacial tension back to acceptable, stable limits. As previously mentioned, in order to form
microemulsions, it is required that the concentration of surfactants be greater than that required
to reduce the oil/water interfacial tension to zero and to cover the total interfacial area of all
dispersed droplets. The transient negative interfacial tension that is generated facilitates the
spontaneous break-up of droplets.
The nature of the thermodynamic stability of microemulsions has long been studied. The
stability can be attributed to the fact that the interfacial tension is low enough that the increase in
interfacial energy accompanying dispersion of one phase in the other is outweighed by the free
energy decrease that is associated with the entropy of dispersion.79 Furthermore, the free energy
decrease that accompanies adsorption of surfactant molecules from a bulk phase favors the
existence of a large interfacial area and hence plays a maj or role in stabilizing microemulsions.79
One must also understand the role of cosurfactants in microemulsion formulations. The
addition of a short-chain alcohol to a surfactant solution to enhance microemulsion formation has
long been practiced." This cosurfactant (short-chain alcohol) serves to 1) fluidize the interface,
2) decrease interfacial viscosity, 3) destroy the lamellar liquid crystalline structures, 4) provide
additional interfacial area, 5) reduce electrical repulsion between droplets and also between polar
head groups of surfactants by acting as charge screeners and decreasing surface charge density,
and 6) induce the appropriate curvature changes."
In 1972, Gerbacia and Rosano86 inVCStigated the formation and stabilization of
microemulsions, with emphasis on the role of the cosurfactant (in this case, pentanol). They
suggested that a critical aspect of the mechanism of formation of microemulsions involves the
diffusion of the pentanol through the interface. This process has been found to be a necessary,
but not sufficient condition for microemulsification. In essence, the alcohol transiently lowers the
interfacial tension to zero as it diffuses through the interface, thereby inciting the spontaneous
dispersion of one phase into the other. The surfactant then acts to stabilize the system against
coagulation and coalescence. NMR data and calculation of free energies of adsorption of the
pentanol into the interface have proven that a strong association between the surfactant and
cosurfactant is not necessary for microemulsion formation.86
The stability of the phases of surfactant in microemulsions results from the competition
between entropic and elastic contributions to the free energy." The two intrinsic parameters that
ultimately affect the structure of the aggregates existing in solution are the mean bending
modulus, ic, and the Gaussian bending modulus, K The bending energy, Fb, is directly related to
the mean bending modulus and the Gaussian bending modulus, as can be seen below:
Fb, 1 -~T C2 2CO,)2 1 2C
where C1 and C2 are the local principal curvatures of the surfactant layer and Co is the
spontaneous curvature. Bellocqs states that the contribution of the bending energy to the total
free energy is a crucial determining factor in the type and characteristic size of the structure. In
Equation (1-12) above, the first term represents the amount of energy needed to bend a unit area
of interface by a unit curvature amount, and the second term is important to the change of the
membrane topology and the phase transition.88 The spontaneous curvature, Co, is determined by
the nature of the interactions between the surface-active molecules; i.e., the competition in
packing of the polar heads and the hydrocarbon tails of the surfactant. If the dominant
interactions are between the polar heads, then the surfactant orientation will be concave to water
and a water-in-oil microemulsion will be formed, whereas, if the dominant interactions are
between the hydrocarbon tails, the surfactant orientation will be convex to water and an oil-in-
water microemulsion will be formed.
The addition of cosurfactant (short-chain alcohol) can have a profound effect on the
curvature. Bellocq reported that there is a very efficient lowering of the bending constant, K, of
the surfactant film with the dilution of the system with short-chain alcohols.88 This lowering of K
was attributed to thinning of the interface, and this attribution was confirmed with deuterium
The chain length of the cosurfactant (alcohol) was found to be critical in microemulsion
formation. The chain length determines what types of phases (bicontinuous, lamellar, sponge,
vesicle, etc.) will be of importance. These phases play an important role in the type, structure and
size of the microemulsion that will be formed.89 In Simplistic terms, a short-chain cosurfactant
acts to prevent the formation of or destroy lamellar liquid crystals. Since there is a significant
difference between the surfactant chain length and the short-chain alcohol, there is a tail wagging
effect due to the fact that the excess hydrocarbon tails have more freedom to disrupt molecular
packing through conformational disorder, increased tail motion, and penetration and/or buckling
of the chain into the monolayer.90 This motion of the hydrocarbon tail prevents or disrupts
ordering of the molecules and therein prevents formation of or destroys lamellar liquid crystals
and enhances microemulsion formation.
It must be noted that cosurfactant addition to microemulsion-forming systems is typically
only applicable for ionic surfactant systems. Microemulsions that incorporate non-ionic
surfactants may be formed without the need for cosurfactant addition; especially in the case of
non-ionic surfactants of the polyethylene oxide adducts (POE). This is because these surfactants
are composed of a homologous series of varying composition and molecular weight," which
serves the same purpose of enhancing the interfacial film fluidity. The important factor in the
formation of these types of microemulsions is temperature, because this class of materials is
solubilized in water by means of hydrogen bonding between the water molecules and the POE
chain. Hydrogen bonding is a temperature-sensitive phenomenon, which decreases with
increasing temperature. Therefore, the temperature conditions under which a microemulsion is
formed are important to the type of microemulsion that is formed. Above a characteristic
temperature, which is commonly known as the phase inversion temperature (PIT),91 the non-
ionic surfactant changes its affinity from the water phase to the oil phase. Below the PIT, O/W
microemulsions will be formed and above the PIT, W/O microemulsions will be formed.
1.3.1 Formation of Microemulsions
Now that the some of the basic principles of microemulsions have been discussed, it is
easier to understand what conditions must be met for microemulsion formation and why they are
required. There are three maj or factors that must be considered in order to form
First, given the importance of achieving an ultra-low surface tension at the oil/water
interface, surfactants must be carefully chosen so that this may be accomplished. Secondly, there
must be a large enough concentration of surfactants to stabilize the newly formed interface such
that phase separation does not occur. It must be mentioned that the type, structure and
characteristics (e.g. Hydrophilic-Lipophilic Balance (HLB), degree of ionization, etc.) of the
surfactants may potentially play a maj or role in determining just how high the surfactant
concentration needs to be to stabilize the interface. The third required condition for forming
microemulsions is that the interface must be fluid (flexible) enough to facilitate the spontaneous
formation of micro-droplets with a small radius of curvature (50 500 A+). That is where
cosurfactant structure can become very important.
1.3.2 Applications of Microemulsions
Microemulsions have a range of industrial applications. They are useful in technologies
such as enhanced oil recovery,92 pharmaceuticals,93 COSmetics,94, 95 food sciences,96 and
detergency.97 98 Microemulsions have been extensively studied in regards to their use as drug
delivery vehicles, and are now gaining attention as possible mediums for use in detoxification of
blood to remove free drug from the blood stream of overdose patients.99
Drugs that have significant hydrophobic functionality have been shown to partition into the
corona (area at the interface) and/or interior core of O/W microemulsions.99 It is generally
accepted that this is largely due to hydrophobic interactions. Recall that the formation of
microemulsions leads to the generation of a large interfacial area. It is believed that this large
interfacial area facilitates the uptake of relatively large amounts of drug into the microemulsion
in a time efficient manner, thereby significantly decreasing the bulk drug concentration.
As early as 1943, Dr. T. P. Hoar and J. H. Schulman75 discovered that he could prepare
transparent water-in-oil dispersions of nanometer size, which displayed stable thermodynamic
characteristics (i.e., the water droplets remained stably dispersed indefinitely if left unperturbed,
with respect to temperature, pressure, or compositional conditions). Anomalous systems could
also be prepared in which oil droplets are dispersed in water. Later, other scientists, including Dr.
Stig Friberg,89 found that they could develop transparent nanometer size dispersions of one
medium within another continuous medium, which were not thermodynamically stable, but were
kinetically stable (i.e., given time, there will be a breakdown in the stability of the dispersion that
will lead to phase separation. Such systems have come to be referred to as nano-emulsions
(thermodynamically unstable systems). Thus, both Schulman's microemulsions and nano-
emulsions start in the nanometer range, but the microemulsions are thermodynamically stable
and maintain the same size whereas the nano-emulsions coalescence and display an increase in
size with time, which ultimately causes phase separation.
In order to sufficiently comprehend the difference between microemulsions and nano-
emulsions, there must be clarification of the nomenclature of the two. As previously mentioned
microemulsions is a misleading title considering that their average diameter ranges from 10 -
100 nm. Light scattering and X-ray analysis have indicated that microemulsions are, in fact,
coarse mixtures as opposed to molecular dispersions.7 Nano-emulsions lie within the same size
range, but may have diameters that considerably exceed the size of a microemulsion droplet (20-
500 nm). The emphasis must be placed on the fact that nano-emulsions are in fact emulsions of
nano-size, meaning that they are kinetically stable, (thermodynamically unstable) heterogeneous
systems in which one immiscible liquid is dispersed as droplets in another liquid (as emulsions
Although they are on similar size scales, microemulsions and nano-emulsions have
markedly different characteristics (see Table 1-8). Nano-emulsions are formally defined as
thermodynamically unstable, generally opaque, sub-micron-sized (20 500 nm) systems that are
stable against sedimentation and creaming.100 They may have the appearance of microemulsions,
but they do not necessarily require as much surfactant concentration in their preparation.1oo
As early as 1981, Rosano et al8 6 found that certain microemulsion systems, which they
termed "unstable microemulsions", displayed significantly different characteristics from
traditional microemulsions. These systems were dependent upon the order of addition of
components (i.e., the mixing protocol) and their formation was contingent upon having a large
enough concentration gradient to allow diffusion of amphipathic materials across the oil/water
El-Aasser et at. lot performed studies of a miniemulsification process, which produced O/W
miniemulsions (another term for what we refer to as nano-emulsions in this paper) having an
average droplet diameter in the size range of 100 400 nm. The miniemulsions that they
produced were generated by means of a mixed emulsifier protocol, which was comprised of a
mixture of ionic surfactant and long-chain fatty acid in concentrations of 1-3 % by weight in the
oil phase. They stated four "distinct and significant" differentiating aspects between the
necessary conditions of preparation for miniemulsification systems and traditional
microemulsion systems, which may also be produced by mixed emulsifier systems:
1) Concentration of the mixed emulsifier system: only 1-3 wt% (with respect to the oil
phase) is sufficient for the formation and stabilization of miniemulsions, whereas
microemulsions typically require 15-30 wt %.
2) Droplet size: their miniemulsion droplets range from 100-400 nm in diameter, as
opposed to microemulsions, which range from 10-100 nm in diameter.
3) Chain length of the cosurfactant (fatty alcohols or acids in this case): miniemulsions
require at least a 12-carbon chain length, whereas microemulsions can be prepared
with alcohols of shorter chain lengths.
4) Order of mixing of the ingredients (mixing protocol): successful production of
miniemulsions requires that the ionic surfactant and the fatty alcohol be initially
mixed in the water phase for 30 minutes to 1 hour at a temperature above the melting
point of the fatty alcohol, prior to the addition of the oil phase, whereas order of
mixing does not affect microemulsion formation.
El-Aasser et at. lot performed an array of experiments to help them to better understand the
miniemulsification process. They found that the process was a spontaneous phenomenon based
on results yielded by both dynamic and static spinning drop experiments. The oddity in this
discovery lay in the fact that the measured interfacial tensions were unexpectedly high, ranging
from 5 to 15 dynes/cm. They attributed this finding to the formation of emulsion droplets by
diffusion of the oil (in this case, styrene) from drops into the adj acent liquid crystal structure of
the mixed emulsifier system. They also stated that the presence of mixed emulsifier liquid
crystals, which was confirmed by birefringence observations, significantly improved the
emulsification process and led to greater emulsion stability. They concluded that the mechanism
of formation of miniemulsions was by swelling of the mixed emulsifier liquid crystals by oil (in
this case, toluene). This swelling of the liquid crystalline structure led to its break-up or sub-
division to form small emulsion droplets, which were stabilized by the adsorption of the mixed
emulsifier complex at the oil-water interface.10
Although emulsion stability is generally known to increase with droplet surface charge, the
miniemulsions that El-Aasser prepared displayed contradictory behavior; their stability increase
corresponded to a reduction in surface charge. These results suggested that the steric component
of stabilization was the dominating factor.'01
The mechanism that has been attributed to nano-emulsion breakdown in a ternary system
of brine, oil, and non-ionic surfactant is a 3-stage proceSS.102 The first and last stages of the
droplet growth process were found to be due to Ostwald ripening, whereas the droplet size
distribution of the second stage became too broad compared to the expected theoretical
distribution (as predicted by the Lifshitz- Slezov-Wagner theory) to be due to Ostwald ripening.
Katsumoto et al 102 made this assessment after plotting the cube of the z-average radius, rz, as a
function of time, and obtaining a linear relationship. In addition, their group found that the
volume of bound water on miniemulsion droplets plays an important role in obtaining a
homogeneous miniemulsion.32 The storage temperature of the miniemulsion solution and the
molecular weight of the surfactant were determined to significantly affect the system's stability:
as storage temperature is decreased, the rate of coalescence increases, and as the molecular
weight of the surfactant is increased, the rate of coalescence decreases. The former Einding is due
to an increase in surface tension because non-ionic surfactants become more water-soluble as
temperature is decreased. The latter finding is due to steric effects that become predominant as
the surfactant size is increased.
Nano-emulsions may be formed by a few different experimental methods: condensation,103
low-energy emulsifieation methods involving phase inversion,100, 104 Or by high energy input
during emulsifieation.los Forgiarini et at. too reinforced the concept that was proposed by El-
Aasseriol concerning the importance of the mixing protocol in the formation of nano-emulsions.
They found that they only obtained dispersions of nanometer droplet size when the nano-
emulsion was formed via stepwise addition of water to a solution of the surfactant in oil. If other
methods of addition were used, the droplet size ranged from 6 10 Clm.
Izquierdo et at. 104 analyzed the formation and stability of nano-emulsions that were
prepared by the phase inversion temperature (PIT) method, in which emulsions were formed at a
temperature near the PIT and then rapidly cooled to room temperature (250C) by immersion in
an ice bath. They proposed that a change in the interfacial curvature is critical to nano-emulsion
formation, and that the presence of lamellar liquid crystals was probably a necessary, but
insufficient requirement for preparation of nano-emulsions.100 They concluded that the key factor
for nano-emulsion formation should be credited to the kinetics of the emulsification process.
Nano-emulsions have possible applications as drug delivery vehicleS,105, 106 in drug
targeting, as reaction media for polymerization, in personal care and cosmetics, and in
agrochemicals. It is also plausible that nanoemulsions may be used in most industries where
extraction will ultimately be required. This idea is based on the premise that the nanoemulsions
may be designed so that their stability characteristics will coincide with the requirements of the
m olec ul e ...........................................m icelle
Figure 1-1. Schematic diagram of a surfactant molecule, micelle, and reverse micelle.
I~\ / /Magnetic
I Su rface Tensiocn
/ // Se11-Dltfusion
Concentration of Surfactant
Figure 1-2. Properties of surfactant solutions showing abrupt change at the solution critical
micelle concentration (cmc).
Mi cell e
/ I Monomer
Figure 1-3. Schematic design of micellar solution showing the three maj or species that are in
dynamic equilibrium: 1) monomers, 2) micelles, 3) adsorbed film.
Figure 1-4. Schematic diagram of the four major micellar structures: A) spherical micelle, B)
cylindrical, rod-like micelles, C) unilamellar vesicle, D) lamellar micelle.
Fast relaxation time, microseconds
Slow relaxation time, milliseconds to minutes
Figure 1-5. Mechanisms for the two characteristic relaxation times for a micelle in a surfactant
solution, zl and z2, above cmc.
Aggregation number, n M
Figure 1-6. Typical size distribution curve of aggregates in a micellar solution according to the
Aniansson-Wall model of stepwise micellar association. Region (I) corresponds to
monomers and oligomers; Region (III) to abundant micelles with a Gaussian
distribution around the mean aggregation number; and Region (II) to the connecting
"wire" (heat transfer analogy) or "tube" (mass transfer analogy) between Regions (I)
N Na N
Na+ Na' Na
Figure 1-7. Schematic of sodium counter-ion "cloud" around SDS spherical micelle.
Figure 1-8. Schematic diagram of the adsorption of surfactant monomers from the bulk to the
oil/water interface during emulsion formation
Figure 1-9. The emulsion droplet size in the hexadecane/SDS solution system after 30 s
emulsification at: (A) 50 mM, (B) 100 mM, (C) 200 mM, (D) 300 mM, and (E)
~> o ~ O
Figure 1-10. Schematic depiction of the Stern-Graham model of the electrical double layer.
Figure 1-11. Schematic diagram of an oil-in-water (O/W) microemulsion
+ I I \Macroemnulsions (IFT = 1 mnN/m)
0 Droplet radius
Mi croemul si ons
(R* = 10-100 nm, IFT 10-3 mN/m)
Figure 1-12. Thermodynamic explanation for behavior of macroemulsions and microemulsions
Table 1-1: Summary of methods used to produce emulsions
Metlul Related to Drops mainly EnergX Mode of Restrictions"
Metlul disrupted by" density Operatione
1. Shaking 4a CD L B N
2. Pipe Flow
a. Laminar 5 V L-M C V
b. Turbulent 4a T L-M C N
3. Injection l0a -L C
a. Simple stirrer 1,2b T,V L B,C
b. Rotor-stator (5) T.V M-H B,C
c. Scraper 5 V L-M B,C V
d. Vibrator 8a ? L BC N
5. Colloid Mill 2a,4c,6 V M-H C V
6. Ball and roller mills 5 V M B(C) V
1. High-press. (2b) T,C,V H C N
a. Vibrating knife 4d C,T M-H C W
b. Magneto-striction C M-H B,C W
9. Electrical l0b Elec. Charge M B(C) Several
10. Aerosol to liquid
a. Mechanical 3 -L-M B.C
b. Electrical 9 -M BC Several
11. Foaming or boiling Spreading L-M (W)
12. Condensation Several
a V-viscous forces in laminar flow, T-turbulence, C=cavitation
b L=low, M-moderate, H=high
a B=batch and C-continuous
d The continuous phase should be V-viscous, N-not too viscous, W=aqueous
Table 1-2: Common tests for determining, emulsion tvoe (W/O or O/W144
Without shaking, a drop of oil is placed in the
emulsion. If W/O type, the added oil dissolves rapidly
in the emulsion and disappears. If O/W type, the added
oil floats on top of the water continuous emulsion.
Electrical conductivity is measured. If W/O,
conductivity is low (like oil). If O/W, conductivity is
high (like water).
Viscosity is measured while a few drops of water are
added to the emulsion. If W/O type, added water
increases viscosity by adding more droplets to the
dispersed phase. If O/W type, added water decreases
viscosity (or shows little change) by slightly diluting the
A parallel beam is shined through an emulsion of low
turbidity. If the beam converges, an O/W type emulsion
is present. If it diverges, it is W/O type. This is due to
the relative refractive indices of oil and water.107
Without shaking, a few drops of water soluble dye are
added. If O/W type emulsion, the dye dissolves rapidly
in the continuous phase, changing color. If W/O type,
the dye dissolves very slowly in the dispersed phase,
and most will sink through the emulsion.
Filter paper is dipped into the emulsion. If O/W type,
paper turns pink immediately from being wetted. If
W/O type, paper stays dry (blue) for a long time since
the continuous phase does not wet it.
Electrical Conductivity Test
Refractive Index Test
Dye Solubilization Test
Filter Paper Test
Table 1-3: Types of breakdown processes occurring in emulsions
Breakdown Type Description
Buildup of an equilibrium droplet concentration gradient
within the emulsion. This phenomenon results from
No change in droplet size external force fields, usually gravitation, centrifugal, or
(or size distribution) electrostatic, acting on the system. "Creaming" is a
special case in which the droplets collect in a
concentrated layer at the top of an emulsion.
No change in basic droplet size,
but ithbuidupof ggrgats o This process is called flocculationn" and results from
the existence of attractive forces between the droplets.
droplets in the emulsion
This process also occurs when creaming or sedimentation
Flocculated droplets in an results in a close-packed array of droplets and these
aggregate coalesce to form larger droplets coalesce. The limiting state is the complete
droplets separation of the emulsion into two immiscible bulk
This process does not involve actual coalescence of
droplets, but rather the transfer of dispersed phase across
continuous phase after solubization occurs. If the
Average droplet size increases --
emulsion is polydisperse, larger droplets will form at the
due to the two liquids forming the
emlso expense of smaller droplets due to the difference in
ermulscion bin ot ttal chemical potential for different size droplets (Ostwald
ripening). In principle, the system will tend to an
equilibrium state in which all the droplets have
combined and are one large droplet, or a separated phase.
This is a questionable "breakdown" process since
essentially another emulsion is formed. The inversion
Emulsion type inverts from W/O
to O/W process can be brought about by numerous parameter
9 changes, which will bue discussed in detail later in this
Table 1-4. Factors influencing the stability of emulsions
Description of Effect
For mechanical stability, a surfactant film with strong lateral
intermolecular forces and high film elasticity is desired. A mixture
of two or more surfactants is preferred over a simple surfactant
(i.e., lauryl alcohol + sodium lauryl sulfate).
Significant only in O/W type emulsions, because of conductivity in
continuous phase. In the case of nonionic emulsifying agents, charge
may arise due to adsorption of ions from the aqueous phase. The
repulsion or attraction can be influenced by changing the thickness
of the double layer, which is described below.
A higher viscosity reduces the diffusion coefficient of the
dispersed droplets, resulting in reduced frequency of collision and
lesser coalescence. Viscosity can be increased by adding natural or
synthetic thickening agents. Viscosity also increases as the number
of droplets increases; so many emulsions are more stable in
concentrated form than when diluted.
Uniform size distribution is more stable than an emulsion with the
same average droplet size but having a wider size distribution.
As volume of dispersed phase increases, stability decreases
Phase inversion can occur if dispersed phase volume is
Usually, as temperature increases, emulsion stability decreases
because of increased frequency of collision.
Addition of polymer that adsorbs at interface can influence stability.
Polymer chains can prevent coalescence due to bulkiness, but
they can also enhance flocculation and decrease stability.
Physical nature of
the interfacial film
Viscosity of the
continuous phase or
of dispersed droplets
Phase volume ratio
Table 1-5. Parameters that affect phase inversion in emulsion and the effect they have.
Effect on phase inversion
W + O + emulsifier & W/O
O + W + emulsifier & O/W
Bancroft's Rule Making the emulsifier more oil soluble tends
to produce a W/O emulsion and vice- versa
Oil/Water Ratio increased in an O/W emulsion +W/O emulsion and
vice-versa, as described above in the text
If surfactant can be dissolved at least partially in either water or oil
Bancroft's Rule If surfactant is dissolved in water+ O/W emulsion
Depends on the surfactant and its temperature dependence.
If emulsion is O/W type with polyoxyethylenated nonionic
surfactant, phase inverts to W/O with increase in temperature due
to increased hydrophobicity of the surfactant.
Strong electrolytes polyvalentt Ca) added to O/W (stabilized by
ionic surfactant) + inversion to a W/O type
Because of decrease in double layer thickness around oil
droplets, droplets coalesce and become the continuous phase.
Order of phase addition
Nature of emulsifier
Phase volume ratio
Phase in which
emulsifying agent is
Addition of electrolytes
Table 1-6. Commonly used physical methods of demulsification
Basic centrifugation techniques are applied to separate emulsions of either O/W or
Centrifugal methods are advantageous when:
1. Viscosity of continuous phase is not too high
2. Droplets are above a certain minimum size dependent on the viscosity
3. Density difference between continuous and dispersed phase is low
Inexpensive, simple process in which time is the only parameter Only useful for larger
size droplets, and usually used only after one of the other methods has formed large
Filter with large surface area is used to collect droplets until enough droplets combine to
form a large droplet, which breaks and moves downstream Wetting of the fibers by the
coalescing dispersed phase is desirable Fiber bed thickness is not a factor, as coalescence
occurs on the front face of the fibers only
More efficient than gravitational settling, but not very efficient for small drops
Low intensity of vibration is necessary for coalescence of droplets High
intensity would cause loosely held floccules to separate
Emulsion passes through a granular bed which coalesces droplets May be either
gravity or pressure driven process Common filters include anthracite coal, sand, and
gravel May have a combination of beds with different filter material at each stage
Widely used on large industrial scale due to low cost of electricity Works on both O/W
and W/O type emulsions For O/W, droplets move towards electrode of opposite charge
and coalesce For W/O, electric field induces a charge on the droplets which causes
them to collide and eventually coalesce at the oppositely charged electrode Since O/W
has a high conductivity continuous phase, charge dissipates rapidly but droplets attract
due to rapid travel of charge through the medium For W/O with a low conductivity
continuous phase, droplets hold charge for a long period of time which allows time for
droplets to travel to the electrode In general, W/O type emulsion coalescence is faster
than O/W coalescence
Heating above 70oC will rapidly break most emulsions Coalescence is increased at
higher temperatures due to diffusion of droplets
Freezing of water droplets in a W/O emulsion will cause ice to form, which expands
and breaks the film + oil envelope. Requires repeated freezing and thawing due to
elasticity of oil envelope Generally uneconomical due to apparatus for repeated
freezing and thawing
Gently shaking or
Granular bed floatation
Table 1-7. A summary of HLB ranges and their application
HLB Range Application
3 to 6 W/O emulsifier
7 to 9 Wetting agent
8 to 18 O/W emulsifier
13 to 15 Detergent
15 to 18 Solubilizer
Stability Thermodynamically Thermodynamically
Droplet size 10 100 nm 20 500 nm
Surfactant Concentration Usually require 10 30 wt% Can be formed with
surfactant 4 8 wt % surfactant
Formation Independent of mixing Depends on mixing
Table 1-8. Microemulsions vs. Nano-emulsions.
Characteristic Mi croemul si ons
A NOVEL METHOD TO ELUCIDATE THE PRESENCE OF SUB-MICELLAR
AGGREGATES INT SURFACTANT SOLUTIONS
It is generally believed that surfactant molecules in micellar solutions exist in equilibrium
in three states: 1) as surfactant molecules that are adsorbed at the interface, 2) as monomers that
are dispersed in the aqueous phase, and 3) as micellar aggregates.l Little, if any, attention has
been given to sub-micellar aggregates, which may in fact be a significant fourth state of
existence, particularly in micellar solutions with short relaxation times (< 100 msec).
Given that micellar solutions are used in various technological and biological processes,
one must consider the potential impact of the often overlooked sub-micellar aggregates. It has
been shown that the monomeric form of surfactants displays significantly different properties in
solution when compared to micelles, especially when considering the disruption of biomembrane
structures and when using micelles for solubilization of proteinS.109, 110
Midura and co-workersll illustrated how one might manipulate the relative concentrations
of monomeric versus micellar forms of various surfactants. They have shown that the monomeric
concentration can be determined by filtering the surfactant solution through ultracentrifuge tubes
having a nanoporous membrane with a nominal molecular weight cutoff that is smaller than the
combined molecular weight of the surfactants in the micellar aggregate (see schematic diagram
in Figure 2-1). They have also shown that the critical micelle concentration (cmc) of a surfactant
can be determined by means of ultrafiltration. "l However, they did not acknowledge the
contribution of sub-micellar aggregates, which will conceivably play a role in such processes as
biomembrane disruption hemolysiss), solubilization of hydrophobic molecules in aqueous
solutions and in drug delivery via micelles.
When one considers the dynamic nature of micelles, it is conceptually obvious that at any
given time, there must be sub-micellar aggregates present in solution in significantly large
quantities, particularly if micelles break and form rapidly (i.e. relaxation times less than 100
msec). If micelles were infinitely stable, then one would only see the existence of monomers and
micelles in the solution. But if the micelles become more and more unstable, the concentration of
sub-micellar aggregates must increase in the solution. In order to prove this hypothesis, we have
performed ultrafiltration experiments of sodium dodecyl sulfate (SDS) surfactant solutions
followed by analysis of the filtrate by the dye complexation method to determine the
concentration of SDS. We have taken ultracentrifugation tubes having a 10,000 molecular weight
cutoff (i.e. molecules or aggregates that exceed this molecular weight, aggregates greater than 34
molecules, will not pass through the filter), and measured the filtrate concentration for SDS
concentrations ranging from 1 mM to 100 mM. Given that SDS has a molecular weight of
288.34 grams/mole and an aggregation number of~-65 molecules/micelle, micelles definitely
will not pass through the ultracentrifugation membrane filter. If the notion that micelles exist in
only three equilibrium states is true, then one would expect the concentration of SDS in the
filtrate to increase as a function of total SDS concentration up to the critical micelle
concentration (cmc), and then for the SDS concentration in the filtrate to remain constant with
respect to increasing total SDS concentration beyond the cmc. If sub-micellar aggregates are a
significant entity (i.e. a fourth equilibrium state) in the micellar phenomena, one might expect
that as the total concentration of SDS exceeds the cmc, the concentration of SDS in the filtrate
will increase, albeit at a different slope (lesser) than the pre-cmc slope due to the presence of
sub-micellar aggregates that are made up of 34 molecules or less (which may pass through the
10,000 MWCO ultracentrifugation filter membranes).
Here, we present the results of our findings, which support the existence of sub-micellar
aggregates and allude to their significance in technological and biological processes. To take our
hypothesis one step further, we examine the effect of micellar dynamics on sub-micellar
aggregate concentration by stabilizing SDS micelles with dodecanol (C120H) and
dodecyltrimethylammonium bromide (C12TAB), (as shown by their relaxation timeS2), and by
changing the counter-ion of the dodecyl sulfate.
2.2 Experimental Procedure
Ultrapure sodium dodecyl sulfate (SDS) from MP Biomedicals, Inc. (Solon, OH) was used
as received. The following chemicals were also used without further purification: n-
dodecyltrimethylammonium bromide (C12TAB) from Tokyo Kasei Kogyo Co. (Tokyo, Japan)
and 1-dodecanol (C120H) from Acros Organics (New Jersey). Double distilled, deionized
Millipure water was used for all solutions.
Centricon YM-3 and YM-10 ultracentrifugation filter tubes, having a 3,000 and a 10,000
molecular weight cutoff (MWCO), respectively, were purchased from Fisher Scientific. Two
milliliters of SDS solutions of various concentrations (ranging from 1 mM to 50 mM) were
placed into the top portion of the ultracentrifugation tubes and subsequently centrifuged at
~2900g (10,000 MWCO) or ~4500g (3,000 MWCO) for approximately 10 minutes so that less
than 10% of the total solution volume was collected as filtrate. All samples were centrifuged in a
bench top IEC Clinical Centrifuge (Damon/IEC Division, Needham Hts, Mass). The filtrate was
collected in the bottom attachment and diluted to the micromolar concentration regime for
analysis by a slightly modified dye complexation method112 and compared to a previously
prepared calibration curve.
2.2.3 Two-Phase Dye Transfer (Methylene Blue Complexation) and UV-Vis Analysis
Methylene blue dye, chloroform, sodium phosphate monobasic, and sulfuric acid were
purchased from Fisher Scientific. Methylene blue reagent was prepared using these materials
according to standard preparation procedure.113 Two milliliters of methylene blue reagent were
added to two milliliters of the diluted filtrate from the ultrafiltration experiments. Two milliliters
of chloroform were added and the solution was shaken on a Vortex mixer for approximately
thirty seconds. Any SDS that was in the fi1trate completed through electrostatic interaction with
the positively charged methylene blue, became oil soluble, and thereby partitioned into the
chloroform organic phase. The solution was allowed to phase separate and the organic phase was
removed and placed into a separate test tube. This process was repeated two more times and the
organic phase was then analyzed by UV-Visible spectrometry at 652 nm and the SDS
concentration was determined upon comparison with a calibration curve. A Hewlett Packard HP
8453 UV-vis spectrometer was used for all UV-Vis analysis.
SDS solutions were also prepared with 1-dodecanol (C120H) or n-dodecyltrimethy-
lammonium bromide (C12TAB) at various SDS:C12X (X= OH or TAB) ratios. These solutions
also underwent ultrafiltration, dye complexation, and subsequent UV-Vis analysis to determine
the SDS concentration in the fi1trate. It must be noted that the dye complexation method was
specific to SDS; therefore the presence of any C12TAB or C120H in the filtrate did not interfere
with the detection of SDS to any significant degree.
Ten milliliter samples of 25 mM SDS, 25 mM SDS + 6.25 mM C120H, and 50 mM SDS +
6.25 mM C12TAB were placed into 100-mL graduated cylinders and capped. Each cylinder was
vigorously shaken 10 times by hand and the volume of the foam is recorded immediately after
shaking. Each solution is tested at least five times and the reproducibility is better than & 2 ml.
2.2.5 Fabric Wetting
A commercially gained 50:50 cotton:polyester blend fabric of I in.2 was placed on the
surface of pure water (control) or surfactant solution at 250C. The surfactant solutions that were
used were 50 mM SDS and 50 mM SDS + 12.5 mM C12TAB. The water or surfactant solution
displaces air in the cotton surface by a wetting process and when sufficient air has been
displaced, the fabric starts sinking. The residence time of fabric on the surface of the solution
before it was completely immersed was measured as wetting time in this study. This wetting time
in each solution was measured at least 5 times.
2.2.6 Dynamic Surface Tension
Dynamic surface tension was measured using the maximum bubble pressure technique.
The pressure required to form a new bubble in solution is measured by a pressure transducer, and
the reading is transmitted to an oscilloscope. For these experiments, the dynamic surface tension
was measured for micellar solutions consisting of 50 mM SDS and 50 mM SDS + 12.5 mM
C12TAB. All dynamic surface tension measurements were taken using a 22 gauge needle tip with
a gas flow rate of 3 cm3/min (Which corresponds to 6 to 13 bubbles per second or approximately
77 to 167 msec per bubble residence time at the needle tip). This flow rate was chosen because at
higher low rates the nitrogen gas forms a continuous j et in the surfactant solution at the needle
tip. At lower flow rates, the results are similar to equilibrium surface tension results.
2.3 Results and Discussion
2.3.1 SDS Surfactant Solutions
During the 1970s, Aniansson and coworkers discovered the existence of two (fast and
slow) relaxation processes and developed a model of the kinetic process of micelle formation and
disintegration.21 The first maj or assumption of this model was that the free surfactant monomers
are assumed to be completely dissociated and the size distribution of the aggregates in a
surfactant solution is assumed to follow the behavior that is shown by the solid line, A, in Figure
2, where monomers and micelles are the predominant species. The curve shown by the dashed
line, B, in Figure 2-2 represents a micellar solution where a substantial fraction of surfactant
exists as sub-micellar aggregates in the solution. This representation suggests that the sub-
micellar region (Region II) is larger and that the actual micellar region (Region III) is broader as
compared to the generally accepted model for micellar solutions.
Aniansson's model suggests that the only important contributing species in micellar
solutions are the micelles themselves and monomeric forms of the surfactant as shown
schematically by Figure 2-3A. Over the years many researchers have based their thermodynamic
models of micelles on such a perspectivell4 (i.e. monomers and micelles as two main species in
We propose here that sub-micellar aggregates are another maj or component of micellar
solutions as shown by Figure 2-3B. If one were to filter either solution from Figure 2-3A or 2-3B
through a nanoporous filter with pore size smaller than the size of micelles, then in the case of
Figure 2-3A, there would only be monomers in the filtrate as reflected by Figure 2-3C; whereas,
in the case of Figure 2-3B, there would be both monomers and sub-micellar aggregates in the
filtrate as reflected by Figure 2-3D. If the surfactant concentration in the filtrate were plotted as
a function of the total surfactant concentration, in the first case, one would expect for the
surfactant concentration in the filtrate to increase proportionally to the total surfactant
concentration up to the cmc, and then the surfactant concentration in the filtrate would remain
the same irrespective of the total surfactant concentration as shown by Figure 2-3E. If the
second case were true, one would expect the same initial behavior (i.e. increase of surfactant
concentration in the filtrate proportional to total surfactant concentration up to the cmc), but
beyond the cmc, the filtrate concentration should continue to increase with a different slope due
to the fact that sub-micellar aggregates (but not the micelles) can pass through the pores (see
Figures 2-3D and 2-3F). In Figure 2-3F, the plot suggests that the monomeric contribution to the
filtrate surfactant concentration is represented by the region from point Q to point P, whereas the
sub-micellar aggregate contribution is represented by the region from point R to point Q.
The concept of sub-micellar aggregates contributing to micelle formation has been
considered previously by Zana39 and by Kahlweit,22, 42 but only for very high (~ 25 times the
cmc) concentrations. It has been well established that micelles have two characteristic relaxation
times associated with them: a fast relaxation time (referred to as zl), which represents the time
that it takes for one surfactant monomer to diffuse into or out of a micelle, and a slow relaxation
time (referred to aS z2), which represents the time that it takes for a single micelle to fully break
down or to fully form. According to Aniansson's model, the slow relaxation time, z2, Should
increase with increasing surfactant concentration.21 However, it has been reported that for some
ionic surfactants, such as SDS, z2 fifSt increases, passes through a maximum, and then decreases
again.38, 39, 41 This behavior in the slow relaxation process of ionic micelles is not predicted in the
Aniansson-Wall model. Kahlweit and coworkers, using their own T -jump and p-jump results,22,
42, 43 COncluded that in ionic surfactant systems at high concentration, the reaction path for the
formation of micelles must be different than that at low concentration. Based on this conclusion,
they came up with a new model for micelle formation (Figure 2-4).
This model is based upon the principle that ionic micelles, including sub-micellar
aggregates, can be considered as charged particles. When ionic surfactant molecules such as SDS
are added to water, the surfactant molecules dissociate into negatively charged dodecyl sulfate
molecules and their positively charged counter-ions. These counter-ions are present in solution
as a cloud surrounding the negatively charged micelle. At low surfactant counter-ion
concentration, the micelles are stable with respect to coagulation due to repulsive electrostatic
forces. Consequently they can grow only by stepwise incorporation of monomers according to
Aniansson's model. As more and more surfactant is added into the system, the counter-ion
concentration also increases, which compresses the electrical double layer around the micelles
and reduces charge repulsion, allowing the micelles to come closer to each other so that
attractive dispersion forces (i.e., Van der Waals forces) lead to a reversible fusion-fission
coagulation according to
Ak Al 7 Al k I = I (2-1)
where k and I are classes of sub-micellar aggregates. Kahlweit42 then represented the micelle
formation reaction path by two parallel resistors, R1 and Rz (Figure 4b), and compared the
formation of micelles to the discharge of a capacitor through two parallel resistors," so that the
change in the monomer concentration with time was given by
dlnA, 1 1
dt r,, r,,
where t21 TeferS to the reaction pathway corresponding to the stepwise formation of micelles by
addition of one monomer at a time, and z22 TeferS to the reaction pathway which corresponds to
the merging of sub-micellar aggregates to form micelles.
At low surfactant concentration, and hence at correspondingly low counter-ion
concentration, Rzis very high due to electrostatic repulsion between sub-micellar aggregates, so
stepwise aggregation dominates and RI determines the rate of micelle formation. As the
surfactant concentration is increased, the counter-ion concentration also increases, and hence, R1
increases as Rz decreases. The concentration where R1 equals Rz is the point where 1 = z2 paSSeS
through a minimum and z2 is highest (for the SDS micelle, this occurs at 200 mM).116 If the
counter-ion concentration is still further increased, R1 becomes so high that R2 determines the
rate of micelle formation according to the reaction mechanism in Equation 2-1). Kahlweit' s
model suggests that the concentration of sub-micellar aggregates in micellar solutions only
becomes significantly large at very high (25 X cmc or above 200 mM) surfactant (and counter-
ion) concentrations. Here, we propose that sub-micellar aggregates are present in relatively large
concentrations even at lower concentrations (3-4 times the cmc). For example, in a 25 mM SDS
solution, the sub-micellar aggregates account for ~ 11-12 mM SDS.
In order to determine if sub-micellar aggregates are indeed a significant component in
micellar equilibrium, SDS solutions were prepared at concentrations below and above the
reported cmc value."' Upon ultrafiltration and analysis of SDS in the fi1trate, we have found that
the SDS concentration in the fi1trate increases nearly proportionally to the total SDS
concentration up to the cmc value. However, beyond the cmc, the SDS concentration does not
remain constant but continues to increase with a decrease in the slope of the curve (Figure 2-5).
Since the SDS concentration in the filtrate does not remain constant beyond the cmc (as
would have been expected if there were no sub-micellar aggregates in the system), we have
concluded that the increase in SDS concentration in the filtrate must be due to the presence of
sub-micellar aggregates that are made up of fewer than 35 SDS molecules. This graph is striking,
as it suggests that in a 50 mM SDS system, given a cmc of ~ 8 mM, more than one-third of the
surfactant molecules (~ 17-18 mM) are in the form of sub-micellar aggregates of less than 35
molecules. This tells us that sub-micellar aggregates represent a significant portion of the
micellar solutions. This is the first conclusive evidence that sub-micellar aggregates represent a
significant portion of micellar solutions and thereby, cannot be ignored. Such a finding has
potential significance in applications related to flux of oil soluble drugs with respect to
controlled drug delivery (sub-micellar aggregates would carry a large percentage of the drug to
the target organs), hemolysis of red blood cells (a system with a large sub-micellar aggregate
population would cause much more hemolysis than a system that has few to no sub-micellar
aggregates), and even possibly in predicting the anti-microbial efficiency of a given solution that
consists of surface active bactericidal agents.
In order to further prove our hypothesis, we decided to make the SDS micelles more stable
by adding C120H or C12TAB in various mole fractions. The Shah research group has previously
shown that one may tailor micellar stability by the addition of long chain alcohols or oppositely
charged surfactants2, 28 (Such as alkyltrimethylalmmonium bromides, commonly referred to as
CnTABs) as shown in Figure 2-42 and that the stability is especially enhanced when the additives
have the same chain length as the SDS.30 The long chain alcohols enhance micellar stability
through charge shielding and the long chain TABs enhance micellar stability through
electrostatic interactions between their positively charged head group and the negatively charged
head group of SDS. We have noticed that the addition of C12TAB tends to better stabilize the
micelles when compared with C120H.
Given this previous knowledge, we began by adding C12TAB in increasing mole fractions
to the SDS system to see if the concentration of SDS in the fi1trate decreases. As shown in Figure
2-7, the filtrate concentration does indeed decrease for C12TAB mole fractions up to 20 mole%
and subsequently levels off. It is important to note that by adding up to 20 mole % C12TAB, the
concentration of SDS in the fi1trate was reduced from 26 mM to ~3 mM. This is an amazing
Ending which suggests that the addition of the C12TAB made the micelles so stable that only 3
mM SDS was free, presumably as SDS monomers, to pass through the fi1ter (i.e. the
SDS+C12TAB micelles are behaving somewhat like rigid spheres).
If this is the case, then if we reproduce Figure 2-5, but with the addition of 20 mole %
C12TAB and C120H for all SDS concentrations, then one would expect that the curve beyond the
cmc should shift towards zero slope, with SDS having the steepest slope, C12TAB having the
near zero slope, and C120H falling in between the two curves. As can be seen by Figure 2-8, the
addition of 20 mole % of C120H and C12TAB did indeed decrease the slope beyond the cmc,
with C12TAB leading to a virtually zero slope line!
This confirms our hypothesis, but in order to completely eliminate any possible doubt, we
decided to perform one more critical experiment. If there are indeed no sub-micellar aggregates
in the 80:20 SDS:C12TAB system, then if these solutions were filtered through a filter that has an
even smaller molecular weight cut-off (MWCO) than 10,000, one would expect that the SDS
concentrations in the filtrate for a given SDS/C12TAB system should be the same irrespective of
the MWCO. We ran the samples in 3,000 MWCO where aggregates with less than 11 SDS
molecules can pass through the pores of the membrane tubes at ~ 4500g for 10 minutes and
plotted the results together with the 10,000 MWCO results and as seen in Figure 9, the SDS
filtrate concentrations in the two different filter sizes were approximately the same in all cases
where C12TAB was added. In the case of 50 mM SDS (with no added C12TAB), as expected,
there is a significant difference between the filtrate concentration from the 3,000 MWCO tubes
versus the 10,000 MWCO tubes. This suggests that in mixed systems of SDS and C12TAB, there
are no aggregates larger than 11 molecules, whereas in pure SDS micellar solutions, there are
aggregates at least up to 34 molecules and possibly having even higher aggregation numbers.
Various other physical properties of SDS systems, including osmotic pressurell6 and
conductivity,120 have been shown to increase proportionally to the total SDS concentration up to
the cmc and subsequently display a change in slope just as seen in our ultrafiltration studies. We
believe that the presence of sub-micellar aggregates contributes significantly to the deviation
from "ideality" that is observed in the osmotic pressure of SDS as reflected in the work of Amos
and coworkers.116 We are also convinced that the rise in conductivity beyond the cmc is partially
due to the increasing concentration of sub-micellar aggregates in addition to the number of
Micelles have also been used throughout the years as a vehicle for carrying out various
reactionsls and it has been shown that the reaction rates are dependent upon the micellar
stability.ll We believe that the underlying factor here is the contribution of sub-micellar
aggregates, which may very well solubilize some of the reactants and thereby significantly
influence the reaction kinetics. Therefore, this is yet another application of micelles where the
presence of sub-micellar aggregates must be considered when utilizing various surfactant
sy stem s.
Sodium dodecyl sulfate was used as the chosen surfactant for these studies because it is an
extensively studied surfactant.46, 119-121 However, Midurall illustrated that this phenomenon of
increasing fi1trate concentration beyond the cmc holds true for at least two other surfactant
systems: Triton X-100 and Chaps ((3-[(3 -cholamidopropyl)-dimethylammonio]- 1-
propanesulfate). This shows that the presence of sub-micellar aggregates is not limited only to
ionic surfactant, but that they are present in non-ionic surfactant systems as well. Based on these
Endings, care must be taken when modeling surfactant micellar solutions and when designing
micellar solutions for usage in controlled drug delivery, anti-microbial solutions, for
solubilization and denaturing of proteins," and when considering the hemolytic activity of a
given surfactant system. These Eindings also provide great insight into the actual mechanism of
micelle relaxation in surfactant solutions and allows for further correlation to various
technological processes, such as foaming, detergency, fabric wetting, and emulsification.
2.3.2 Effect of Counter-lons on Sub-Micellar Aggregate Concentration
Another interesting aspect of micellar solutions is the effect of counter-ions on the micellar
stability. Pandey et. al.125 have shown how counter-ions affect surface and foaming properties of
dodecyl sulfates. They have illustrated how changing the counter-ion in dodecyl sulfates from
sodium to lithium, cesium or magnesium leads to distinct differences in the corresponding
dynamic surface tension of the solutions. Dynamic surface tension is a measure of the actual
surface tension of the interface at a specific point in time where new liquid/liquid or gas/liquid
interfaces are rapidly being generated in a surfactant solution. Dynamic surface tension directly
reflects the surfactant concentration at the interface at that point in time, and hence the
availability of monomers and sub-micellar aggregates to diffuse to and stabilize the newly
created interfaces such as those created in the generation of foams and emulsions. As such, it is
heavily dependent upon micellar stability in that a more unstable micelle will provide more
monomers and sub-micellar aggregates to diffuse to the interface. Dynamic surface tension can
be measured by the maximum bubble pressure methodl26 and can be represented by using the 6
parameter which normalizes the dynamic surface activity with respect to the equilibrium surface
activity as follows:
6 = yd Teq w eq), (2-3)
where yd is the dynamic surface tension, yeq is the equilibrium surface tension measured by the
Wilhelmy plate method, and 7, is the surface tension of pure water at 250C. The value of 8 = 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 and the monomers are diffusing fast,
whereas 8 = 1 (7d = 7,) 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.
Pandey et. al 122 meaSured the dynamic surface tension of four dodecyl sulfate solutions
(having lithium, sodium, cesium, and magnesium as counter-ions) and reported the 6 parameter
as shown in Table 2-1 below. The 6 parameter values are lower and similar for lithium dodecyl
sulfate (LiDS) and sodium dodecyl sulfate (referred to here as NaDS to distinguish the counter-
ion), while they are higher for cesium dodecyl sulfate (CsDS) and magnesium dodecyl sulfate
(Mg(DS)2), Suggesting a higher dynamic surface activity of LiDS and NaDS. This finding
suggests that the LiDS and NaDS have a lower micellar stability, while the CsDS and Mg(DS)2
have a higher micellar stability. Readers may refer to reference 33 for a more detailed
explanation of the factors responsible for the counter-ion effects on micellar stability. Pandey et.
al found that the trends in dynamic surface tension correlated well to the foamability behavior in
these systems as well.
The Shah research group has shown that more stable micelles tend to have low
foamability, but high foam stability.2 The stability of foam depends on how quickly liquid is
drained from the foam lamellae.90 Nikolov and Wasanl23 extensively studied the micellar
structure inside the thin liquid film of the foam lamellae and they showed that the drainage of the
liquid film can be explained by a layer-by-layer thinning of ordered structures of micelles inside
the film. This structured phenomenon is a reflection of the micellar effective volume fraction,
stability, interaction, and polydispersity.
Based on these studies of counter-ion effects on micellar stability, we decided to determine
the relative concentrations of the sub-micellar aggregates in LiDS, NaDS, CsDS, and Mg(DS)2.
We prepared 25 mM solutions of the LiDS, NaDS, and CsDS and a 12.5 mM solution of the
Mg(DS)2. The Mg(DS)2 WAS prepared at half the concentration of the other solutions because it
has two dodecyl sulfate chains in every molecule. After centrifugation in the 10,000 MWCO
filter tubes, we measured the filtrate surfactant concentration by the dye complexation method.
We were pleased to discover that the trends in filtrate concentrations relative to counter-ion
correlated well with the dynamic surface tension values reported by Pandey et al.122 (See Figure
Figure 2-10 clearly shows that the filtrate surfactant concentrations are significantly higher
for LiDS and NaDS than they are for CsDS and Mg(DS)2. This behavior gives further credence
to our speculation that the presence of sub-micellar aggregates is directly linked to the micellar
stability of a given surfactant system.
2.3.3 Importance of Sub-Micellar Aggregates in Technological Processes
The Shah research group has shown significant evidence correlating micellar stability to
various technological processes including foamability,124, foam stability,2 emulSion droplet
size,46 fabric wetting,125 and detergency.126 The effect of micellar stability on these processes was
explained on the basis of the micelles ability to break and supply monomers to the bulk that can
adsorb at the interfaces that are created in each application. For example, in the case of
foamability, a less stable (more labile) micelle will break rapidly, giving up its monomers to
stabilize the foam against instantaneous breakdown. However, this explanation has been met
with a bit of skepticism over the years because the timescale of micellar breakdown
(milliseconds) is so much shorter than the timescale of these processes. This argument does have
merit and until now, there was no sufficient explanation for how micellar stability extended to
effect technological applications. We have shown here the intimate relationship between micellar
stability and sub-micellar aggregates. It follows that the operating molecular mechanism in the
dependence of the aforementioned technological processes on micellar stability is directly related
to the flux of not only monomers, but also to a larger extent, to the flux of sub-micellar
aggregates to the newly created interfaces in each of the processes. The more labile the micelle,
the higher is the concentration of sub-micellar aggregates and higher is the flux of the monomers
to the interface.
A foam is a coarse dispersion of a gas in a liquid with the gas making up most of the phase
volume and with the liquid in thin sheets, lamellaee" between the gas bubbles.127 Foamability,
the degree to which a surfactant solution is able to generate foam, is a relevant property for many
industrial applications, including detergency, food processing, and mineral floatation. One of the
maj or factors that affect foamability is the ability of the stabilizing agent (surfactant in this case)
to adsorb at the newly created air/water interface to prevent immediate breakdown of the foam.63
Therefore, foamability is highly dependent upon the concentration of monomors and sub-
micellar aggregates which can readily provide the monomers to the interface. Figure 2-11 shows
a schematic diagram of the lamellae of a foam.
In 1991 Oh and Shahl24 Showed that foamability is influenced by the average lifetime of a
micelle. We have shown that the sub-micellar aggregate concentration is very high in solutions
of SDS alone, lower in solutions of SDS + C120H, and almost non-existent in solutions of SDS +
C12TAB. After measuring the foamability of each of these systems (25 mM SDS, 25 mM SDS +
6.25 mM C120H, and 25 mM SDS + 6.25 mM C12TAB) we found that the SDS system, which
has the highest concentration of sub-micellar aggregates, also displayed the greatest foamability
as shown in Figure 2-12. The foamability decreases with increasing micellar stability, with the
SDS + C120H micellar solution generating less foam than the SDS solution, and the SDS +
C12TAB micellar solution generating the least amount of foam. Since the SDS + C12TAB
solution had few, if any, sub-micellar aggregates, only free monomers were available to adsorb
at the air/water interface of the foam. The free monomer concentration is very low in this
solution, and as such, the foamability is low.
188.8.131.52 Fabric wetting
Fabric, or textile, wetting is another process where the presence of sub-micellar aggregates
is important. Due to the large surface area of fabrics, equilibrium conditions are rarely attained in
the time allowed for wetting in practical processes.3 As such, the kinetics of surfactant adsorption
at the solid/liquid interface of the fabric is a controlling parameter in fabric wetting. If there is a
high concentration of surfactant that is available to adsorb at the interface, making the fabric
more water-wettable, the wetting time will be short. The hydrophobic tails of the surfactant
molecules adsorb onto the fabric and lower the interfacial tension so that water can penetrate into
the interstitial spaces of the fabric weave. Therefore, one would expect a solution of SDS (50
mM), which has a high sub-micellar aggregate concentration, to have a faster wetting rate than a
solution of 50 mM SDS + 12.5 mM C12TAB. Wetting experiments were performed on these
systems, and as expected the fastest wetting time was found for the SDS solution, as can be seen
in Figure 2-13. The fabric wetting time was also measured in pure water as a control.
184.108.40.206 Dynamic surface tension
Dynamic surface tension is a measure of the ability of surfactant molecules to adsorb at
newly created interfaces under dynamic conditions. Dynamic surface tension can be measured by
determining the maximum bubble pressure when gas is bubbled through a surfactant solution at a
specific flow rate. When there is a significant concentration of monomers and sub-micellar
aggregates, the dynamic surface tension is low because the flux of monomers to the interface is
high. The dynamic surface tension was found to increase with increasing micellar stability so
that a solution of 50 mM SDS exhibited a lower dynamic surface tension than a mixed micellar
system of 50 mM SDS + 12.5 mM C12TAB as shown in Figure 2-14. This is an interesting
finding because the equilibrium surface tension of an SDS solution will always be lower than
that of an SDS/C12TAB mixed solution. Therefore, one must take care when choosing surfactant
solutions for various dynamic processes, because if the micelles are too stable, there will be a
low concentration of sub-micellar aggregates, and the solution will not be able to effectively
lower the surface or interfacial tension.
Here, we have presented the first conclusive evidence of sub-micellar aggregates as a
significant component in micellar solutions. The following conclusions can be drawn based
upon the results reported here:
1. The generally accepted notion that surfactant solutions consist of only three
compartments, namely, adsorbed film at air/water interface, monomers, and micelles, is
incorrect and we have shown that sub-micellar aggregates are a significant entity in
surfactant solutions making up as much as one-third of the surfactant concentration for
example, as in 50 mM SDS.
2. We have shown here that micellar dynamics are intimately linked to the presence of sub-
micellar aggregates (i.e. the more stable a micelle is, the less is the concentration of sub-
micellar aggregates in the system) and that by stabilizing SDS micelles through the
addition of dodecanol (C120H) or dodecyltrimethylammonium bromide (C12TAB), we
can effectively eliminate sub-micellar aggregates and reduce the monomeric surfactant
concentration to values as low as 3 mM.
3. It is known that counter-ions affect micellar stability. We have shown that counter-ions
also affect the concentration of sub-micellar aggregates in dodecyl sulfate systems (i.e.
counter-ions such as Mg2+ and Cs which enhance micellar stability, were shown to have
lower concentrations of surfactant in the filtrate, whereas, Li and Na which form
relatively unstable micellar systems, have higher concentrations of surfactant in the
filtrate) which correlates well with previously reported dynamic surface tension data.122
4. We have shown that sub-micellar aggregates, or micellar fragments, exist in the micellar
solution even at low concentrations, such as 25 100 mM.
5. We have determined that it is the presence of sub-micellar aggregates that provides the
missing link in our understanding of the effect of micellar stability on technological
processes. We have shown that when there is a high concentration of sub-micellar
aggregates, the foamability is the highest, the fabric wetting rate is the fastest, and the
dynamic surface tension is the lowest.
Figure 2-1. Schematic diagram of the ultracentrifugation process. Sample is placed in top portion
of tube and the tube is centrifuged at ~2900g (for 10,000 MWCO ultracentrifuge
tubes) or ~4500g (for 3,000 MWCO ultracentrifuge tubes) for 10 minutes so that less
than 10% of the solution volume is collected as filtrate.
Force = 2900 g
r' /' 20 \
Aggregation number, n M
Figure 2-2. Size distribution curves of aggregates in a micellar solution. The solid curve
represents a typical size distribution curve of aggregates in a micellar solution
according to the Aniansson-Wall model of stepwise micellar association. Region (I)
corresponds to monomers and oligomers; Region (III) to abundant micelles with a
Gaussian distribution around the mean aggregation number; and Region (II) to the
connecting "wire" (heat transfer analogy) or "tube" (mass transfer analogy) between
Regions (I) and (III). The dashed curve represents our hypothesized size distribution
for a micellar solution containing a significant concentration of sub-micellar
Figure 2-3. Schematic diagrams of surfactant solutions, filtration of solutions, and plot of filtrate
concentration as a function of total surfactant concentration. A) Schematic diagram of
surfactant solution above cmc consisting of three components; adsorbed film of
surfactant molecules, surfactant monomers, and micelles; B) Schematic diagram of
surfactant solution above cmc consisting of four components; adsorbed film of
surfactant molecules, surfactant monomers, sub-micellar aggregates and micelles; C)
Schematic illustration of filtration of solution from Figure 3A where only surfactant
monomers are present in the filtrate and micelles were not able to pass through the
filter; D) Schematic illustration of filtration of solution from Figure 3B where
surfactant monomers and smaller sub-micellar aggregates are present in the filtrate
and micelles and larger sub-micellar aggregates were not able to pass through the
filter; E) Expected results of a plot of surfactant concentration in the filtrate versus
total surfactant concentration for a system like the one shown in Figure 3A. F)
Expected results of a plot of surfactant concentration in the filtrate versus total
surfactant concentration for a system like the one shown in Figure 3B. The monomer
contribution to the filtrate is represented by the region from point Q to point P and the
sub-micellar aggregate contribution to the filtrate surfactant concentration is
represented by the region from point R to point Q.
Sub -mi cellar
Figure 2-3C Figure 2-3D
Figure 2-4. Schematic representation of the two possible reaction paths for the formation of
micelles (a) and the corresponding resistances (b): (1) formation by incorporation of
monomers and (2) formation by reverse coagulation of sub-micellar aggregates.
Conc. SDS in
0 10 20 30 40 50
total [SDS] (mM)
Figure 2-5. Filtration of SDS through 10,000 MWCO ultracentrifuge tubes for ~10 minutes at
2900*g. The point at which the slope changes is considered the critical micelle
concentration (cmc). The dotted line is the result that one would have expected if
there were no sub-micellar aggregates present in the system.
25 mM SDS, T, = 1 ms.
25 mnM SD)S + 1.15 mM CzOH, ty = 230 ms.
~IB 25 mM S)S + 10 mM3 C2TAB, r2 = 2000 ms
Figure 2-6. Tailoring of micellar stability by the addition of 1-dodecanol (C 120H) or n-
dodecyltrimethylammonium bromide (C12TAB).
Total [SDS] = 50 mMI
0 5 10 15 20 25 30
mole % C12TAB in soln
Figure 2-7. Filtrate of SDS+C12TAB through 10,000 MWCO ultracentrifuge tubes for ~10
minutes at 2900g. The C12TAB mole fraction was increased from 5 mole% to 25
mole%. The total SDS concentration is fixed at 50 mM.
35 '" "
B 25 (
0 20 40 60 80 100 120
total [SDS] (mM)
OSDS alone 480:20 SDS:C12TAB mixed micelles X 80:20 SDS:C120H
Figure 2-8. Filtration of SDS alone or SDS + C12X (X = OH or TAB) through 10,000 MWCO
ultracentrifuge tubes for ~10 minutes at 2900g. The C120H and C12TAB were added
at a molar ratio of 80:20 SDS:C12X (i.e. the concentration of C120H or C12TAB in
each system is 20 mole % of the total surfactant concentration (SDS+C12X)).
7.5mMSDS/1.875mM 10mMSDS/2.5mM 25mMSDS/6.25 mM 50mMSDS/12.5mM 50mMSDS
C12TAB C12TAB C12TAB C12TAB
H3000 MWCO 10,000 MWCO
Figure 2-9. SDS concentration in the filtrate for 80:20 SDS:C12TAB systems after filtration
through 3,000 and 10,000 MWCO tubes, as compared to pure SDS solutions (50
mM). Samples in 3,000 MWCO ultracentrifuge tubes were centrifuged at ~ 4500g
and samples in 10,000 MWCO ultracentrifuge tubes were centrifuged at ~ 2900g for
Total Concentration = 25 mM for all samples except
Mg(DS)2, which has a total concentration = 12.5 mM
Figure 2-10. Filtrate surfactant concentrations for 25 mM lithium dodecyl sulfate (LiDS), sodium
dodecyl sulfate (NaDS), and cesium dodecyl sulfate (CsDS) and 12.5 mM
magnesium dodecyl sulfate (Mg(DS)2).
Figure 2-11. Schematic depiction of foam column generated by passing air through a surfactant
solution (left) and magnified view of foam lamella, thin sheet of surfactant solution
between adjacent air bubbles (right).
25 mM SDS
High conc. sub-micellar aggs.
Low conc. sub-micellar aggs.
Figure 2-12. Foamability of SDS micellar solution and SDS + C12X mixed micellar solutions (X
= OH or TAB)
Low conc. sub-micellar aggs.
High Fabric wetting time
High conc. sub-micellar aggs.
Low Fabric wetting time
50 mM SDS
50 mM SDS/12.5
Figure 2-13. Wetting time of lin2 Strips of 50:50 cotton:polyester blend fabric in pure water, 50
mM SDS, and 50 mM SDS + 12.5 mM C12TAB.
22 gauge needle
45 _C6 13 bubbles/sec
Air flow rate = 3 cc/min
50 mMI SDS 50 mMI SDS + 12.5 mMI C12TAB
Figure 2-14. Dynamic surface tension of solutions of 50 mM SDS and 50 mM SDS + 12.5 mM
Table 2-1. Dimensionless dynamic surface tension (6) of different counter-ions of dodecyl
sulfates (50 mM) at a bubble lifetime of 50 msec (from ref 33).
Ion 6 parameter
DETERMINATION OF DRUG AND FATTY ACID BINDING CAPACITY TO PLURONIC
F l27 INT MICROEMULSIONS FOR DETOXIFICATION
Drug overdose incidences are a common and problematic occurrence both nationally and
globally. Many life-threatening drugs do not have specific pharmacological antidotes to reverse
the toxic effects that result when an overdose occurs.99 Attempts are currently underway to
develop procedures to detoxify blood in a timely and efficient manner.128-130 Therefore, the
development of an effective methodology for the removal of free drug from the blood of an
overdosed patient in a timely manner (less than 15 minutes) is critically important. In the past
few years, efforts have been underway to use nanoparticulate systems to accomplish this task.
Microemulsions are one of the systems that are currently under investigation. Upon injection of a
biocompatible, nontoxic microemulsion in the blood of an overdosed person, the microemulsion,
having extremely high interfacial area, can effectively adsorb and solubilize drug molecules, and
thereby quickly decrease the concentration of free drug molecules in the blood. However, in
order to fully grasp their function as toxicity reversal agents, one must understand the molecular
mechanism of drug uptake and be able to determine and manipulate the contributing interfacial
Preliminary results from pH studies have led us to believe that electrostatic forces can play
a significant role in adsorption of drug onto the microemulsion. Amitriptyline Hydrochloride,
shown in Figure 3-1, is an antidepressant and as of yet, there is no efficient method to reverse the
effects of an overdose in a patient; therefore it is the target drug for the experiments reported
here. Amitriptyline has a pKa of approximately 9.4 so that at physiological pH (~ 7.4), it will be
positively charged and can thereby interact through electrostatics with a negatively charged
microemulsion. These microemulsions are composed of Pluronic F l27, Ethyl Butyrate, and
Sodium Caprylate fatty acid (which gives the negative charge) and are prepared in Phosphate
Buffered Saline at pH 7.4. The obj ective of this study is to develop a better understanding of the
important interactions that occur between the microemulsion and the drug.
We have shown, through turbidity analysis experiments, that there is a linear relationship
between the Amitriptyline Hydrochloride solubilization capacity (i.e. the amount of
Amitriptyline that the microemulsion can accommodate before turbidity occurs) of the
microemulsions and Pluronic surfactant concentration up to a certain Pluronic Fl27
concentration. Above that critical Pluronic F l27 concentration, further titration with
Amitriptyline never yields turbidity. We have also seen that turbidity is not observed in systems
that do not have sodium caprylate present. Based on these findings we have concluded that at the
critical Pluronic concentration, there is no longer any free unassociatedd) sodium caprylate
molecules in the bulk phase, presumably due to binding of all fatty acid molecules with Pluronic
molecules. Therefore, we are able to determine how many molecules of sodium caprylate and
Amitriptyline are associated with each Pluronic molecule. Each Pluronic F l27 molecule can
associate with approximately eleven molecules of sodium caprylate and twelve molecules of
Amitriptyline at the critical concentration (i.e. there appears to be a nearly 1:1 association of
sodium caprylate to Amitriptyline). This yields further credence to ultrafiltration studies that we
have done as a function of pH which show that electrostatic interactions are important in
Amitriptyline binding to microemulsions produced by Pluronic F l27 and fatty acid soap. The
findings of this study will provide substantial information regarding the mechanism of reduction
of overdosed drugs and will allow us to approximate the uptake capacity of a particular
3.2 Experimental Procedure
3.2.1 Materials. Pluronic surfactants were obtained from BASF Inc. (Mount Olive, NJ).
Pluronic was used as a nonionic surfactant composed of a symmetric triblock copolymer of
propylene oxide (PO) and ethylene oxide (EO). The polypropylene oxide block was sandwiched
between the more hydrophilic poly(ethylene oxide) blocks. The block copolymer was denoted by
(EO)x(PO),(EO)x, where x and y are the number of units of EO and PO, respectively.
Amitriptyline Hydrochloride, sodium caprylate, sodium decanoate, and sodium dodecanoate
were purchased from the Sigma Chemical Co. (St. Louis, MO). Ethyl butyrate was purchased
from ACROS Organics (New Jersey). Sodium phosphate monobasic, sodium phosphate dibasic,
sodium chloride, and potassium chloride which were used to prepare the phosphate buffered
saline were purchased from Fisher Scientific Inc. (Suwanee, GA). Double distilled, deionized
Millipure water was used for all solutions.
3.2.2 Microemulsion Preparation. Oil-in-water microemulsions were prepared by first
solubilizing the appropriate concentration (3 9 mM) of Pluronic Fl27 surfactant in phosphate
buffered saline at pH 7.4 (physiological pH). Sodium caprylate (fatty acid surfactant) was then
added to this Pluronic solution in concentrations ranging from 25 100 mM. Lastly, ethyl
butyrate (oil) was added to the solution and the system was stirred until it became clear. The
ethyl butyrate concentration was fixed at 110 mM for all experiments in which microemulsions
were used. The microemulsions were subsequently allowed to equilibrate for at least one day
pnior to use.
3.2.3 Turbidity Analysis. Micelles, mixed micelles and microemulsions were prepared
with varying compositions of Pluronic Fl27, and/or Sodium Caprylate, and/or Ethyl butyrate.
The aqueous phase was phosphate buffered saline (PBS) with a pH ~ 7.4. Ten milliliters of the
micelle or microemulsion sample was placed into a vial. The solution was titrated with 0.2 M
Amitriptyline (prepared in PB S) until the onset of turbidity was observed visually. The systems
were sensitive enough that the transition from clear to turbid was very sharp (i.e. occurring over
a change in volume of 50 microliters or less). In some systems, prior to the system reaching
turbidity, upon each incremental addition of Amitriptyline, the solutions would exhibit a
momentary cloudiness, but gently swirling would lead to a return in clarity. During titration, if
the initial cloudiness was not observed upon the additions of Amitriptyline, copious amounts of
drug was added to that system; if turbidity was not observed, then the system was categorized as
one where turbidity would never occur.
3.2.4 Dynamic Surface Tension. Dynamic surface tension was measured using the
maximum bubble pressure technique. The pressure required to form a new bubble in solution is
measured by a pressure transducer, and the reading is transmitted to an oscilloscope. For these
experiments, the dynamic surface tension was measured for microemulsions consisting of fixed
sodium caprylate (100 mM) and ethyl butyrate (110 mM) concentrations and increasing
concentrations of Pluronic F l27. All dynamic surface tension measurements were taken using an
18 gauge needle tip with a gas flow rate of 5 cm3/min (Which corresponds to 3 to 10 bubbles per
second or approximately 100 to 333 msec per bubble residence time at the needle tip). We chose
this flow rate because at higher low rates the nitrogen gas forms a continuous jet in the surfactant
solution at the needle tip. At lower flow rates, the results are similar to equilibrium surface
3.2.5 Foamability. Twenty milliliter samples of microemulsions consisting of fixed
sodium caprylate (100 mM) and ethyl butyrate (110 mM) concentrations and increasing
concentrations of Pluronic F l27 were placed into 100-mL graduated cylinders and capped. Each
cylinder was vigorously shaken 10 times by hand and the volume of the foam is recorded
immediately after shaking. Each solution is tested at least three times and the reproducibility is
better than + 2 ml.
3.2.6 Fabric Wetting. A commercially gained cotton fabric of 1 in.2 was placed on the
surface of microemulsion solution at 250C. The microemulsions used consisted of fixed sodium
caprylate (100 mM) and ethyl butyrate (110 mM) concentrations and increasing concentrations
of Pluronic F l27. The surfactant solution displaces air in the cotton surface by a wetting process
and when sufficient air has been displaced, the cotton starts sinking. The residence time of cotton
fabric on the surface of the solution before it was completely immersed was measured as wetting
time in this study. This wetting time in each microemulsion solution was measured at least 3
3.2.7 Surface Tension. Surface tension measurements were carried out to determine the
critical micelle concentration (cmc) using the Wilhelmy plate method. In this method, the plate is
lowered into surfactant solutions of known concentrations and the corresponding output from a
gram-force sensor holding the plate is sent to a transducer and then to a voltage readout. The
system was calibrated using two known solutions (water at 72.5 mN/m and acetone at 23 mN/m).
The platinum plate was heated between each reading to clean off anything that may have
adsorbed onto the plate.
3.3 Results and Discussion
3.3.1 Effect of Sodium Caprylate Concentration on Drug and Fatty Acid Binding to
As previously reported,99 we have taken a systematic approach to design a biocompatible
microemulsion system that would effectively reduce the free concentration of target drugs in the
blood. This microemulsion system is composed of Pluronic Fl27 as the surfactant, sodium
caprylate (SC) fatty acid as the co-surfactant, ethyl butyrate (EB) as the oil phase and is prepared
in a phosphate buffered saline solution at pH 7.4. Given that Pluronic Fl27 is a block copolymer
and sodium caprylate is a co-surfactant, if we can understand the nature of the polymer-
surfactant interactions in this microemulsion, then we can have a better understanding of the
structure of the microemulsion and the molecular mechanism of uptake of the drug. For many
years now, polymer-surfactant interactions have been studied extensively in relation to various
interfacial processes.84, 131-137 One of the methods of analyzing polymer-surfactant interactions is
through titration studies.54, 68 Here, we take various microemulsion compositions and titrate them
with concentrated Amitriptyline solutions to turbidity. We are using the results of these studies to
determine the pertinent stoichiometric ratios in our optimal microemulsion formulations.
For our initial titration studies we took the various microemulsion components and titrated
them individually. So our first titrations were of sodium caprylate (SC) solutions in phosphate
buffered saline (PBS) (pH 7.4). In this study, we found that at 100 mM SC, turbidity occurred
when 1 molecule of AMT was added for every 100 molecules of SC.
Next, we titrated systems containing only Pluronic Fl27 in PBS (pH 7.4) with 0.2 M
AMT. In these systems we found that turbidity was never obtained, irrespective of how much
AMT was added to the system. Then we added the ethyl butyrate oil to the Pluronic Fl27
systems and titrated these solutions with 0.2 M AMT. Once again, turbidity was never obtained.
Finally, we added our last component, the sodium caprylate fatty acid, to the system and found
that upon titration with 0.2 M AMT, turbidity was seen in these systems. For these systems, the
sodium caprylate concentration was held fixed at 100 mM, the ethyl butyrate concentration was
held fixed at 110 mM, and the Pluronic F l27 concentration was varied from 3 mM to 9 mM. One
of the interesting observations that we noticed in these titrations was that turbidity was achieved
for every Pluronic F l27 concentration up to 8 mM. Above 8 mM F l27, turbidity was never
achieved (see Figure 3-2).
Our next experiment involved titration of Pluronic F l27 and sodium caprylate mixed
micellar systems (i.e. no oil is present). In this case, the sodium caprylate concentration was held
fixed at 100 mM and the F l27 concentration was varied from 1 mM to 9 mM. We were
somewhat surprised to see that the lack of oil in these systems did not seem to affect the amount
of AMT needed to induce turbidity (i.e. the graph for the mixed micellar system is nearly the
same as that of the microemulsion system (see Figure 3-3).
These experiments provided us with two important insights. First, turbidity is only
observed in the systems where the sodium caprylate is present. Based on this finding, we can
conclude that the turbidity is arising from AMT forming a complex with the SC. Secondly, in the
systems where Pluronic Fl27 is present with SC, turbidity is observed up to some critical Fl27
concentration, above which turbidity is no longer observed. Based on this finding, we can
conclude that the critical F l27 concentration is the concentration at which no more SC exists as
free monomers in the bulk solution and that the turbidity is a result of the AMT completing with
the free SC in the bulk. Figure 3-4 provides a schematic illustration of this hypothesis.
In order to test our hypothesis we did the turbidity experiments for various sodium
caprylate concentrations. If our hypothesis is correct, we would expect for the critical Fl27
concentration to decrease proportionally to the decrease in SC concentration. As can be seen in
Figure 3-5, the decrease in the critical concentration of F 27 is indeed nearly proportional to the
decrease in the SC concentration. The critical Fl27 concentration is never reached in the system
where the SC concentration is 125 mM because above a F 27 concentration of 9 mM, the
solution becomes a gel.