Hydrocarbon Catanionic Surfactants

Journal of Colloid and Interface Science 234, 117–121 (2001) doi:10.1006/jcis.2000.7284, available online at http://www.idealibrary.com on

Winsor I⇔III⇔II Microemulsion Phase Behavior of Hydrofluoroethers and Fluorocarbon/Hydrocarbon Catanionic Surfactants Jimmie R. Baran, Jr. Advanced Materials Technology Center, 3M, Company, 3M Center, Building 201-1W-28, Maplewood, Minnesota 55144 Received April 13, 2000; accepted October 16, 2000

The Winsor type phase behavior of hydrofluoroethers (HFEs) has been evaluated using several fluorocarbon/hydrocarbon catanionic surfactants. It was found that these systems could be rendered temperature insensitive by changing the identity of the ions. ° C 2001

Academic Press

Key Words: microemulsions; hydrofluoroethers; catanionic surfactants.

INTRODUCTION

Systematic studies of microemulsion formation involving hydrocarbons have been reported many times in the literature (1). There have also been studies on fluorocarbon microemulsions (2). Due to the enhanced solubility of gases in fluorocarbon fluids, much interest has been shown in formulating microemulsions of these fluids for biomedical applications, such as synthetic blood (3). This report focuses on a new class of oils, hydrofluoroethers (HFEs), and their use in microemulsions. HFEs are environmentally friendly, nonflammable, non-ozone-depleting solvents that have recently been marketed by 3M, Co. (4). HFEs generally have the structure Rf –O–Rh ,

[1]

where Rf is a fluorinated hydrocarbon that may contain branching within the chain and/or may contain other heteroatoms. The Rh group is a hydrocarbon that also may contain branching within the chain and/or other heteroatoms. To date, no systematic study of microemulsion behavior with HFEs has been reported, in part due to difficulty in identifying useful surfactants for these systems. A systematic approach to selecting and optimizing surfactants for microemulsion formation would be to study the classical Winsor type phase behavior of the systems (5). In order to obtain meaningful results from this type of investigation, all system variables except one should be held constant. In this study, only the salinity is varied within the systems of interest. At low salinity, Winsor type I (oil-in-water, O/W) microemulsions are formed. As the salinity is increased, both the extent of solubiliza-

tion and the opacity of the microemulsion increase. At a certain salinity, there is a transition to a Winsor type III (middle-phase) system, which begins with the middle phase having a water/oil volume ratio (WOR) near infinity. As the salinity is increased, the system gradually passes through the optimum state, where the middle-phase WOR = 1, and ultimately, to a system where the middle-phase WOR approaches 0. Further increases in the salinity generate Winsor type II (water-in-oil, W/O) systems with decreasing opacity and water solubilized in oil. For optimum system composition, the salinity required is called the optimum salinity, S∗ . The solubilization parameter, σ , is a measure of the surfactant’s solubilizing power and is defined as the volume of water (or oil) solubilized per unit weight of surfactant plus cosurfactant (if any). The optimum solubilization parameter, σ ∗ , occurs when equal volumes of oil and water are solubilized and simultaneously the I/III and III/II interfacial tensions, γ ∗ , are equal. The range of salinities, 1S, over which one has a Winsor type III system has also been described in these studies since it is an easy method for the assessment of the sensitivity to system variation in that large values of σ * give small values of 1S (6). A generic diagram of such a system is shown in Fig. 1. Winsor type III optimum middle-phase systems were studied in this work since these systems have the surfactant equipartitioned into both the excess aqueous and oleic phases. Another useful property of type III systems is the inverse relationship between efficiency of solubilization and interfacial tension, i.e., the better the solubilization the lower the interfacial tension (7, 8).

EXPERIMENTAL

Quaternary ammonium hydroxides were obtained from Aldrich Chemical Co. and used without further purification. Fluorinated acids were obtained from 3M, Co. and Aldrich Chemical Co. and used without further purification. The catanionic surfactants were generated in solution and were not isolated. The neutralization of the fluorinated acid was followed by IR and pH. HFE 7100 (perfluorobutyl methyl ether) and HFE 7200 (perfluorobutyl ethyl ether) are commercially available from 3M, Co.

117

0021-9797/01 $35.00

C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.

118

JIMMIE R. BARAN, JR.

solution. Based upon the weight of consumed perfluorooctanoic acid, an additional 16.83 g of Millipore 18 MÄ water was added to produce a 4 wt% solution of TBAPFO (based upon consumed perfluorooctanoic acid). The reaction was followed by IR. As the acid is consumed, the carbonyl peak moves from ∼1774 to ∼1692 cm−1 , indicative of salt formation. Synthesis of Tetrapropylammonium Perfluorooctanoate (TPrAPFO) TPrAPFO was prepared in the same manner described above, except that 1.56 g perfluorooctanoic acid was reacted with 3.64 g 1 M aqueous solution of tetrapropylammonium hydroxide to produce a pH 7 solution. A total of 51.25 g Millipore 18 MÄ water was added to produce a 4 wt% solution of TPrAPFO (based upon consumed perfluorooctanoic acid). Again, the reaction was followed by IR. Upon neutralization of the acid, the carbonyl peak moves from ∼1774 to ∼1693 cm−1 . Synthesis of Tetrapropylammonium Perfluorodecanoate (TPrAPFD)

FIG. 1. Classical phase behavior for varation in electrolyte concentration with accompanying interfacial tensions and solubilization parameters for an HFE/water/surfactant system.

Formulation of Equilibrium Systems The systems were formulated in 5-mL pipettes with the tips flame-sealed. After the components were combined in the pipettes, the open end of each pipette was sealed with a cork. Reproducibility of solubilization parameters is limited by one’s ability to read phase volumes to ±0.01 mL. The phase behavior of all systems was studied at 10, 25, and 40◦ C. The surfactants and NaCl were always predissolved in the aqueous phase, and all systems, as initially constituted, contained equal volumes of aqueous and HFE phases (WOR = 1). All systems were shaken multiple times, and sufficient time was allowed for the initially formed unstable macroemulsions to decay to thermodynamically stable microemulsion systems. Depending upon the system in question, this time varied from overnight to days. In all studies the total surfactant concentration was constant at 2 wt% of the aqueous phase, unless noted otherwise. Synthesis of Tetrabutylammonium Perfluorooctanoate (TBAPFO) TBAPFO was prepared by the neutralization of tetrabutylammonium hydroxide with perfluorooctanoic acid. Typically, 1.26 g perfluorooctanoic acid was added to 30.0 g Millipore 18 MÄ water and neutralized with 1.91 g ∼40 wt% aqueous solution of tetrabutylammonium hydroxide. This produced a pH 7

TPrAPFD was prepared in the same manner described above, except that 1.59 g perfluorodecanoic acid was reacted with 3.19 g 1 M aqueous solution of tetrapropylammonium hydroxide to produce a pH 7 solution. A total of 49.30 g Millipore 18 MÄ water were added to produce a 4 wt% solution of TPrAPFD (based upon consumed perfluorodecanoic acid). This solution was quite viscous and was ultimately diluted to 2 wt% TPrAPFD with Millipore 18 MÄ water for the studies discussed in this article, due to concerns about the ability to deliver a constant volume of such a viscous surfactant solution. Again, the reaction was followed by IR. Upon neutralization of the acid, the carbonyl peak moves from ∼1774 to ∼1680 cm−1 . Synthesis of Tetrapropylammonium Perfluorohexanoate (TPrAPFH) TPrAPFH was prepared in the same manner described above, except that 1.83 g perfluorohexanoic acid was reacted with 5.59 g 1 M aqueous solution of tetrapropylammonium hydroxide to produce a pH 7 solution. A total of 65.33 g Millipore 18 MÄ water was added to produce a 4 wt% solution of TPrAPFH (based upon consumed perfluorohexanoic acid). The reaction was followed by IR. Upon neutralization of the acid, the carbonyl peak moves from ∼1773 to ∼1693 cm−1 . Synthesis of Hexadecyltrimethylammonium Perfluorooctanoate (HDTMAPFO) HDTMAPFOA was prepared the same as above, except that 1.38 g perfluorooctanoic acid was reacted with 8.56 g 10 wt% aqueous solution of hexadecyltrimethylammonium hydroxide to produce a pH 7 solution. A total of 47.35 g Millipore 18 MÄ

119

PHASE BEHAVIOR OF HYDROFLUOROETHERS

water was added to produce a 4 wt% solution of HDTMAPFO (based upon consumed perfluorooctanoic acid). Again, the reaction was followed by IR. Upon neutralization of the acid, the carbonyl peak moves from ∼1774 to ∼1692 cm−1 . DISCUSSION

Microemulsions with HFEs could not be formed using only hydrocarbon surfactants, either ionic or nonionic. Microemulsion formation occurred only with large quantities of nonionic, fluorinated surfactants.1 These systems permitted only a small quantity of water in HFE or HFE in water to be microemulsified. The solubilization could be enhanced by the addition of organic solvents that are compatible with HFEs, such as alcohols or glycol ethers. Large quantities of water and HFEs could be microemulsified in a Winsor type III system (middle phase) by using catanionic surfactants (9), wherein at least one part of the catanionic surfactant is fluorinated.2 Catanionic surfactants are formed as a 1 : 1 complex of two oppositely charged surface active agents, for instance by the neutralization of an acid (carboxylic, sulfonic, etc.) with a quaternary ammonium hydroxide (see [2]) (10). RCOOH + R0 R00 R000 R0000 NOH → RCOO−+ NR0 R00 R000 R0000 + H2 O

[2]

They may also be formed by the reaction of a cationic surfactant with an anionic surfactant (see [3]) (11). RCOO− M+ + R0 R00 R000 R0000 N+ X− → RCOO−+ NR0 R00 R000 R0000 + MX

[3]

This method produces an electrolyte, as well, which is undesirable in the study of the effect of salinity on the microemulsion phase behavior. It has generally been found that these surfactants are quite surface active and are not very soluble in water, due to the presence of the two large, hydrophobic parts of the molecule. Many studies have been done on the precipitation (12) of these complexes or simply on their aqueous (13) or nonaqueous (14) phase behaviors. Bourrel et al. described the microemulsionforming ability of some catanionic surfactants (15). Catanionic surfactants containing a fluorocarbon chain were ideal for microemulsifying HFEs. Since HFEs contain parts that are fluorocarbon in nature and hydrocarbon in nature, neither of these two types of surfactants would be expected to work well independently. Instead, a surfactant (or surfactant system) 1 Commercial fluorinated surfactants are sold under the FLUORAD (3M, Co.) or ZONYL (E.I. DuPont de Nemours and Co.) trade names. Nonionic fluorinated surfactants, such as Fluorad FC171 or FC170C and ZONYL FSO and FSN, are useful. 2 The first time that these types of surfactants were identified in the literature as “catanionic” was in Ref. (9).

TABLE 1 Surfactant Structures and Abbreviations Hydrocarbon cation

Fluorocarbon anion

Abbreviation

C5 F11 CO− 2 C7 F15 CO− 2 C9 F11 CO− 2 C7 F15 CO− 2 C7 F15 CO− 2

TPrAPFH TPrAPFO TPrAPFD TBAPFO HDTMAPFO

(C3 H7 )3 N+ (C3 H7 )3 N+ (C3 H7 )3 N+ (C4 H9 )4 N+ C16 H33 (CH3 )3 N+

containing both a fluorocarbon part and a hydrocarbon part is best. This report details only those results obtained with the negatively charged part of the catanionic surfactant as the fluorocarbon moiety. Alternatively, one could use a fluorinated quaternary ammonium hydroxide to neutralize a hydrocarbon acid, but there is greater variability in available fluorinated acids than in available fluorinated quaternary compounds. Therefore, a more thorough and systematic study can be done with compounds that are fluorinated on the anionic part of the molecule. Listed in Table 1 are the hydrocarbon cations and fluorocarbon anions used to form the catanionic surfactants detailed in this report, along with the abbreviations used throughout this report to describe the surfactants. As can be seen in Table 2, TBAPFO and HFE 7100 produced typical Winsor type phase behavior (I–III–II) only at the lower temperature of 10◦ C. As expected, since the optimum salinity was low (∼0.1 wt%), the optimum solubilization parameter was large, leading to a small salinity window. At 25 and 40◦ C, only a Winsor type II (w/o) system was observed. This behavior has previously been described as “negative optimum salinity” and can be interpreted as TBAPFO preferentially partitioning into the oil phase at these temperatures (16). The same behavior is seen for TBAPFO with HFE 7200. TPrAPFO displayed phase behavior that was unexpectedly different from that of TBAPFO with HFE 7100. It appears that the temperature coefficient for partitioning between the water and oil phases of TPrAPFO and TBAPFO are opposite in sign. As can be seen in Table 3, TPrAPFO exhibits typical Winsor type phase behavior at 10 and 25◦ C with HFE 7100 and at 10◦ C with HFE 7200. Within the TPrAPFO/HFE 7100 system, the optimum salinity is observed to increase with temperature. TABLE 2 TBAPFO with HFEs 7100 and 7200 HFE

S*

σ*

1S

7100 (10◦ C) 7100 (25◦ C) 7100 (40◦ C) 7200 (10◦ C) 7200 (25◦ C) 7200 (40◦ C)

0.15 —a —a 0.37 —a —a

8.90 —a —a 3.90 —a —a

0.25 —a —a 0.65 —a —a

a

Only W/O microemulsions found at all salinities.

120

JIMMIE R. BARAN, JR.

TABLE 3 TPrAPFO with HFEs 7100 and 7200

TABLE 4 HFE 7100 with Various HC/FC Catanionic Surfactants

HFE

S*

σ*

1S

Surfactant

S*

σ*

1S

7100 (10◦ C) 7100 (25◦ C) 7100 (40◦ C) 7200 (10◦ C) 7200 (25◦ C) 7200 (40◦ C)

3.25 3.45 —a 3.95 —a —a

2.2 1.30 —a 1.5 —a —a

10.0+ 9.0 —a 8.5+ —a —a

TPrAPFD (1 wt%, 10◦ C) TPrAPFD (1 wt%, 25◦ C) TPrAPFD (1 wt%, 40◦ C) TPrAPFH (10◦ C) TPrAPFH (25◦ C) TPrAPFH (40◦ C) HDTMAPFO (10◦ C) HDTMAPFO (25◦ C) HDTMAPFO (40◦ C)

3.95 2.90 —a —b —a —a gel gel gel

4.70 3.90 —a —b —a —a

0.75 3.0 —a —b —a —a

a

Only O/W microemulsions and middle-phase systems without obtaining an optimum formulation.

The “+” in the tables indicates that the middle-phase microemulsion is still present at 12.5% NaCl. At 40◦ C with HFE 7100 and at 40 and 25◦ C with HFE 7200, the system only exhibits Winsor type I (O/W) and a Winsor type III (middle phase) that does not attain optimum formulation. This is interpreted as TPrAPFO preferentially partitioning into the water as the temperature increases, the opposite of what was observed with TBAPFO. One will also note that the optimum salinity of both TBAPFO and TPrAPFO increases in going from HFE 7100 to HFE 7200. This is attributed to the increased hydrophobic characteristics of HFE 7200. It should also be pointed out that the optimum solubilization parameter of TBAPFO is larger than that of TPrAPFO with both HFE 7100 and HFE 7200. This may be, in part, due to the complex interaction between optimum salinity and the optimum solubilization parameter, i.e., usually, the lower the optimum salinity, the larger the solubilization parameter. It is not a normal feature of surfactants that one can change a part of the molecule (such as the cation, in this case) and reverse the sign of its temperature coefficient. This would indicate that a mixture of TBAPFO and TPrAPFO could be formulated such that the system would exhibit temperature insensitivity (see below). This insensitivity is usually accomplished by mixing ionic surfactants with nonionic ones, as these two classes of surfactants have temperature coefficients that differ in sign. Ionic surfactants preferentially partition into the aqueous phase with an increase in temperature, while nonionic surfactants preferentially partition into the oil phase as the temperature increases. One example where the oil/water partitioning can be balanced within a single molecule is in sugar-based surfactants (17). The –OH groups make these surfactants more water soluble, while the attached hydrophobes will make the surfactants more oil soluble. Several other HC/FC catanionic surfactants were made and studied with HFEs. The data for these surfactants are presented in Table 4, along with their temperature dependence. Much like TPrAPFO, TPrAPFD does not exhibit an optimum salinity up to 12.5 wt% NaCl at 40◦ C, but the two surfactants show opposite temperature sensitivities at 25 and 10◦ C. TPrAPFD partitions into water at lower temperatures, while TPrAPFO partitions into the HFE at lower temperatures. This is unexpected since a longer hydrophobe chain in a surfactant

a Only O/W microemulsions and middle-phase systems without obtaining an optimum formulation. b Only “past” optimum middle-phase systems and W/O microemulsions are obtained.

should preferentially partition into the oil over one containing a shorter chain. The observed behavior is perhaps more consistent with reduced HFE compatibility with the longer fluorocarbon chain rather than the notion that the longer chain surfactant is more hydrophilic. Therefore, some mixture of these two surfactants should also demonstrate temperature insensitivity (see Table 5). The solubilizing ability of a surfactant is known to increase as the length of the hydrophobe tail increases (18). That this also holds true for these systems is made apparent by comparing the data for TPrAPFO and TPrAPFD. TPrAPFH has a shorter fluorinated chain, therefore the middle phases are smaller than those produced with TPrAPFO. At 40 and 25◦ C TPrAPFH produces systems that show middle phases, but they contain very little oil and therefore an optimum system TABLE 5 HC/FC Catanionic Surfactant Mixes with HFE 7100 Surfactants

S*

σ*

1S

TPrAPFO/TPrAPFD (3/1, 25◦ C) TPrAPFO/TPrAPFD (1/3, 25◦ C) TPrAPFO/TPrAPFD (1/1, 10◦ C) TPrAPFO/TPrAPFD (1/1, 25◦ C) TPrAPFO/TPrAPFD (1/1, 40◦ C) TBAPFO/TPrAPFO (3/1, 25◦ C) TBAPFO/TPrAPFO (1/3, 25◦ C) TBAPFO/TPrAPFO (1/1, 10◦ C) TBAPFO/TPrAPFO (1/1, 25◦ C) TBAPFO/TPrAPFO (1/1, 40◦ C) TPrAPFO/TPrAPFH (3/1, 25◦ C) TPrAPFO/TPrAPFH (1/3, 25◦ C) TPrAPFO/TPrAPFH (1/1, 10◦ C) TPrAPFO/TPrAPFH (1/1, 25◦ C) TPrAPFO/TPrAPFH (1/1, 40◦ C)

3.50 3.50 3.45 3.40 —a 0.094 1.60 0.76 0.50 0.60 5.20 7.55 —a 6.10 —b

1.65 2.50 2.70 1.85 —a 7.10 1.80 3.25 2.55 2.25 0.95 0.54 —a 0.64 —b

7.0 4.5 8.5 6.5 —a 0.02 3.0 0.85 0.70 0.55 10.0+ 9.0+ —a 10.0+ —b

a Only O/W microemulsions and middle-phase systems without obtaining an optimum formulation. b Only “past” optimum middle-phase systems and W/O microemulsions are obtained.

121

PHASE BEHAVIOR OF HYDROFLUOROETHERS

is never obtained. At 10◦ C, again middle phases are produced throughout most of the salinity range scanned, but the middle phases contain very little water. This results in systems that are past an optimum salinity. This behavior is similar to that of TPrAPFO, indicating that the temperature coefficients of these two surfactants have the same sign. Other catanionic surfactants with even shorter fluorinated tails (perfluorobutanoate and perfluoropropionate) demonstrated much smaller middle phases, and only at higher salinities. HDTMAPFO represents a nonspherical hydrocarbon cation. Interestingly, this surfactant produced only a very stable gel/ precipitate mixture in the middle-phase microemulsion regime at all temperatures studied. This gel was present over the entire salinity range studied. Surfactant Mixtures Table 5 lists mixtures studied and their temperature behavior. Concentrating first on the TPrAPFO/TPrAPFD systems, one notes very little change in optimum salinity between the different system compositions and the different temperatures. This is due to the opposite signs of the temperature coefficients of these two surfactants. The fact that the optimum salinity does not significantly change over a large range of compositions indicates that the magnitudes of the temperature coefficients must be quite similar. The TBAPFO/TPrAPFO system behaves much more as expected. Varying the surfactant composition at a given temperature, the more hydrophobic the surfactant mixture (i.e., the more TBAPFO in the mixture) the lower the optimum salinity and the larger the optimum solubilization parameter. Comparing the optimum salinities of the same composition at different temperatures, one once again sees the effect of the temperature coefficients being of opposite signs. The optimum salinity at 25◦ C is lower than that at 10 and 40◦ C. The fact that the optimum salinity does change, although only slightly, indicates that the magnitudes of the temperature coefficients are not the same. Finally, analyzing the TPrAPFO/TPrAPFH system, one sees that at a constant temperature the optimum salinity increases as the surfactant mixture becomes more hydrophilic (i.e., more TPrAPFH), as expected. With a change in the cation from tetrapropylammonium to tetrabutylammonium, the surfactant changes enough to create a system which does not produce a middle phase at any salinity. Changing the anion from a C6 to a C10 does not greatly affect the ability to produce a middle phase: it only affects the ability to form an optimum system. It appears that none of these surfactants produces an optimum system at 40◦ C with the studied HFEs, yet they all produce an optimum system at 25◦ C. Unlike TPrAPFO and TPrAPFD, TPrAPFH does not produce an optimum system at 10◦ C. Therefore, it appears that the shorter the fluorocarbon tail, the

narrower the temperature range in which the surfactant is effective at forming optimum systems. CONCLUSIONS

The results presented here show that Winsor type phase behavior can be obtained with hydrofluoroethers, as long as the surfactant system is correctly formulated. Since hydrofluoroethers contain both fluorocarbon and hydrocarbon moieties, a surfactant system that contains both hydrocarbon and fluorocarbon characteristics is preferred. This can be accomplished most efficiently by preparing catanionic surfactants containing a hydrocarbon ion with a fluorocarbon counterion. It was found that these systems could be made temperature insensitive. The compositional phase behavior in relation to temperature of these systems is currently being studied and will be reported in the future (19). ACKNOWLEDGMENT The author thanks 3M Company for the opportunity and financial support for this work.

REFERENCES 1. Bourrel, M., and Schechter, R. S., “Microemulsions and Related Systems: Formulation, Solvency and Physical Properties,” Surfactant Science Series Vol. 30. Dekker, New York, 1988. 2. (a) Gambi, C. M. C., Giri, M. G., Carla, M., and Senatra, D., Phys. Rev. E 56, 4356 (1997); (b) Giannetti, E., Chittofrati, A., and Sanguineti, A., Chim. Ind. 79, 611 (1997); (c) Baglioni, P., Gambi, C. M. C., Giordano, R., and Senatra, D., J. Mol. Struct. 383, 165 (1996). 3. LoNostro, P., Choi, S.-M., Ku, C.-Y., and Chen, S.-H., J. Phys. Chem. B 103, 5347 (1999), and references therein. 4. 3M Specialty Chemicals Division, “Product Information Sheet,” 1996. 5. Winsor, P. A., “Solvent Properties of Amphiphilic Compounds.” Butterworths, London, 1954. 6. Barakat, Y., Fortney, L. N., LaLanne-Cassou, C., Schechter, R. S., Wade, W. H., Weerasooriya, U., and Yiv, S. H., Soc. Pet. Eng. J., 913 (1983). 7. Huh, C., J. Colloid Interface Sci. 71, 408 (1979). 8. Israelachvili, J., in “Surfactants in Solution” (K. L. Mittal and P. Bothorel, Eds.). p. 3. Plenum, New York, 1986. 9. Jokela, P., Jonsson B., and Khan, A., J. Phys. Chem. 91, 3291 (1987). 10. Zemb, T., Dubois, M., Deme, B., and Gulik-Krzywicki, T., Science 283, 816 (1999). 11. Khan, A., and Marques, E., in “Specialist Surfactants” (I. D. Robb, Ed.). Chapman & Hall, Cambridge, UK, 1997. 12. Stellner, K. L., Amante, J. C., Scamehorn J. F., and Harwell, J. H., J. Colloid Interface Sci. 123, 186 (1988). 13. Bergstr¨om, M., and Pedersen, J. S., Langmuir 15, 2250 (1999). 14. Huang, J.-B., Zhu, B.-Y., Mao, M., He, P., Wang, J., and He, X., Colloid Polym. Sci. 277, 354 (1999). 15. Bourrel, M., Bernard, D., and Graciaa, A., Tenside Deterg. 21, 311 (1984). 16. Baran, J. R., Jr., Pope, G. A., Wade, W. H., and Weerasooriya, V., Environ. Sci. Technol. 30, 2143 (1996). 17. Baran, J. R., Jr., Oh, K. H., Wade, W. H., and Weerasooriya, V., J. Dispersion Sci. Technol. 16, 165 (1995). 18. Rosen, M. J., “Surfactants and Interfacial Phenomena,” 2nd ed., p. 176. Wiley, New York, 1989. 19. Baran, J. R., Jr., manuscript in preparation.