Emulsions under elevated pressure and temperature conditions

Emulsions under Elevated Pressure and Temperature Conditions II The Model System Water (Electrolyte)-Octanoic Acid-Sodium Octanoate-n-Heptane at 20°C M E R E T E H. SELLE, J O H A N SJOBLOM, 1 AND R O A L D S K U R T V E I T Department of Chemistry, University of Bergen, N-5007 Bergen, Norway

Received September 24, 1990; accepted November 20, 1990 The systemwater (electrolyte)/sodium octanoate/octanoicaeid/n-heptane has been investigatedwith respect to phase behavior, emulsion formation, and emulsion stability. The microemulsion phase will extend as an L3 phase toward the n-heptane apex. The existence of this phase will strongly influence the stability of the emulsions. Two-phaseregions L2 + L3 and L~ + L 3 will cause inversion and instability of the emulsions. The existence of a lamellar liquid crystalline phase (D phase) will for the highest contents of the emulsifiermixture enhance the stability of the oil-continuous (W/O) emulsions © 1991 Academic Press, Inc.

INTRODUCTION

equilibria Friberg has very clearly shown that the lyotropic liquid crystalline D-phase will have a stabilizing effect on emulsions ( 7 - 8 ) . This is obviously due to the multilayer surfactant formation around the dispersed droplets which will provide the droplets with an enhanced surface rigidity (9) and also reduce the attraction potential between the droplets (10). In a previous paper in this series we investigated the emulsion stability in the system water (electrolyte)-hexadecanoic acid-sodium hexadecanoate-decane at 70°C ( 11 ). In accordance with Friberg, we found that the existence o f a lamellar liquid crystalline phase ( D ) caused a substantial increase in the stability of the emulsions. Especially oil-continuous emulsions were stabilized by the occurrence of this third phase. Further, we observed that electrolytes inverted the emulsions from water-continuous to oil-continuous. The efficiency o f counterions increased in the sequence A13+ > Ca 2+ > Na +. This observation correlates with the physicochemical properties of the surfactant mixture (i.e., the sodium salt and the free fatty acid). The interfacial packing of the free hexadecanoate ion, its metal ion

It is by now well known that different types of surfactants play a significant role in the stabilization of emulsions. In order to obtain a basic understanding of the mechanics behind the role of the surfactant, two different main approaches have emerged. The chemistry of surfactants in solution and the chemistry of interfacial films are both areas upon which the research has been focused in order to clarify basic stability criteria for emulsions. In a study of the solution chemical behavior of surfactants, phase diagrams are very convenient for an understanding of some of the intricate interactions between surfactants, solvents (in most cases water), and polar additives ( 1 - 3 ) . In phase diagram studies two fundamental aspects of emulsion stability have emerged. Shinoda et al. (4-6) have convincingly shown the importance of HLB temperature on emulsions stabilized by nonionic surfactants. Close to the phase inversion temperature ( P I T ) the dispersed droplets have been shown to be less stable toward coalescence. Utilizing phase To whom correspondence should be addressed. 36 0021-9797/91 $3.00 Copyright © 199l by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and InteoCace Science, Vol. 144, No. 1, June 1991

EMULSION STABILITY soap, and the undissociated corresponding fatty acid will be completely decisive for the stability of the system. In this system where the components consist of long hydrocarbon tails (C15) the interaction between the nonpolar chains will of course facilitate the formation of a rather well-defined interfacial region separating the hydrocarbon and aqueous regimes. In our present investigation dealing with the system water (electrolyte)-octanoic acid-sodium octanoate-n-heptane, we have approximately halved the length of the hydrocarbon moiety of the surfactant and cosurfactant. In this way the molecules will be much more mobile which will influence the nature of the interfacial zone. Further, the critical micelle concentration of sodium octanoate (NaCs) will be substantially higher at 20°C than that for sodium hexadecanoate (NaC16) at 70°C. In this way the influence of a higher monomeric concentration on the emulsion stability will also be illustrated. From phase diagrams the association tendency of the long-chain surfactant and the corresponding fatty acid as well as the shortchain surfactant in combination with its homologous fatty acid can be combined. Both surfactant mixtures form ordinary micelles, mesophases, and inverted micelles. The most conspicuous difference in the phase behavior at the investigated temperatures is the capacity of the organic solution phase to incorporate water. In the sodium octanoate/octanoic acid case the salient part of this phase has properties obviously similar to those of an anomalous phase (L3) discovered in a variety of model systems consisting of water/ oil / surfactant (12-16). Comparison with our previous model emulsions ( 11 ) will hence involve the influence of different phase equilibria as well as different dynamical behavior. EXPERIMENTAL

Materials. Sodium octanoate (NaCs) and octanoic acid (HC8) were from Fluka. The salt was >99% pure and the acid was >98% pure. n-Heptane and NaC1 were both Merck's

37

proanalysis quality. The n-heptane was >99% pure and NaC1 was 99.5% pure. NaC1 was dried at 70°C. All the chemicals were used without further purification. Water was distilled in a Fi-Stream apparatus and its conductivity was < 10-6 f~-i cm-1. Phase behavior. The components were weighed on an analytical balance and placed in glass vials. The samples were equilibrated at 20°C for 1 week. The samples were inspected visually and also through two crossed polarizers to detect lamellar liquid crystalline phases. Preparation of emulsions. The samples were equilibrated for 1 week before emulsification. After this the emulsification was undertaken with a rotor. The speed was about 6000 rpm. The emulsification time was 50 s and the emulsification was repeated five times. Determination of emulsion type. The emulsion type was measured with a dilution method using oil or water, respectively. The emulsion type was also checked by measuring resistance. High resistance indicates oil-continuous emulsions and low resistance indicates water-continuous emulsions. Emulsion stability. The emulsions were stored in scaled tubes with glass stoppers at 20°C. The rate of separation of the emulsion was followed by measuring the volume of dispersed phase as a function of time. RESULTS

Phase Equilibria in the System WaterSodium Octanoate-Octanoic Acid This phase diagram is given in Fig. 1 and has been determined by Ekwall et al. (17). The system is built up by two homogeneous solution phases L1 and L2, where L1 is an ordinary micellar solution and L2 contains reversed micelles. The L: phase has a very pronounced extension toward the water apex. This part of the solution phase can take up 80% of water and it is obvious that the solution properties of this channel are different from the other parts of the L2 phase. This is why we term this part Lz/L3, where L3 usually repJournal of Colloid and Interface Science, Vol. 144, No. 1, June 1991

38

SELLE, SJOBLOM, AND SKURTVEIT Caprytic acid

10-Y/~90

40

60

F

i?

Woter

No-caprylate

FIG. 1. Phase diagram of water/sodium octanoate/octanoic acid according to Ekwall et al. (17). The axis refers to percentage by weight.

resents the anomalous phase occurring in similar systems. The system contains three further lyotropic liquid crystalline phases, D, E, and F. The E and F phases consist of rods in a hexagonal array, where the insides of the rods consist of hydrocarbon chains (E phase) and water (F), respectively. The D phase is a lamellar phase with lipid bilayers and intervening water layers. Phases B and C noted in Fig. 1 are not stable phases but dispersions. A similar phase diagram has been corrected accordingly (18).

the solution channel in the system H20 (electrolyte)-sodium octanoate-octanoic acid. With a higher content of salt the molar ratio between octanoate and octanoic acid will gradually increase. Without salt this ratio is

HC8

Phase Equilibria in the System Water (Electrolyte)-Sodium OctanoateOctanoic Acid-n-Heptane We have chosen three different, but fixed, ratios by weight between sodium octanoate and octanoic acid, 1:1, 2:5, and 1:5. We can see from Fig. 1 that line 1 ( 1:1 ) will cross the D-phase. Line 2 (2:5) will be in the microemulsion channel (Lz/L3), and line 3 (1:5) will traverse the two-phase reigon (L1 + L2). Figure 2 shows the influence of electrolyte (0, 10 -3, 0.1, and 0.5 m) on the location of Journal of Colloid andlnterface Science, Vol. 14,1, No. 1, June 1991

Brine

NaCs

I~G. 2. Shift in optimal water solubilization under different salinity conditions in the system brine/sodium octanoate/octanoic acid. e, H20, m, 10-3m; O, 0.I m, and [], 0.5 m NaCL Temperature, 20°C.

EMULSION STABILITY

39

n-Heptane

~

LI+L2/L3

,

~

n-Heptane

L+D

Water

/ NaC8/HC8 = 1/1 /

\ n-Heptane

-~

~

~-

~

~x

1 mmolal NaC1 (aq)

NaC8/HC8 = 1/1

0.5 m NaCI (aq)

NaCs/HC8 = 1/1

0.1 m NaCI (aq)

FIG. 3. Extension of the microemulsion phase L2/L3 in the system water/sodium octanoate/octanoic acid/n-heptane for different salinities. A ratio of equal amounts (by weight) of surfactant and cosurfactant has been used.

~ 0 . 4 and with 0.5 m NaC1 present the ratio has increased to ~ 0 . 6 . Figure 3 extends the narrow solution phase Lz/L3 toward the n-heptane comer at different levels of electrolyte. For salinities between 0.1 and 0.5 m NaC1 the one-phase region disappears for the investigated NaCs/HC8 ratio and

is replaced by a two- or three-phase region (Ll + L2/L3) or (LI + L2/L3 + D ) . It should be mentioned that for the other ratios between the sodium octanoate and octanoic acid used in this study the equilibria with n-heptane do not show any D phases. The only one-phase region is the organic solution phase close to Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991

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SELLE, S J O B L O M , A N D S K U R T V E I T

the mixed emulsifier'/n-heptane axis. This so- that closest to the heptane/water axis the lution phase is capable of holding varying emulsions are water-continuous. When the amounts of water (electrolyte), normally be- content of emulsifier is increased there is a tween 5 and 20 wt%. transition to oil-continuous emulsions. The location of this boundary is slightly dependent on the level of the electrolyte. All in all, the Emulsions Formed in the Model System position and extension of the L2/L3 phase in Water (Electrolyte)-Sodium Octanoatethe quarternary phase diagram will be decisive Octanoic Acid-n-Heptane for the phase inversion contour, as is pointed Figure 4 shows compositions investigated out in the next section. The emulsions stabiwith regard to emulsion formation and gives lized by equal amounts of sodium octanoate the results for the three different ratios of sur- and octanoic acid were investigated in greatest factant and cosurfactant. We have, however, detail. omitted the influence of electrolyte on the The picture that emerges for these compoemulsion inversion composition. We have in- sitions without electrolyte is as follows. When vestigated the influence of 10 -3, 0.1, and 0.5 the dispersed phase is 10% heptane, all emulm NaC1 on the inversion and found that in sions investigated are water-continuous, but the case of Figs. 4a and 4c, i.e., 1:1 and 1:5, the most stable emulsions are obtained for 2% the addition of NaC1 shifted the boundary be- emulsifier. When the content of the hydrotween oil- and water-continuous emulsions carbon phase is raised to 30%, the highest statoward the brine/n-heptane axis. But it was bility of the water-continuous emulsions is still possible to detect water-continuous emul- obtained for emulsifier contents of 0.5 and 2% sions at high contents of n-heptane, i.e., up to by weight. For 5% by weight a destabilization 80% by weight. For the intermediate ratio of the emulsion with a subsequent phase tran2:5 the electrolyte will depress the boundary sition will occur. With 20% emulsifier the between the two kinds of emulsions, which emulsions will be oil-continuous and rather means that water-continuous emulsions could instable. For equal amounts (by weight) of oil not be detected above 20 or 30% by weight of and water, 2% emulsifier will produce the most n-heptane. stable O / W emulsions. For 70% heptane the Figure 5 summarizes the results of the pattern above is repeated. For 2% emulsifier emulsion study, showing the contours of the stable water-continuous emulsions are obphase inversion boundary where the type of tained. After this the stability will decrease and, emulsion is changed. It is a consequent feature in this case, a transition to oil-continuous n-Heplane

n-Heptane

n Heptane

\ Water

NaCs/HC8 - I/I

Water

NaC,~/HCs = 2/5

WaFer

NaCs/HC,~ = 1/5

FIG. 4. Phase inversion of e m u l s i o n s in the system w a t e r / s o d i u m o c t a n o a t e / o c t a n o i c a c i d / n - h e p t a n e for different ratios (by weight) o f surfactant a n d cosurfaetant. O, oil-in-water ( O / W ) emulsions. O, waterin-oil ( W / O ) emulsions. Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991

EMULSION STABILITY

41

Phase inversion area O/W

O/W

2% emulsifier

W/O

5% emulsifier

18% emulsifier

/ /

lOG

8,o

~52o o

W/O

10% emulsifier

. . . . . . . . . . . . . . . . . . .

10

20

30

i

40

50

T . . . . . . . . . . . .

i

i

0

6

8

hours

2

4 minutes

i,,,v~

0,0

....

2,5

k ....

i ....

5,0 7,5 minutes

80 3 6o 4o 20 0

i '7

10,0

0

10

20

30

hours

FIG. 5. Stabilityand phase inversionas a function of time and emulsifiercontent of emulsions containing 70/30 ratio of n-heptane and water.

emulsions will occur before 10% emulsifier. For this emulsifier content the emulsions are rather unstable but stability is substantially increased when the content of emulsifier is raised to 20% by weight. We have also investigated the influence of electrolyte on the emulsions stabilized by equal amounts of soap and fatty acid. The parallels to the electrolyte-free emulsions are obvious. The stabilities are comparable and the phase inversion sequences coincide with regard to position in the phase diagram. However, the phase transition boundary is gradually shifted somewhat toward the heptane/water axis when the amount of electrolyte is increased in the aqueous phase. We have also evaluated the stability of the emulsions on the basis of the other ratios of emulsifiers, i.e., 2:5 and 1:5 between sodium octanoate and octanoic acid. These emulsions have been investigated in the presence and absence of sodium chloride (as above). It is not so easy to find any deviating features for these emulsions containing more fatty acid. The same stability pattern is obtained and the inversion takes place when the analytical content of the emulsifier is increased~ DISCUSSION For an appropriate understanding of the emulsion stability and the mechanics behind stability it is important to view the different

kinds of structures that occur in the organic solution phase L2. It is obvious that for different ratios between soap and acid, the emulsion will when separated into two (or three) phases have different equilibrium conditions. In this survey we concentrate upon the structures in the L2 phase.

The Organic Solution Phase L2 The complexation between the fatty acid and the sodium salt has been thoroughly studied by Friberg et al. (19-22). He found that the molar ratio of octanoic acid:sodium octanoate was 2:1 in water-free solutions. The experimental evidence for this molar ratio is provided by IR investigation. Friberg et al. observed a linear decrease in the out-of-plane deformation vibration of the hydroxylic group at 950 cm l when sodium octanoate was added. At the phase boundary on the binary octanoate/octanoic acid axis the signal is totally absent. At this point all acid molecules are complexed with the salt. In a recent paper Ahlnfis and S6derman (23) report on 13C N M R chemical shifts, multifield 13C relaxation, and self-diffusion data on the same system. They conclude that all sodium octanoate and octanoic acid are present in the complexes. Further, they found that the aggregation number of the soap-acid complex was constant although the soap concentration was alJournal of Colloid and Interface Science,

Vol.

144, No.

I, June

1991

:~ o~

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SELLE, S J O B L O M , A N D S K U R T V E I T

tered. All in all, this investigation agrees with Refs. (19-22). When water is added to the binary sodium octanoate/octanoic acid solutions, reversed micellar aggregates start to form. In recent pap e r s the structures in the organic solution phase have been determined by the Lund group (23-25). The addition of water to the octanoate/octanoic acid solution seems to bring about an aggregation where reversed mieelles are formed. The addition of water does not induce any discontinuity in the physiochemical properties up to about 40% by weight of H20. At this point the spherical aggregates could be explained by a radius of ~ 3 0 A. At concentrations above 40% by weight H20 the structure is most likely bicontinuous. In the solution channel (termed L2/L3) an L3 structure is the most probable. This structure is considered to be built up as a bilayer shaped close to a minimal surface, i.e., zero mean curvature. The oil domains form "the surface" and the channels are the aqueous regimes. The properties of these layers have been investigated and the general conclusion is that the interfacial layer is highly flexible and undergoes continuous changes, in forms of frequent oscillations. The appearance of this L2/L3 phase is in accordance with the general correlation between the L3 phase and the presence of a lamellar liquid crystalline phase as pointed out by Andersson et al. (16). Against this background the interpretation of the electrolyte addition presented in Fig. 2 is rather straightforward. In order to maintain the ultimate extension toward the water apex the ratio between surfactant and cosurfactant will vary when NaC1 is added. With more sodium chloride added, more sodium octanoate is needed in relation to octanoic acid. For necessary repulsion (maximum water uptake) and retained surfactant parameter ( ~ 1, L3 structure) the presence of NaC1 in the microemulsion must be compensated with more ionic surfactant. Similar behavior is to be expected by lamellar phases. The chemical parameter for surfactant and cosurfactant association into certain kinds of Journal of ColloM and Interface Science, Vol. 144, No. 1, June 1991

geometrical aggregates is the molecular geometry factor vefr/(l*a), where v is the molecular volume of the hydrocarbon tail, a is the head group area, and l is the length of the hydrocarbon tail. Small values of this ratio have been shown to give rise to "normal" water-soluble aggregates while high values ( > 1 ) render reversed structures soluble in oil (2629). In Fig. 3 two different parameters affect the interfacial curvature, i.e., the binding of electrolyte to the ionic interface (reduction in a) and the penetration of n-heptane into the hydrocarbon part (increase in Vefr). With the penetration of n-heptane into the structure, the surfactant parameter will increase and reversed structures should form. Obviously the electrolyte (NaC1) will affect the system in the same way since the presence of counterions at the interface will reduce the lateral repulsion between the head groups, resulting in a lower value of a. It is therefore not so surprising that the anomalous phase L3 in Fig. 3 exists only for low salinities for the given ratio NaCs/ HCs. At 0.5 m the phase has most likely shifted to a higher surfactant/cosurfactant ratio. It should be mentioned that the addition of n-heptane will also most likely influence the partitioning of the free fatty acid between the interfacial regime and the hydrocarbon bulk. In this way the surfactant/cosurfactant packing will be disturbed at the interface in relation to the heptane-free system.

Properties of Emulsions The general feature found for the emulsions in the system water (electrolyte)/sodium octanoate/ocanoic acid/n-heptane is that stable oil-in-water emulsions are found at lower emulsifier contents, i.e., for the analyzed 0.5 and 2% by weight. When the emulsifier content is increased up to 5 and 10%, the emulsions are less stable and eventually a phase transition to oil-continuous emulsions occurs. For the lowest contents of emulsifier these emulsions are often rather unstable. With an increasing analytical content of the emulsifier

EMULSION

STABILITY

43

mixture the stability of the oil-continuous emulsions is often increased. The dominant factor with regard to emulsion type and stability is the existence of the L2/L3 phase in the model system investigated. It is clearly demonstrated that as soon as this microemulsion phase participates in the chemical equilibria the emulsion is destabilized and an inversion takes place. This result is in complete agreement with the result by Shinoda et aL (4-6). Thus, the most important aspect of the destabilization will be the extension of the twophase region where the L3 phase is in equilibrium with the oil-rich L2 or the water-rich L1. The direction of these tie lines will directly determine the boundary for the inversion visualized in Fig. 4. The importance of the D phase for the stability of oil-continuous emulsions is displayed by the combination of Figs. 3 and 4. Figure 3 clearly shows that the stability of the emulsions containing 70% heptane is substantially increased when the amount of emulsifier is raised to 20%. In the phase diagram (Fig. 4) one enters a multiphase region where a lamellar liquid crystalline phase is involved in the phase equilibria. Although we have not mapped the phase equilibria in detail for the other combinations of the emulsifier mixture, it is obvious that the D phase will, when disturbing the phase equilibria, substantially enhance the stability of the emulsions. This will then take place for the higher emulsifier contents.

tures of emulsified systems. First, the oil-inwater emulsions are electrostatically stabilized and, hence, destabilized by an addition of electrolyte. The efficiency of the series A13+ > Ca 2+ > Na + clearly demonstrates the validity of the DLVO theory for these emulsions. With the addition of electrolyte, an inversion to oil-continuous structures is promoted, which can be seen as a result of a changed HLB value of the surfactant mixture. Under these conditions the packing of the molecules is different and a more efficient molecular close packing of the salt and the acid can be obtained at the O / W interface. Under such conditions and at equilibrium a third phase in equilibrium with the LI and L2 phase will occur. This liquid crystalline phase (D phase) most substantially enhances the stability of the oil-continuous emulsions. For the shorter surfactant (C8) mixture the situation is rather different. In this case the surfactant system (in the presence of n-heptane) is at its HLB temperature. For nonionic surfactant systems Shinoda found that emulsions at the PIT (HLB temperature) are unstable and tend to invert. In this article this discussion is extended to involve an ionic surfactant system. We have convincing evidence for the initiation of the demulsification process as soon as the phase equilibrium in the quarternary system oil/water/emulsifier 1/emulsifter 2 involves the microemulsion phase L3. Hence it is most important to map the phase equilibria and to follow the direction of the tie lines connecting the L3 phase with an excess LI or L2 phase for an understanding of the A Comparison of Properties of Emulsions stability/instability of emulsions. In this way in Two Model Systems the crucial parameter for emulsion stability will be the content of emulsifier since this will In this series we have by now investigated determine how close the compositions are to emulsion properties in two model system, i.e., the L3 phase. It is obvious that the high dythe system water (electrolyte)-sodium octa- namics of the components in this solution noate (NaC8)-octanoic acid (HC8)-n-hep- phase will strongly counteract emulsion statane and water (electrolyte)-sodium hexa- bility and promote an efficient breakup of the decanoate-hexadecanoic acid-n-decane. The emulsion in order to form the dynamic mitemperatures were 20 and 70°C, respectively. croemulsion phase. In the model system with The system based on the long-chain surfac- these shorter surfactants the D phase will play tant (C16) mixture reveals many common fea- a stabilizing role at higher surfactant concenJournal of Colloid andlnterface Science, Vol. 144, No. 1, June 1991

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SELLE, SJOBLOM, AND SKURTVEIT

trations. Also, in this case we found the D phase to enhance the stability of oil-continuous emulsions substantially. ACKNOWLEDGMENT Roald Skurtveit thanks the Norwegian Research Council for Science and the Humanities (NAVF) for financial support. REFERENCES 1. Ekwall, P., in "Advances in Liquid Crystals" (G. H. Brown, Ed.), Vol. 1, p. 1. Academic Press, New York, 1975. 2. Lindman, B., and Wennerstr6m, H., Top. Curr. Chem. 87, 1 (1980). 3. SjiSblom,J., Stenius, P., and Danielsson, I., in "Nonionic Surfactants. Physical Chemistry" (M. Schick, Ed.), p. 369. Dekker, New York, 1987. 4. Shinoda, K., J. Colloid Interface Sci. 25, 369 (1967). 5. Shinoda, K., and Saito, H., J. Colloid Interface Sci. 30, 258 (1969). 6. Shinoda, K., "Proceedings, 5th Inter. Congr. Surface Activity," Vol. 2, p, 275. Butterworths, London, 1968. 7. Friberg, S., Mandell, L., and Larsson, M., J. Colloid Interface Sci. 29, 155 (1969). 8. Friberg, S., J. Colloidlnterface Sci. 37, 2914 ( 1971 ). 9. Krogh, N., Fette Seifen Anstriehm. 77, 267 (1975). 10. Friberg, S., Jansson, P.-O., and Cederberg, E., Z Colloid Interface Sci. 55, 614 (1976). 11. Skurtveit, R., Sj~blom, J., and Holland, H., J. Colloid Interface Sci. 55, 614 (1989). 12. Mitchell, D. J., Tiddy, G. J. T., Waring, L., Bostock, T., and McDonald, M. P., ,L Chem. Soc. Faraday Trans. 1 79, 975 (1983).

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13. Persson, K. T., and Stenius, P., J. Colloid Interface Sci. 102, 527 (1984). 14. Fontell, K., in "Colloidal Dispersions and Micellar Behaviour," ACS Symposium Series 9, p. 270. Amer. Chem. Soc., Washington, DC, 1975. 15. Olsson, U., Shinoda, K,, and Lindman, B., J. Phys. Chem. 90, 4083 (1986). 16. Andersson, D., Wennerstr6m, H., and Olsson, U., J. Phys. Chem. 93, 4243 (1989). 17. Ekwall, P., Mandell, L., and Fontell, K., Mol. Cryst. Liq. Cryst. 8, 157 (1969). 18. Frirnan, R., Danielsson, I., and Stenius, P., J. Colloid Interface Sci. 85, 442 (1982). 19. Friberg, S., Mandell, L., and Ekwall, P., Acta Chem. Scand. 20, 2632 (1966), 20. Ekwall, P., and Mandell, L., Kolloid-Z. Z. Polym. 233, 937 (1967). 21. Friberg, S., Mandell, L., and Ekwall, P., Kolloid-Z. Z. Polym. 233, 955 (1965). 22. S6derlund, G., and Friberg, S., Z. Phys. Chem. (neue Folge) 70, 39 (1970). 23. Ahln~is,T., and S6derman, O., Colloid Surf 12, 125 (1984). 24. Ahln~is,T., S~Sderman,O., Hjelm, C., and Lindman, B., J. Phys. Chem. 87, 822 (1983). 25. Ahln~is, T,, S6derman, O., Walderhaug, H., and Lindman, B., "Surfactants in Solutions" (K. L. Mittal and B. Lindman, Eds.), Vol. 1, p, 107. Plenum, New York, 1984. 26. Tartar, H. V., J. Phys. Chem. 59, 1195 (1955). 27. Tanford, C., "The Hydrophobic Effect," Wiley, New York, 1973. 28. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. N., ,L Chem. Soc. Faraday Trans. 2 72, 1525 (1976), 29, Israelachvili, J. N,, Marcelja, S., and Horn, R., Q. Rev. Biophys. 13, 121 (1980).