The effect of salt concentration, temperature, and additives on the solvent property of aerosol OT solution

The Effect of Salt Concentration, Temperature, and Additives on the Solvent Property of Aerosol OT Solution 1 K t ) Z O SHINODA AND H I R O N O B U KUNIEDA Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan Received December 15, 1986; accepted February 4, 1987 The effects of salt concentration, temperature, and additives on the phase behavior and HLB temperature of a brine/Aerosol OT/C~oHz2 system have been studied. The most important factor which affects the solvent property of a surfactant solution is its hydrophile-lipophilebalance (HLB) in solution. Information obtained may be useful to design organized surfactant solutions as biomimetic solvents. © 1987Academic Press, Inc.

INTRODUCTION

The solvent power of a surfactant phase toward water and oil is large when the hydrophile-lipophile property o f the surfactant balances for a given system (1-3). Namely, surfactants whose hydrophile-lipophile property is balanced are useful. Ordinary ionic surfactants are too hydrophilic on the one hand, and the CMC of balanced nonionic surfactants in oil is too high on the other hand. Fatty acid salts and glycerol monoesters are relatively well-balanced surfactants and used in biological systems in which pH, salt concentration, etc. are kept fairly constant. Hence, Aerosol OT (sodium 1,2-bis(2ethylhexyloxycarbonyl)- 1-ethanesulfonate, AOT), which is a balanced ionic surfactant (4, 5), has been used frequently to study reversed micelles (6-10). The typical feature of the AOT molecule is its two hydrocarbon chains. If an ionic surfactant possesses two hydrocarbon chains, such as sodium 2-hexyldecyl sulfate, the hydrophile-lipophile property is similarly balanced (3). The present investigation has been undertaken to study the effects of salt concentration, l We are very happy to participate in celebrating the 65th birthday of Professor Egon Matijevic.

temperature, additives, etc. on the phase behavior and solvent properties of AOT, i.e., a balanced ionic surfactant. An understanding of the phase behavior under various conditions and additives is indispensable in utilizing microemulsions as reaction media as well as versatile solvents. EXPERIMENTAL

Pure-grade AOT was obtained from Tokyo Kasei Kogyo Co., Ltd. The solution behavior of AOT in water-oil is largely affected by a small amount of impurities, especially salt (11). It was purified as follows. Benzene solutions of 15% AOT were shaken with water to remove water-soluble impurities. Emulsification occurred during the extraction so that W/O cream was settled and the drained water phase was discarded. This procedure was repeated three times. In other hydrocarbon systems, a large amount of water is solubilized at lower temperatures than at those of the benzene system. Therefore, water-soluble impurities cannot be extracted effectively. After evaporating the benzene, AOT was dissolved in 25 wt% aqueous methanol solution and oilsoluble impurities were extracted three times with petroleum ether. The solvent was evaporated and finally AOT was dried over P205

586 0021-9797/87 $3.00 Copyright © 1987 by Academic Press, Inc. All fights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 118, No. 2, August 1987

PROPERTIES OF BALANCED SURFACTANT

587

in vacuo to constant weight. The procedures are confirmed to be useful to purify AOT by Carnali et al. (12). Procedures to determine the phase boundary are described in former papers (4, 11).

fraction of surfactant phase in the three-phase region rapidly increases, because a large amount of oil and water is swollen in the surfactant phase, the volume of oil and water phases decreases, and finally one surfactant phase is obtained. Although the shape of the RESULTS AND DISCUSSION three-phase region is not completely symmetThe phase diagram o f a brine/AOT (2 wt%/ rical, it resembles the phase equilibria o f a wasystem)/decane system as a function of the salt ter/nonionic surfactant/oil as a function of concentration is shown in Fig. 1. The solution temperature (13). The change in volume fracbehavior of AOT is changed from water sol- tions of respective phases at 2 wt% AOT/sysuble to oil soluble with an increase in salt con- tem as a function of the salt concentration is centration as expected. There is an interme- shown in Fig. 2a. Aqueous micellar solution diate three-phase region consisting of water, phase (Win) coexists with an excess oil phase surfactant, and oil phases. The three-phase re- at low salt concentrations whereas nonaqueous gion is almost parallel to the horizontal axis. reversed micellar solution phase (O,1) coexists It means that brine can be considered to be a with an excess water phase. Although notations are different, these Win, Om, and D (surpseudo one component. The effect of the concentration of AOT on factant) phases continuously change with the the solubility curves is shown in Fig. 2b, in salt concentration as is shown in Fig. 2a. which the brine/decane ratio is unity correJudging from this phase behavior, it is clear sponding to the broken line in Fig. 1. Above that two critical endpoints between O - D and the saturation concentration of AOT in brine D - W exist at constant temperature. It means + oil, a surfactant phase appears. The volume that the critical solution phenomena occurs

[.4

i

i

AOT

i

i

2wt%/system

42oc

~ 0.6

0.2 O0 H20

0.2

0.4 weight

0.6 froction

0.8

1.0 CmH22

FIG. 1. Phase diagram for a brine/AOT/decanesystem as a function of salt concentrationat 42°C. The concentration of AOT is 2 wt%/system. Wm(D),aqueous micellar solution. Ore(D), nonaqueous reversed micellar solution. O, W, excessoil and water. D, surfactant phase. Journal of Colloid and Interface Science, Vol. 118, No. 2, August 1987

588

SHINODA AND KUNIEDA

®

@ I

i

i

I

[

i

2wt %/system

AOT

i

Om

/. /o

c 0

0

Wm I

I

volume

+o+o

---... '~[

I i

I0

0

i

~ Om+W

"i

I

i

NoCI oq. : C10H22: [: I (wt)

froction

Wm "1" 0

I

2

I

I

4 6 8 conch, of AOT wt%/system

FIG. 2. (a) The change in volume fractions of respective phases as a function of salt concentration. The composition corresponds to the broken line in Fig. 3b. (b) The effect of concentrations of AOT and salt on the equiweight mixture of brine and decane as a function of salt concentration at 42°C. The broken line corresponds to that in Fig. 1.

b e t w e e n O - D a n d D - W at b o t h l o w e r a n d u p p e r l i m i t s o f t h e t h r e e - p h a s e r e g i o n (4, 13, 14). O n t h e o t h e r h a n d , A O T c h a n g e s f r o m l i p o p h i l i c to h y d r o p h i l i c w i t h rise in t e m p e r -

100[,

,

ature in a brine/AOT/oil system at constant salt concentration (4). Namely, AOT is oil soluble, forming a reversed micelle at lower temperatures, whereas it is water soluble, forming

i

,

,

I

80tt

Aerosol OT 3.5wt%/syst~.=/lm

Om*W

I

0 0 . 5 % NaC I

0.2

I

0.4 Weight

I

0.6 Fraction

I

0.8

.0 isooctane

:FIG. 3. Phase diagram for a 0.5 wt%/AOT/isooctane system as a function of temperature. The concentration of AOT in the system is 3.5 wt%. Journal of Colloid and Interface Science, Vol. 118, No. 2, August 1987

PROPERTIES OF BALANCED SURFACTANT normal micelle at higher temperatures. A three-phase region appears at temperatures in between (4). The effect of temperature is opposite to that in nonionic surfactant systems. When the concentration of A O T is relatively high, a one-phase region is observed in a wide range of water/oil ratio as is shown in Fig. 3. The microstructure of this one phase was investigated by the self-diffusion technique (12). As is shown in Fig. 3, a narrow one-phase region is parallel to the horizontal axis. It is intuitive that two critical endpoints of O - D and D - W also exist at constant salt concentration from Figs. 1-3. In a water/nonionic surfactant/ oil system, the temperature range of the threephase region is fixed at constant pressure. Therefore, it is not easy to change the position of three-phase region, but in the A O T system it can be done. Once the salt concentration is fixed, the A O T system can be regarded as a pseudo three-component system. If the kind ofoil is changed, the temperature of the threephase region also changes as does that of a water/nonionic surfactant/oil system, but the effect of temperature is opposite (15). Amphiphilic additives also change the threephase (HLB) temperature. The temperature went down about 15°C, if 10% of A O T (10 wt%/system) is replaced by sodium dodecyl sulfate. On the other hand, the temperature went up when 10% of A O T is replaced by a lipophilic amphiphile such as decanol. The effect of salt concentration on the H L B temperature was, for example, about 40°C higher when the NaC1 concentration changed from 0.2 to 0.6 wt% in a brine/AOT/C6Hl4 system (4). The effect of various additives on the H L B of surfactant in the system should be taken into consideration to avoid phase separation when a well-balanced surfactant solution is used as a reaction medium. In this respect, 2hexyldecyl sulfate (or sulfonate) has no ester

589

group and is more stable than AOT. It is a well-balanced surfactant, simple to synthesize from 2-hexyldecanol, and its solvent power is large (3). Other ionic surfactants that can be used to biomimetic solvents are CnH2n+~[OCH2CH-(CH3)]m-SO4Na or Cal/2. These surfactants are soluble in hard water as well as hydrocarbon depending on the salt concentration (3, 16). ACKNOWLEDGMENTS The authors thank Mr. K. Hanno for his experimental cooperation. The financial support from the Asahi Glass Foundation and the Ministry of Education is gratefully acknowledged. REFERENCES 1. Shinoda, K., and Saito, H., J. Colloid Interface Sci. 26, 70 (1968). 2. Shinoda, K., and Kunieda, H., J. Colloid Interface Sci. 42, 381 (1973). 3. Shinoda, K., and Shibata, Y., Colloids Surf 19, 185 (1986). 4. Kunieda, H,, and Shinoda, K., J. Colloid Interface Sci. 75, 601 (t980). 5. Frank, S. G., and Zografi,G., £ Colloid Interface Sci. 29, 27 (1969). 6. Ekwall, P., Mandell, L., and Fontell, K., J. Colloid Interface Sci. 33, 215 (1970); 44, 318 (1973). 7. Eicke, H., and Christen, H., J. Colloid Interface Sci. 48, 28! (1974). 8. Eicke, H., and Rehak, J., Helv. Chim. Acta 59, 2883 (1976). 9. Martin, C. A., and Magid, L. J., J. Phys. Chem. 85, 3938 (1981). 10. Luisi, P. L., Angew. Chem. 97, 449 (1985). 11. Kunieda, H., and Shinoda, K., £ Colloid Interface Sci. 70, 577 (1979). 12. Carnali, J. O., Ceglie, A., Lindman, B~,and Shinoda, K., Langmuir 2, 417 (1986). 13. Kunieda, H., and Friberg, S. E., Bull. Chem. Soc. Japan 54, 1010 (1981). 14. Herrmann, C.-U., Klar, G., and Kahlweit, M., J. Colloid Interface Sci. 82, 6 (1981). 15. Figure 6 in Ref. (4). 16. Shinoda, K., Maekawa, M., and Shibata, Y., Z Phys. Chem. 90, 1228 (1986).

Journal of Colloid and Interface Science, Vol. 118, No. 2, August 1987