Aerosol OT aggregation in water and hydrocarbon solution from NMR self-diffusion measurements

Aerosol OT Aggregation in Water and Hydrocarbon Solution from NMR Self-Diffusion Measurements Recent theoretical work has very substantially improved our general understanding of various features of amphiphile self-association, such as micellization, microemulsions and phase diagrams (1-8). Theoretical calculations treating the electrostatic interactions with the PoissonBoltzmann equation have predicted micellization equilibria and phase diagrams in good agreement with experimental observations (5-7). Self-diffusion studies for single chain ionic surfactants, have given results in close agreement with the theoretical ones (9). For surfactants with bulky nonpolar parts, the theory predicts an entirely different aggregation pattern which should be directly amenable to testing by self-diffusion studies. Aerosol OT, sodium di-2-ethylhexylsulfosuccinate, is a common and widely studied surfactant which has a crosssectional area which is much larger for the nonpolar part than for the polar part. The phase diagram of the Aerosol OT-water system is dominated by an extensive lamellar phase region (10, 11), while in the presence of a third component like a hydrocarbon, there is a very extensive solution phase extending from the hydrocarbon-Aerosol OT axis versus the water corner (12) (cf. Fig. 1). Aerosol OT is characterized by a low aqueous solubility indicating that normal micelles are energetically unfavorable. This is supported by the low solubilizing capacity of aqueous Aerosol OT (AOT) solutions (12). In Fig. 2, self-diffusion coefficients (cf. below) of the surfactant anion are presented as well as those of solubilized p-xylene (PX). Because of the limited extension of the L~ phase, these data have a much lower precision than those o f our previous studies (13-15). Therefore, we will mainly restrict ourselves to qualitative conclusions. DAox is at first roughly independent of concentration and then it decreases to fairly low values, demonstrating that there is a marked self-association. Plotting DAOr versus the inverse AOT concentration (16), the critical micelle concentration is obtained to be ca. 2.6. X 10-3 rn. Reported CMC data show a considerable spread in the range 2.5-6 X 10 -3 M (17). Surface tension measurements (18, 19) have given CMC values identical with our value, while conductance studies (11, 20, 21) have given values around 0.006 M. Miller and Dixon (2 I) argue that the value obtained from surface tension is the better one. In our self-diffusion study as well as in studies using other techniques, the CMC is much less well-defined than in studies of single-chain surfactants with similar CMC's. From his theoretical work,

Jrnsson (7) estimated the CMC to lie in the range 0.0030.006 m. Upper limits of the micellar self-diffusion coefficients are given by the values of DAox while lower limits were obtained using the normal two-site model (9) taking the diffusion coefficient of nonassociated Aerosol OT from the low concentration data and taking the concentration of nonassociated Aerosol OT to be equal to the CMC. The value so obtained is D m i e = 0.07 _+ 0.01 × 10-9 m 2 sec-1. An analysis of the diffusion-concentration behavior of AOT through the normal two-site model and a single monomer-micelle equilibrium (n. M ~ M,, where n is the aggregation number) similarly yields Dmo. . . . . 0.35 X 10-9 m 2 sec-1, Dmic ~ 0.065 × 10-9 m 2 sec-~, the best fit being obtained for n = 12-16. These calculations were made according to the strategy outlined in Ref. (22) using a specially written FORTRAN 77 program ASSOC (P. Stilbs, unpublished). Not knowing micellar shape, neither the micellar size nor the aggregation number can be deduced from Dmi¢ alone. However, it is quite clear that these results are not consistent with a minimal spherical miceUe. From the geometrical shape (volume, cross-sectional area) of the AOT molecule, the aggregation number for a spherical micelle can be estimated to be 6 (7), a value in agreement with findings from work with space-filling models (cf. Ref. (23)). For oblate shaped micelles, the obstruction effect on the diffusion coefficients is large (24) and the relatively low D~ic value may, therefore, be indicative of disk-shaped micelles. This is also consistent with the small extension of the L~ phase, since disk micelles are energetically unfavorable with respect to the lamellar phase (4); rod micelles, on the other hand, can according to both experimental and theoretical studies have a wide range of existence (4, 23). Further studies using other techniques are required to establish the shape of AOT micelles. This is of interest in connection with attempts to understand the relation between surfactant molecular shape and micellar shape. Although the solubilizing capacity of AOT micelles, as reflected in the extension region o f the L~ phase, is quite low, the actual solubilizate partitioning between micelles and water, at least for p-xylene (px), appears to be similar to that of "normal" single-chain surfactants like sodium dodecyl sulfate, sodium octylbenzenesulfonate, etc. (9, 13, 15, 23). Using the two-site model, one may estimate the

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Journal of Colloid and Interface Science. Vol.99. No. 1, May 1984

NOTES

291 ]so-octane

p-Xylene

,oo%,,

,'o

"%0

Water

Aerosol OT

\LI' Water

2'0

'

40

'

6'0

'I/

8'0 ' 100 Aerosol OT

FIG. 1. Ternary phase diagrams for Aerosol OT-hydrocarbon-water systems: (a) p-xylene and (b) isooctane. For clarity, only the single phase regions are given, i.e., L] and L2 isotropic solution phases, D lameUar liquid crystal, I cubic liquid crystal, and F reversed hexagonal liquid crystal. The LI region extends to about 1% AOT in water and 0.2% in hydrocarbon and is too small to be properly represented on this scale. (The figures were kindly provided by Dr. Krister Fontell.)

fraction of p-xylene which is solubilized from the Vmi c and Dpx data. At micelle concentrations between 0.02 and 0.03 m, the data in Fig. 2 correspond to a fraction of 9094% miceUafly bound p-xylene molecules. These figures can be normalized to a form of partition equilibrium constant, as required in the pseudo-phase model for solubilization: K = m

1-p

concentration is low and the situation corresponds closely, in agreement with previous work (12, 26, 27), to an idealized case of W/O microemulsions. This applies also at very high water concentrations (above 50% by weight). The situation is very different from that of microemulsions with butanol or pentanol as cosurfactant. Here, both hydrocarbon and water self-diffusion are very rapid over wide concentration ranges, corresponding to an effectively

Vmic

where p is the degree of binding to the micelles (0 ~ p < 1) and Vaq and Vmi c a r e the volumes of aqueous and micellar "phases." These data then transform into K-values of the order of 1000, which can be compared to data for solubilization of benzyl alcohol, benzene, and toluene into SDS micelles, corresponding to K ~ 33, 93, and 310, respectively (13). Addition ofp-xylene reduces DAOT,corresponding to an increased micellar size and/or a decreased concentration of monomeric Aerosol OT. The I~ solution phase was investigated for two hydrocarbons, p-xylene (phase diagram in Ref. (12)) and isooctane (Ref. (25) and unpublished work by K. Fontell). The observed self-diffusion coefficients for water, surfactant, and hydrocarbon are exemplified in Fig. 3. Hydrocarbon self-diffusion can be seen to be more rapid than water self-diffusion by a factor of 10-30; this applies also to other ratios between surfactant and hydrocarbon than those represented in Fig. 3. This indicates that in these solutions, water is confined to micelles of the reversed type in a hydrocarbon continuum. The intermicellar water

ix D/m2 s±1 '

water

2 69 10.9 510-lc

21o<

~

(-p-x,ene~

1(lc o 5.10-11 0'01

0102

0103 mAOT

FIG. 2. Self-diffusion data in the L1 region of the Aerosol OT-D20 system. The open symbols correspond to results for Aerosol OT/p-xylene ratios of ~ 10. p-Xylene diffusion in water is characterized by D ~ 1.0 X 10 9 m 2 sec-t at the measurement temperature of 25°C.

Journal of Colloid and Interface Science, Vol. 99, No. I, May 1984

292

NOTES a). . . . . .

2.1(59 10-9

D/m2 []

s-1 im

br).

iso-octane n i-i

p-xytene []

t2

t2

5.1610

Tfls for the AOT protons. For similar reasons, no evaluation of AOT diffusion data was possible in the AOTiso-octane-H20 system. The AOT signal was here obscured by a considerably larger iso-octane signal and was not monitorable with adequate precision. It was evident, however, that AOT diffusion was of the same low magnitude as in the corresponding p-xylene system.

2.10-1° H20

REFERENCES

o

Io-IO •

ii o

• •

1-120 (ool

o

5.10-I~

•

A0T

(AA)

o

8 A

2.1041 10-'I

I

i

10

3'0 4'o

6

•

t

i

i _

10 20 30 weigh[

• °

percent

i•

i

50

H20

FIG. 3. Self-diffusion at 25°C in the L2 region in the iso-octane and p-xylene-Aerosol OT-water systems. (a) Open symbols: 43/57 AOT/iso-octane weight ratio, filled symbols: 51/49 ratio. (b) Open symbols: 60/40 AOT/pxylene weight ratio, filled symbols: 73/27 ratio. bicontinuous situation (14). For the Aerosol OT systems, no major changes with concentration are observed for any of the self-diffusion coefficients within the 1-2 phases. Therefore, the W/O structure applies over essentially the whole range of existence of the l-a phase. (For water contents below 5%, measurements of water self-diffusion could not be performed.) Assuming that the mobility of the reversed micelles can be approximated to be the same as that of the water molecules, the hydrodynamic radius, rh, was estimated from the Stokes-Einstein equation. Taking the viscosity of the intermicellar solution to be that of the pure hydrocarbon, we estimate rh to vary in the range of 80-250/~ for the p-xylene system and 45-120/~ for the isooctane system. The latter figures are in agreement with those ofZulaufand Eicke (28) obtained from quasi-elastic light scattering. At the present stage, our data do not have sufficient precision or detail to describe the evolution of hydrodynamic radius with composition. In summary, self-diffusion studies of LI and I~ solution phases in Aerosol OT-hydrocarbon-water systems are in agreement with current theoretical models suggesting that a surfactant like Aerosol OT has a strong preference for micelles of the reversed type rather than normal micelles. EXPERIMENTAL The measurement procedures have been described previously (13, 14). The low-concentration Lrdata were usually collected in overnight runs, partly as a result of short

Journal of Colloid and Interface Science, Vol. 99, No. 1, May 1984

1. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., J. Chem. Soc. Faraday Trans. 2 72, t525 (1976). 2. Israelachvili, J. N., Marcelja, S., and Horn, R. G., Quart. Rev. Biophys. 13, 121 (1980). 3. Mitchell, D. J., and Ninham, B. W., J. Chem. Soc. Faraday Trans. 2 77, 601 (1981). 4. Wennerstrrm, H., J. Colloid Interface Sci. 68, 589 (1979). 5. Gunnarsson, G., Jrnsson, B., and Wennerstrrm, H., J. Phys. Chem. 84, 3114 (1980). 6. J6nsson, B., and Wennerstrrm, H., J. Colloidlnterface Sci. 80, 482 (1981). 7. Jrnsson, B., "The Thermodynamics of Ionic Amphiphile-Water Systems. A Theoretical Analysis." Thesis, Lund, 1981. 8. Jrnsson, B., Nilsson, P. G., Lindman, B., Guldbrand, L., and Wennerstrrm, H., in "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), Plenum, New York, in press. 9. Lindman, B., Puyal, M.-C., Kamenka, N., Brun, B., and Gunnarsson, G., J. Phys. Chem. 86, 1702 (1982). 10. Park, D., Rogers, J., Toft, R. W., and Winsor, P. A., J. Colloid Interface Sci. 32, 81 (1970). 11. Fontell, K., J. Colloid Interface Sci. 44, 318 (1973). 12. Ekwall, P., Mandell, L., and Fontell, K., J. Colloid Interface Sci. 33, 215 (1970). 13. Stilbs, P., J. Colloid Interface Sci. 87, 385 (1982). 14. Lindman, B., Stilbs, P., and Moseley, M. E., J. Colloid Interface Sci. 83, 569 (1981). 15. Stilbs, P., and Lindman, B., J. Phys. Chem. 85, 2587 (1981). 16. Lindman, B., and Brnn, B., J. Colloid Interface Sci. 42, 388 (1973). 17. Mukerjee, P., and Mysels, K. J., "Critical Micelle Concentrations of Aqueous Surfactant Systems," NSRDS-NBS 36. Washington, D. C., U. S. Government Printing Office, 1971. 18. Williams, E. F., Woodbury, N. Y., and Dixon, J. K., J. Colloid Interface Sci. 12, 452 (1957). 19. Kitahara, A., Kobayashi, T., and Tachilyana, T., J. Phys. Chem. 66, 363 (1962). 20. Haffner, F. D., Piccione, G. A., and Rosenblum, C., J. Phys. Chem. 46, 662 (1942).

NOTES 21. Miller, M. L., and Dixon, J. K., J. Colloid Interface ScL 13, 411 (1958). 22. Persson, B. O., Drakenberg, T., and Lindman, B., J. Phys. Chem. 83, 3011 (1979), 23. Wennerstr6m, H., and Lindman, B., Phys. Rep. 52, 1 (1979). 24. J6nsson, B., Nilsson, P. G., and Linse, P., in press. 25. Kuneida, H., and Shinoda, K., J. Colloid Interface Sci. 70, 577 (1979). 26. EkwaU, P, J. Colloid Interface Sci. 29, 16 (1969). 27. Eicke, H.-F., Pure Appl. Chem. 52, 1349 (1980). 28. Zulauf, M., and Eicke, H.-F., J. Phys. Chem~ 83, 480 (1979).

293 PETER STILBS

Institute of Physical Chemistry Uppsala University Box 532 S- 751 21 Uppsala, Sweden BJORN LINDMAN Physical Chemistry 1, Chemical Center Lund University Box 740 S-220 07 Lund, Sweden Received January 19, 1983

Journalof Colloidand InterfaceScience,.Vol.99, No. 1, May 1984