Solubilization of 1-hexanol in hexadecyltrimethylammonium bromide, sodium dodecyl sulfate, and sodium decanoate

S o l u b i l i z a t i o n o f 1 - H e x a n o l in H e x a d e c y l t r i m e t h y l a m m o n i u m

Bromide,

Sodium Dodecyl Sulfate, and Sodium Decanoate The solubility of hexanol has been measured in aqueous solutions of hexadecyltrimethylammonium bromide (HTAB), sodium dodecyl sulfate (NaDDS), and sodium decanoate (NaCt0). In the HTAB and NaDDS solutions the hexanol solubility decreased abruptly above a surfactant content of about 0.05 mole kg-1, suggesting a structural change from spherical to anisodiametric micelles. The total solubility of hexanol shows that it must be solubilized both in the palisade layer and in the micellar interior of these spherical micelles. After the structural change has taken place less hexanol is solubilized so that all can be accomodated in the palisade layer. No decrease in solubility has been observed for the NaCl0 system, and the total hexanol solubility is so small that all can be accommodated in the palisade at all NaCl0 concentrations. 5 H ° and ~S ° of solubilization for spherical micelles have been calculated from the temperature dependence of the distribution coefficient. © 1985AcademicPress,Inc. INTRODUCTION

RESULTS AND DISCUSSION

Zana et aL (1, 2) have recently shown that the solubility of l-pentanol and 1-hexanol in micellar solutions of tetradecyltrimethylammonium bromide (TTAB) and sodium dodecyl sulfate (NaDDS) decrease abruptly above a certain surfactant concentration. The data have been taken as evidence for a transition from a spherical to an anisodiametric micellar structure. The molar solubilization ratio, i.e., the ratio of solubilizate molecules to surfactant molecules in the micelles, suggests that the alcohol is solubilized both in the palisade layer and the interior of the spherical micelles, and only in the palisade layer of the anisodiametric micelles. In previous work (3, 4) we have shown that the solubility of I-alcohols in aqueous surfactant solutions is linearly dependent upon the surfactant content, making it possible to calculate the distribution coefficient of the alcohol between water and the micelles. However, for sodium dodecyl sulfate and hexadecyltrimethylammonium bromide a deviation from linearity was observed above a surfactant concentration of about 0.05 mole kg-J. In this paper we have extended previous work by measuring the solubility of l-hexanol in various aqueous surfactant solutions at various temperatures, looking for abrupt changes in the hexanol solubility. The enthalpy and entropy of solubilization have also been calculated.

Figures 1 and 2 show the total solubilities of 1-hexanol as a function of the micellar content of surfactant, i.e., the total surfactant molality minus the surfactant monomer molality. For the NaDDS and HTAB systems an abrupt decrease in the hexanol solubility is observed around a surfactant content of 0.05 mole kg-~, particularly abrupt for HTAB. For the NaCt0 system no such abrupt decrease is seen even at 0.8 m though it appears that the solubility curve is leveling.

0.4

mCsOH tool kg -1

0.3

0.2

0.1

EXPERIMENTAL Sodium dodecyl sulfate was obtained from BDH, "specially pure," hexadecyltrimethylammonium bromide was obtained from Sigma. Both were dried in evacuated desiccator at 50°C. Sodium decanoate (NaC~o) was prepared from decanoic acid (Fluka) as previously described (3). lHexanol (Fluka, puriss grade), was used as received. The solubility of 1-hexanol in various surfactant solutions was measured as previously described (4).

0.05 '

0 I1

0.15 ' ~$urf - mERE

m0t kg-1

~G. 1. The solubility of hexanol in aqueous solutions of hexadecyltrimethylammonium bromide: (11) 293.2 K, (O) 298.2 K, (A) 308.2 K; and aqueous solutions of sodium dodecyl sulfate: (I-4)293.2 K, (O) 298.2 K, (A) 308.2 K. 576

0021-9797/85 $3.00 Copyright © 1985 by Academic Press, Inc. All tights of reproduction in any form reserved,

Journal of CoUoid and Interface Science, Vol, 107, No. 2, October 1985

NOTES !

rnc6oH

!

!

solubilized in the micellar interior of these micelles. For the NaClo system the molar solubilization ratio is less than 1 over the entire range (up to 0.8 mole kg-~). This means that all alcohol molecules can be accommodated at the micellar surface, and the problem of structural changes and their effect on the solubilization of hexanol do not arise. As shown previously it is possible to determine the distribution coefficient o f the alcohol between water and the micelles for the linear part of the phase diagram (3). The Gibbs' energy of the solubilization process can thus be determined:

I

0.8

mo# kg -I 0.6

0.4

0.2

I

I

0.2

0.4

1

577

xmic

I

0.6

AG ° = -RT

0.8

mNa

C10- mcmc

tool k g -1 Fio. 2. The solubility of hexanol in aqueous solutions of sodium decanoate: ( i ) 288.2 K, (e) 298.2 K, (A) 308.2 K.

In K =

-RT

In ~ .

[1]

Here x ~ c and x~q are the mole fractions of alcohol in the micellar and aqueous phases, respectively. The enthalpy and entropy of solubilization can be calculated in the usual way from

2~H ° If the curves are inspected more closely, it will be seen that the molar solubilization ratio is as high as 5 to 6 for the NaDDS and HTAB systems while it is less than 1 for NaCto. For TTAB geometrical calculations have shown that a maximum of 2.5 alcohol molecules can be accommodated at the micellar surface (2). This is approximately correct also for the NaDDS and HTAB micelles. Therefore, a fair amount of the hexanol molecules must be solubilized in the micellar interior. The abrupt decrease in the hexanol solubility has been ascibed to structural changes producing anisodiametric micelles of TTAB and NaDDS (1, 2). The same must be true for the HTAB micelles. The molar solubilization ratio shows that fewer molecules, if any, are

and aS °

A H o - AG °

[21

T

The results are shown in Table 1. Care should be excersised when using Eq. [2] since the micellar aggregation number may change with temperature (5). Direct calorimetric measurements are preferable, and Larsen and Magid (6) have determined ~ H ° for the solubilization ofhexanol in 0.1 m HTAB by this method. Their result, 1.67 kJ mole -l, do not agree very well above the linear solubility region and the results are not directly comparable. The same is

TABLE I Distribution Coefficients, Gibbs' Energy, Enthalpy, and Entropy of Solubilization of Hexanol in Aqueous Solutions of Hexadecyltrimethylammonium Bromide (HTAB), Sodium Dodecyl Sulfate (NaDDS), Sodium Decanoate (NaC~o), and Sodium Deoxycholate (NaDC) Surfactant

T (K)

K

AG ° (kJ mole -~)

z%/-/° (kJ mole -~)

&S ° (J mole -~)

HTAB

293.2 298.2 308.2

738 777 825

- 16.09 - 16.49 - 17.21

6.0

75

NaDDS

293.2 298.2 308.2

720 760 790

- 16.03 - 16.43 -17.08

4.3

70

NaClo

288.2 298.2 308.2

397 455 516

- 14.32 -15.16 -15.99

9.7

83

NaDC a

298.2

500

- 15.4

7.8

78

aDam from Ref. (10). Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985

578

NOTES

true for the calorimetric investigations of Larsen and Tepley (7) and Aveyard and Lawrence (8). Recently, Hayase et al. (9) have determined AH ° and AS ° for the solubilization of some 1-alcohols in NaDDS also by Eq. [2]. They obtained negative AH ° values. However, their data were for systems not saturated with respect to the alcohol. Furthermore, they measured at relatively large temperature intervals, and it also appears that the error in their distribution coefficients must be quite large. All in all the AH ° data for solubilization are not consistent, both positive and negative values have been reported. There is, however, reasonable agreement between our data and those of Spink and Colgan (10) for the solubilization of alcohols in deoxycholate mieelles (see Table I). We tend to believe that these data are the correct ones at least as far as the sign and magnitude is concerned, i.e., positive values of/XH ° and AS ° showing that the solubilization process is entropy-driven. The AH ° values of Table 1 also suggest differences between solubilization in NaDDS and HTAB compared to NaC~0. In the latter case AH ° is approximately twice as large. One possible explanation for the different behavior of the decanoate system is in terms of a specific complex formation between the hydrated carboxylate group and the hydroxyl group of the alcohol at the micellar surface (11, 12). The differences in AS °, on the other hand, are relatively small. Spink and Colgan (10) have shown that the largest contribution to AS ° of solubilization comes from the CH3 group; the CH3 group value is approximately 10 times that of the CH2 group. Since the CH3 group will be solubilized in the interior of the micelle irrespective of the position of the polar group, it is not surprising that AS ° only varies by a small amount, probably reflecting different environments of the polar group. REFERENCES 1. Zana, R., Yiv, S., Strazielle, C., and Lianos, P., J. Colloid Interface Sci. 80, 208 (1981).

Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985

2. Llanos, P., and Zana, R., J. Colloid Interface Sci. 101, 587 (1984). 3. Holland, H., Kvammen, O., Backlund, S., and Run&, K., in "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), Vol. 2, pp. 949-62. Plenum, New York, 1984. 4. Holland, H., Ljosland, E., and Backlund, S., J, Colloid Interface Sci. 101, 467 (1984). 5. Muller, N., in "Micellization, Solubilization, and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, pp. 229239. Plenum, New York, 1977. 6. Larsen, J. W., and Magid, L. J , J. Phys. Chem. 78, 834 (1974). 7. Larsen, J. W., and Tepley, L. B., J. Colloid Interface Sci. 53, 332 (1975). 8. Aveyard, R., and Lawrence, A. S. C., Trans. Faraday Soc. 60, 2265 (1964). 9. Hayase, K., Hayano, S., and Tsubota, H., J. Colloid Interface Sci. 101, 336 (1984). 10. Spink, C. H., and Colgan, S., J. Phys. Chem. 87, 888 (1983). 11. FonteU, K., Mandell, L., Lehtinen, H., and Ekwall, P.,Acta Polytech. Scand. 74, 111 (1968). 12. Rosenholm, J. B., and Lindman, B., J. Colloid Interface Sci. 57, 362 (1976). H. I~ILAND A. M. BLOKHUS O . J. KVAMMEN

Department of Chem&try N-5014 Bergen-University Norway S. BACKLUND

Department of Physical Chemistry Abo Akademi, SF-20500 Abo Finland Received February 18, 1985