a6
Langmuir 1993,9, 86-89
Enthalpic Studies on the State of Water in Sodium Bis(2-ethylhexyl)sulfosuccinate Reversed Micelles Ayako Goto,* Shigeharu Harada,+Teruyuki Fujita,t Youko Miwa,j
Hisashi Yoshioka,s and Hiroshi Kishimotol
Department of Chemistry, School of Food and Nutrition Sciences, and Gradwted School of Nutritional and Environmental Sciences, University of Shizuoka, 52- 1 Yada, Shizuoka 422, Japan, Hamamatsu College, University of Shizuoka, 3-2-3N u n o h h i , Hamamatsu 432,Japan, and Faculty of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori, Mizuhoku, Nagoya 467, Japan Received February 19, 1992. In Final Form: August 28,1992 Thermochemical studies were made on the dissolution state of water in sodium bis(2-ethylhexy1)sulfosuccinate(AOT)/ieooctanesolutions. The solutionenthalpiesof water were determined at 20 O C with a conduction-typemicrocalorimeter by directmixing of water with anhydrousAOT reversed micelle solutione. The observed heats, AHow,were slightly exothermic at low values of molar quantitiee of water, n,,and then turned endothermic and continued to increase nonlinearly with n,until a nearly constant value wae reached. This limiting value for the solution enthalpy of water per mole of AOT was calculated to be 5.3 kJ/mol. The apparent molar enthalpy curve of water was drawn against R (R= molar ratio of water to AOT), and from the apparent quantities the relative partial molar enthalpies were evaluated. The plot of the relative partial molar enthalpy va R showed a maximum at R = ca. 1.1,followed by a eharp fall and the successive gradual decrease at R from 2 to 4. It passed through a plateau between R = 4 and R = 11, and then reached zero at R above 15. These findings led to the conclusion that (1) the dissolution states of water molecules and AOT micelles change discontinuously at R = ca. 1.1 from the immobilized to structured (ionic hydration) states of water, and from small and compact to ordinary spherical micelles for AOT,respectively, and (2) in the resultant water pools, bulk water came to appear above R = 11-15. Similar behavior was observed for deuterium oxide/AOT/isooctane systems, although the height of the maximum which appeared in the relative partial molar enthalpy curve was low in comparison with that of the water systems. enthalpy changes from high to low R. The curve obtained had negative values at low R, showed a steep increase to reach a maximum, and then gradually decreased to zero with increasing R. Although the two-state model shown in Figure l a seemsconventional,our data were interpreted in terms of the three-state model to water partitioned betweenthreeenthalpicstatea(Figure1b).Sincethewater molecules existingin the interfaceof a water pool (sl phaee) interact strongly with ionic head groups of reversed micelles, the Hsl was estimated to be negative on the assumption that H b was zero. On the other hand, the Ha was considered to be positive because the water structure may be destroyed in the e2 phase compared with that in bulk water. The above conclusion was based on the apparent enthalpies; there still remain some problem to be elucidated as follows: (1)The e l and e2 layers are too thin to exist in the ordinary spherical micelles compared with the molecular dimension of the water. (2) The apparent enthalpy is a quantity adequate to reflect the dissolution state of water when the structure of micelles changes on mixing of AOT solutions of different R values. (3) The molar enthalpy of water in the sl phase has not actually been observed. In order to shed more light on the dissolution states of water and reversed micelles, the partial molar enthalpies of water solubilization were determined in this stud; bv a simple experimental procedure, i,e., the dire& mi& if water with h y d r o u s AOT solutions, and as a some modification of the threestate mode1 was made.
Introduction Detailed information on physical properties of water in reversed micellesis suitable for the understandingof states of water in living organismsbecausethe micelles can easily produce water of various states by changing the water content solubilized.' It is, therefore, of interest to study the solution heats of water in reversed micelles. measured the molar enthalpy of solution Aprano et of water in anhydrousAOT reversed micelle solutionswith a flow microcalorimeter, and found that the molar enthalpies were endothermic and increased with a decrease in R, where R means a molar ratio of water to surfactant. They interpreted the phenomenon in terms of a semiempirical model of water partitioning between the two states shown in Figure la. The molar enthalpy of water existing in the inner surfaces of AOT reversed micelles was estimated to be positive on the assumption that the molar enthalpy of bulk water was zero. In a previous paper: the apparent molar enthalpy changes of water were obtained by mixing AOT Solutions having various R values with anhydrous AOT solutions. Since this process involves the transfer of water between anhydrous and water-solubilized micelles, the enthalpy m e against R was drawn by consecutive addition of the
* Towhom correapondence shouldbe a d d r e d at the Department of Chemistrv. Universitv of Shizuoka. + H ~ U X W ~ I ~C~O U U W university ~ of shizuoka. t School of Food and Nutrition Sciences, University of Shizuoka. Graduated School of Nutritional and Environmental Sciences, University of Shizuoka. Faculty of Pharmaceutical Sciences, Nagoya City University. (1) Luisi,P. L.; Gomini, M.; Pileni, M. Pi; bbkson, B. H. Biochirk.
*
Biophys. Acta 1988,947, 2W246. (2) DAprauo, A.; Lizzio, A.; Leveri, V. T. J. Phyu. Chem. 1987, 91,
Experimental Section
(3) Goto, A.; Yoehiokn, H.; Kiahimoto, H.; Fujita, T. Langmuir 1992, 8,441-445.
Materials. T h e d i u m bie(2-ethylhexyl)sulf~uccinate(AOT) obtained from Nikko Chemical Co.,Japan,was dried asdescribed
4749-4761.
0743-7463/93/2409-0086$04.00/0
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1993 American Chemical Society
Langmuir, Vol. 9, No. 1, 1993 87
State of Water in AOT Reversed Micelles
I
Figure 1. Two-state and three-state models of the water pool in the reversed micelle and the molar enthalpies assigned to water in the respective models. (a) Two-state model of the water pool, consisting of water in the s and b phases, which are located near the hydrophilic groups of the reversed micelle and in the core of the water pool, respectively. r shows the distance from the outer surface of the water pool to the interface between s and b phases. H, and Hb represent the molar enthalpies of water in the s and b phases, respectively,where the value of Hb is set at zero. (b) Three-state model of the water pool3with two phases, s l and s2, for water near the hydrophilic groups of the reversed micelle in place of a phase of the two-state model. rl and r2 denote the distances from the outer surface of the water pool to the interface between s l and s2 phases and that between s2 and b phases, respectively. Hex,H82,and H b represent the molar enthalpies of water in the sl, 92, and b phases, respectively, where the value of Hb is set at zero. in the previous paper.'+ Water was distilled by an all-glass distillation apparatus before use. The 2,2,4-trimethylpentane (isooctane) used as bulk solvent was dried as described in the previous paper.'* Deuterium oxide, D20, was obtained from Merck. Preparation of Sample Solutions. The anhydrous AOT solutions were obtained by dissolving AOT in isooctane. The concentration of AOT was always 0.3 mol k g l . Minute contents of water in AOT solutions were determined by using a moisturemeter, CAOC, Mitsubishi Chemical Industry. Calorimetric Measurements. The twin microcalorimeter (model MPL-111, Tokyo Riko) equipped with an ampule system was used. A 15-mL (about 10 g) anhydrous AOT solution (R < 0.1) was put in a sample vessel with a glass ampule in which an appropriate amount of water was sealed. The 15-mL anhydrous AOT solutionwas weighed each time beforemeasurement. Both of the AOT solutions and the water stood for nearly 18 h at 20 "C, and after temperature equilibrium the heat measurement was started by crushing the ampule. The solutions were mixed by a stirrer magnetically connectedto a rotary device. The direct mixing of water with AOT solutions required more time than in our previous study:36 h was needed for complete equilibration when a large amount of water was dissolved. The heats were mol. The solution enthalpies of measured up to n, = 7.8 X le2 DzO were also examined accordingto the procedures mentioned above.
16
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-
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Figure 2. Observed enthalpy change, A H o u , of the reversed micelle solution against the amount of water (H2O) added, h, at 20 "C. The volume and compositionof the solution are 15mL and 0.31 mol kgl AOT/isooctane, respectively.
n, = nD+ no
(3)
n$,* = n a D i+ n,,Hoi
(4) (5)
Results and Discussion In the initial state designated by a superscript i or asterisk before mixing, nD mol of surfactant dissolved in nomol of solventwas put into the vessel of the calorimeter, together with an ampule containing n, mol of water. In the final state designated by a superscript f after mixing, the water was dissolved in the anhydrous surfactant solutions. The enthalpies in the initial and fiial states, H1and IF,are represented as follows
where Hw* and Hwfare the partial molar enthalpies of water before and after mixing,respectively. Hi,He*, and n, are defined as follows: (4) Goto, A.; Kishimoto, H.Bull. Chem. Soc.Jpn. 1989,62,2854-2861. (5)Goto, A,; Kiehimoto, H.J . Chem. SOC.,Perkin Trans. 2 1990,73-
78. (6) Goto, A.; Kiehmoto, H.J . Chem. SOC.,Perkin Trans. 2 1990,891896.
where the subscripts D and o refer to the partial molar enthalpiesof surfactant and solvent, respectively. In the present experimental condition, nD and noare fixed,and therefore n, is also fixed. The observed heat, M o w ,is represented as follows:
Mow=H'-H' Figure 2 shows the plots of the observed heats, mow, vs molar quantities of water, h.The Mowwere slightly exothermic below n, = 2 X 10-3 mol, and then turned endothermic and continued to increase nonlinearly with n,until a nearly constant value was reached. From this value, the enthalpy of HzO per mole of AOT, limk--AHobsd/(molof AOT), was calculated to be 6.3 kJ/mol, which was close to the values evaluated for AOTIheptane solutionsby Aprano et al.2 This value, accordingly, seems to be the solution enthalpy of water characteristicof AOT reversed micelles which exist in the interface between oil and water phases. Similarly to the usual definition of the apparent molar quantity for solute,the apparent molar enthalpy of water,
88 Langmuir, VoZ. 9, No.1, 1993
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Goto et al.
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R 11, resulting in two states of water in the reversed micelle. The above viewpoints for water in the reversed micelle were supported by the partial molar enthalpy of D20.
~~
(13) Zulauf, M.; Eicke, H-F. J. Phys. Chem. 1979,83,480-486. (14) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989,93, 7404.7416.
(15)Marcus, Y. Zon Soluation; Jhon-Wiley t Sone: Chicheater, 1985,
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