ENTHALPIC STUDIES ON THE STATE OF WATER IN SODIUM BIS(2-ETHYLHEXYL)SULFOSUCCINATE REVERSED MICELLES

Thermochemical studies were made on the dissolution state of water in sodium bis(2-ethylhexyl)-sulfosuccinate (AOT)/isooctane solutions. The solution enthalpies of water were determined at 20-degrees-C with a conduction-type microcalorimeter by direct mixing of water with anhydrous AOT reversed micelle solutions. The observed heats, DELTAH(obsd), were slightly exothermic at low values of molar quantities of water, n(w), and then turned endothermic and continued to increase nonlinearly with n(w) until a nearly constant value was 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 vs R showed a maximum at R = ca. 1.1, followed by a sharp 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.