CALORIMETRIC INVESTIGATION OF THE SOLUBILIZATION OF WATER-IN-SODIUM DODECYLBENZENESULFONATE REVERSED MICELLES AND WATER-IN-OIL MICROEMULSIONS IN MIXED-SOLVENT OF N-HEPTANE AND N-PENTANOL

Thermal behavior of sodium dodecylbenzenesulfonate (DDBS) reversed micelles with n-pentanol and n-heptane as mixed apolar solvents was studied by titrating water into anhydrous reversed micelles with a Tronac model 450 calorimeter. The solubilization of water can be described in terms of deformation of molecular structure of water, adsorption of water, ionization of surfactant, dilution of counterions, and swelling of droplets. A specific radius of anhydrous reversed micelles, R(s) = (1 + square-root 6/2) r0, relating to a transition from exothermic type to endothermic type is proposed, where r0 is the molecular radius of water. The partial molar enthalpy of water against R, the molar ratio of water to surfactant, shows an exothermic type with one peak at low P and two peaks at high P, where P is the mass percentage of DDBS and n-pentanol in the mixture. Structural parameters of the droplet, such as radii of the disperse phase (R(d)) and the water core (R(w)), as well as the average aggregation number (N) and the compact effective head group area (a(s) + I(a)a, where a(s) and a(a) are head group areas of surfactant and alcohol per molecule and I is the molar ratio of alcohol to surfactant in interfacial layer), have been derived without any approximation in the densities of the disperse phase. (C) 1994 Academic Press, Inc.