Application of a chemical equilibrium model to the volumes and heat capacities of transfer of benzene from water to aqueous solutions of surfactants

Abstract

In water, hydrophobic solutes and surfactants form mixed micelles. These systems can be conveniently studied through the measurement of thermodynamic functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These functions often go through extrema in the region of the critical micellization concentration (CMC) of surfactants. In the present study, the volumes and heat capacities of transfer of benzene from water to aqueous solutions of octyldimethylamine oxide, octylammonium hydrobromide, and sodium dodecyl sulfate, at 25°C, were measured and compared to the volumes of transfer of benzene from water to aqueous solutions of water-miscible alcohols which have a similar behavior. Applying a simple chemical equilibrium model ( J. Solution Chem. 13, 1 (1984)) to the volumes of transfer, it is shown that the presence of extrema is related in a large extent to the relative hydrophobicities of the surfactant and benzene monomers. This has permitted us to state a general principle which can predict the presence of extrema in transfer functions for most kinds of micellar systems.