THERMODYNAMICS OF MICROEMULSIFICATION WITH IONIC SURFACTANTS∗

Abstract

ABSTRACT A general thermodynamic theory of microemulsions containing ionic surfactant, cosurfactant, and salt is developed. On the basis of this theory, an expression for the equilibrium radius of the globules is derived. This equation, contains the interfacial tension at the surface of the globules, the force which acts between then, as well as their entropy of dispersion in the continuous phase. The thermodynamic treatment provides, in addition, a Gibbs adsorption equation for the interfacial tension and an equation for the force which develops between two neighboring globules due to the existence of double layers. When the shortest distance between globules becomes comparable to twice the Debye length, one can no longer define bulk concentrations for the ionic surfactant and salt. In this case, to facilitate the computation of the electrical potential and of the concentrations, each globule is associated with a cell by distributing the volume of the continuous phase equally among the droplets. The densities of the surfactant and cosurfactant on the surface of the globules are expressed in terms of their concentrations at the periphery of the cell. Mass balances relate the latter concentrations to the total amounts of surfactant and cosurfactant. On this basis, a complete Set of equations is developed from which the eqUilibrium radius and the free energy of the system can be computed. The emphasis of the paper is on basic thermodynamics.