Spinodal decomposition and the liquid–vapour equilibrium in charged colloidal dispersions

Abstract

The possibility of observing a phase separation between a high-density (‘liquid’) phase and a low-density (‘vapour’) phase in charge-stabilized aqueous dispersions of monodisperse spherical colloidal particles which are assumed to interact via the standard DLVO potential has been examined. The free energy of the system is calculated by thermodynamic perturbation theory. The contribution of the repulsive part of the potential is reduced to that of an effective hard-sphere fluid, while the van der Waals attraction is treated in the ‘high-temperature approximation’. The critical-point coordinates and spinodal and coexistence curves have been determined as functions of the potential parameters and show that a reversible ‘liquid–gas’ transition should be observable for physically reasonable values of these parameters. In a well defined range of salt concentrations it is shown that this reversible transition should be clearly distinct from irreversible coagulation, which will always occur beyond a critical salt concentration.