Electrostatic Repulsion in Concentrated Disperse Systems

Electrostatic interactions are considered in the framework of the cell model to predict the osmotic pressure in concentrated disperse systems. A procedure was developed to represent the osmotic pressure as a function of two parameters, namely, the dispersed phase volume fraction and the electric potential attributed to the interface between the continuous and dispersed phases. The procedure is based on a general formula which was derived to express the electrostatic contribution to the osmotic pressure through the electric potential at the cell boundary. The potential of the cell boundary is predicted from the solution of the Poisson-Boltzmann problem which was specified for the cell model approach. The Poisson-Boltzmann problem is solved by a perturbation technique using a normalized interface potential as the perturbation parameter. Three leading terms were obtained in the expansion of the osmotic pressure in terms of the normalized interface potential. Two options for the formation of the interface electric potential are discussed in the analysis of the interface potential dependency on the volume fraction of the dispersed phase. The first one is associated with the difference between the individual ionic distribution coefficients characterizing the equilibrium ratio between the concentrations in the bulk of the constituent phases. The second one deals with preferential adsorption of the carriers having a given electric charge sign. The dependency of the osmotic pressure on the system parameters is discussed and interrelated with other relevant theories. Special discussion is presented concerning the theory's application for the study of hydrocarbon disperse systems, e.g., water-in-oil emulsions. Copyright 2001 Academic Press.