Electrostatic repulsion in concentrated disperse systems - I. Contributionof electrostatic interaction to osmotic pressure

Electrostatic interactions are considered in the framework of the cell mode l to predict the osmotic pressure in concentrated disperse systems, A proce dure was developed to represent the osmotic pressure as a function of two p arameters, namely, the dispersed phase volume fraction and the electric pot ential attributed to the interface between the continuous and dispersed pha ses. The procedure is based on a general formula which was derived to expre ss the electrostatic contribution to the osmotic pressure through the elect ric potential at the cell boundary. The potential of the cell boundary is p redicted from the solution of the Poisson-Boltzmann problem which was speci fied for the cell model approach. The Poisson-Boltzmann problem is solved b y a perturbation technique using a normalized interface potential as the pe rturbation parameter. Three leading terms were obtained in the expansion of the osmotic pressure in terms of the normalized interface potential. Two o ptions for the formation of the interface electric potential are discussed in the analysis of the interface potential dependency on the volume fractio n of the dispersed phase. The first one is associated with the difference b etween the individual ionic distribution coefficients characterizing the eq uilibrium ratio between the concentrations in the bulk of the constituent p hases. The second one deals with preferential adsorption of the carriers ha ving a given electric charge sign. The dependency of the osmotic pressure o n the system parameters is discussed and interrelated with other relevant t heories, Special discussion is presented concerning the theory's applicatio n for the study of hydrocarbon disperse systems, e.g., water-in-oil emulsio ns. (C) 2001 Academic Press.