Thermodynamics of solutions of polyelectrolytes, ionic surfactants and other charged colloidal systems

Abstract

Thermodynamic arguments are used to derive a simple expression for the Donnan equilibrium between a solution of charged colloid plus electrolyte and a solution of electrolyte alone. From it, expressions for the solute chemical potentials and the osmotic pressure are obtained. Polydispersity, excluded volume effects and interactions between the various solute species are allowed for. The treatment has features in common with approaches based on the Poisson–Boltzmann equation but is simpler and is not restricted by considerations of particle geometry. It predicts successfully the characteristic behaviour of dilute solutions of polyelectrolyte plus salt and agrees with experimentally obtained activity coefficients of supporting electrolyte better than a prominent existing model. When the theory is applied to solutions of ionic surfactants the dubious assumptions inherent in some previous approaches are avoided, yet similar equations emerge which allow better for solution non-ideality and give good agreement with experiment. The treatment also permits the application to charged micelles of the methods successfully applied previously to non-ionic micelles. The application to colloid stability is discussed. Finally, the application to polyacid titration behaviour is outlined. This exposes some limitations of the theory and suggests appropriate modifications.