Nonelectrostatic ionic forces between dissimilar surfaces: a mechanism for colloid separation

The interaction between two dissimilar surfaces across an electrolyte is re-examined. The focus is on effects of ion-specific dispersion forces missing from classical electrostatic double-layer theory. The pressure between two flat surfaces is derived by two alternate methods (midpoint and whole domain approaches). Significant differences emerge from expectations of classical theory. These are illustrated by model interactions across electrolytes of mica and oil-like surfaces. A novel consequence that emerges from inclusion of ionic dispersion forces is the possible separation of mixed colloidal suspensions at moderate (0.1 M) concentrations of divalent salt. Repulsion between the model oil and mica surfaces is found to be due to entropic repulsion driven by high adsorption of both counterions and co-ions at the mica surface. Co-ion adsorption is a consequence of electric field reversal ("charge reversal"), caused by attractive ionic dispersion interactions of the counterion to the mica surface. Charge reversal is also found with monovalent electrolyte but only at impracticably high concentrations.