Effect of Nonelectrostatic Ion Interactions on Surface Forces Involving Ion Adsorption Equilibria

The chemical, or chemisorption, part of colloidal interaction free energy is revisited. Consistent incorporation of nonelectrostatic interactions in the chemical potential for the constant potential and charge regulation boundary conditions is developed. This gives rise to shifted adsorption equilibria, and thereby a shift in the predicted surface electrostatic potential. It also results in an additional component previously unaccounted for in the total double layer interaction force. The altered force leads to the need of recalibrating electrostatic surface potentials and equilibrium constants when fitting to experimental force data. A numerical illustration is presented using ionic dispersion potentials for mica surfaces interacting across NaCl at various concentrations. The new force component due to ionic dispersion is typically repulsive and exceeds entropic repulsion in magnitude. These results suggest that the effect of ionic dispersion is more profound than previously believed, even at low electrolyte concentrations.