Consequences of shifted ion adsorption equilibria due to nonelectrostatic interaction potentials in electrical double layers

Abstract

Consistent incorporation of nonelectrostatic interactions in the chemical potential for electrical double layers at interfaces interacting across electrolytes under the constant potential or charge regulation boundary conditions has recently been presented for the case of ionic dispersion potentials. 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 new force component due to ionic dispersion can be repulsive or attractive and can in some cases exceed entropic repulsion in magnitude. The altered force leads to the need to recalibrate electrostatic surface potentials and equilibrium constants when fitting to experimental force data. We explore the implications and consequences for model systems of mica surfaces, to illustrate the effect of salt concentration, and cellulose surfaces to illustrate the effect of pH. The final example is the more complex, asymmetric system of crude oil and glass interacting across salt solutions of varying concentration and pH. The DLVO theory augmented by ionic dispersion potentials is used to fit measured data for ζ-potentials of oil and glass, from which the calculated disjoining pressure isotherms are compared to measured macroscopic oil-glass adhesion data.