Charge reversal of surfaces in divalent electrolytes: The role of ionic dispersion interactions

Surface potentials of alkali earth nitrates at a mica surface are calculated using a modified Poisson-Boltzmann approach that includes nonelectrostatic ion-surface dispersion interactions. New ab initio dynamic polarizabilities are used to determine dispersion interactions. A hydration model describing the hydration shell of cations is presented. Excellent agreement with experiment is achieved, including charge reversal at high electrolyte concentration without the need for site binding models. This suggests that specific ionic dispersion forces provide the mechanism for ion surface binding. An asymptotic surface potential is found in the limit of very high concentration. A Hofmeister series is predicted according to the strength of charge reversal, with Mg > Ca > Sr > Ba. The ion-surface dispersion adsorption energies of hydrated ions appear to explain the apparent repulsive secondary hydration forces observed experimentally between mica surfaces when taken with a surface hydration layer.