Polarization induced ion adsorption on aqueous interfaces: Solvent effect on image charge interactions

Understanding electrostatic interactions near interfaces is of critical importance in broad fields of science. Continuum electrostatics expects ions to be attracted to metal electrodes but repelled from low dielectric surfaces, while recent studies found certain 'chaotropic' ions are adsorbed on both air/water and graphene/water interfaces. Here we systematically study the effect of polarization of electrode, solvent and solute molecules on the adsorption of ions near interfaces with molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two surfaces, exploiting the mirror-expanded symmetry of the exact image charge solution. With neutral surfaces, the image interaction induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the interface. Under external electric field, the adsorption of ions are enhanced by the polarizing electrode. Explicit polarization of the electrolyte is necessary to capture the increased propensity of big halide ions near the electrode seen in experiments. Our analysis shows inclusion of both electrode and electrolyte polarization is of critical importance to model the electrolyte behavior with interfaces.