Molecular Dynamics Study of the Electric Double Layer and Nonlinear Spectroscopy at the Amorphous Silica Water Interface

The electrical double layer (EDL) at the amorphous silica-aqueous electrolyte interface is of long-standing scientific interest and current technological relevance. Using extensive molecular dynamics simulations, we have studied this EDL as a function of salt concentration for a silica surface charge density of -0.82e/nm(2) (e = electron charge). The simulation results can be captured with a simple model by breaking the double-layer region into three zones: an inner region in which the Na+ counterion population is independent of [NaCl] and there are no Cl- co-ions, an intermediate region which hosts a population of nonexchangeable Na+ plus another group of Na+ and Cl- ions whose population is described by a Langmuir adsorption model, and an outer region where the ion distribution is well-described using the Poisson-Boltzmann theory. When the asymptotic [NaCl] >0.17 M, the adsorption of Na+ in the intermediate zone leads to an overcompensation of the negatively charged silica surface. Nonlinear spectroscopic experiments on the water-amorphous silica interface have been interpreted by others using the Gouy-Chapman model at low salt concentration and the constant capacitance model at high salt concentration. We discuss the applicability of these and other models and the implications for interpretation of the results of second harmonic and sum frequency generation experiments.