Interplay of ion availability and mobility in the loss of cation selectivity for CaCl2 in negatively charged nanopores: Molecular dynamics using scaled-charge models

Abstract

Ion transport through charged nanopores is commonly interpreted in terms of the electrical double layer structure, leading to the expectation of cation-selective conduction in negatively charged pores. This picture can break down for multivalent electrolytes, where strong ion–surface correlations modify transport behavior. Here, we study NaCl and CaCl2 conduction through negatively charged silica nanopores using atomistic molecular dynamics simulations with scaled-charge (and also full-charge) ion models. By separating concentration, ci(r), and velocity, vi(r), contributions to the radial particle current density, ji(r) = ci(r)vi(r), we connect static adsorption to dynamic perm-selectivity. We show that strongly adsorbed, but immobilized Ca2+ ions and the low availability of Cl− ions in the surface layer near the charged wall make the contribution of this layer to the total conduction (surface conduction) small. It is the bulk-like electrolyte in the middle of the pore whose contribution (volume conduction) dominates the selectivity behavior of the pore (bulk-like or even slightly anion selective). Although this qualitative mechanism is robust, its detailed manifestation depends sensitively on the balance of ion–surface and ion–water interactions encoded in the force field.