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

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.