Interplay of ion availability and mobility in the loss of cation selectivity for CaCl\textsubscript{2} in negatively charged nanopores: molecular dynamics using scaled-charge models

TL;DR

This study uses molecular dynamics simulations to explore how ion interactions and charge inversion in negatively charged nanopores affect cation selectivity, revealing complex behaviors for multivalent electrolytes like CaCl₂.

Contribution

It demonstrates how ion-surface correlations and charge inversion alter ion transport, providing detailed insights into the mechanisms affecting selectivity in nanopores with multivalent ions.

Findings

NaCl shows conventional cation selectivity in nanopores.

CaCl₂ exhibits near bulk-like or anion-favored transport due to Ca²⁺ immobilization.

The detailed transport behavior depends on ion-surface and ion-water interactions.

Abstract

Ion transport through charged nanopores is commonly interpreted in terms of 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-urface correlations and charge inversion modify transport behavior. Here, we study NaCl and CaCl$_2$ conduction through negatively charged silica nanopores using atomistic molecular dynamics simulations with scaled-charge ion models. By separating concentration and velocity contributions to the radial particle current density, we connect static adsorption to dynamic perm-selectivity. While NaCl exhibits conventional cation selectivity, CaCl$_2$ shows nearly bulk-like or even anion-favored transport due to Ca$^{2+}$ immobilization near the surface and dominant Cl$^-$ conduction in the pore interior following charge inversion. 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.