Intrinsic Limits of Ion Enrichment and Selectivity in Charged Nanopores: Insights from Bulk-Validated Classical Density Functional Theory with Experimentally Calibrated Ion Diameters

Understanding ion enrichment and selectivity in charged nanopores is essential for nanopore-based separation, desalination, and sensing technologies. Here, we employ classical density functional theory with experimentally calibrated effective ion diameters to investigate equilibrium ion distributions in charged cylindrical nanopores. The cDFT framework is first validated against molecular dynamics simulations for bulk primitive-model electrolytes over different ion-size ratios, concentrations, and temperatures. We then examine LiCl and its mixtures with NaCl and KCl over pore diameters of 0.8–6 nm, surface charge densities of -0.01 to -0.1 C/m 2 , and bulk concentrations of 0.001–1.5 M. The results show that Li + /Cl − equilibrium enrichment selectivity can become extremely large under strong surface charge and low ionic strength because of near-complete coion exclusion. In contrast, Li + /Na + and Li + /K + selectivities remain modest, indicating the intrinsic limit of cation-cation discrimination achievable by electrostatic and steric effects alone. We further show that cation selectivity is governed by a competition between electrostatic localization near the charged wall and entropic accessibility in the pore interior, leading to selectivity reversal under extreme confinement and high concentrations. These findings establish quantitative design principles for charged nanopores and clarify the additional physical or chemical mechanisms required to approach biological-level cation selectivity.