Influence of ion pairing on ion and counterion retention in neutral nanopores of gibbsite and kaolinite

Ion pairing plays a crucial role in ion partitioning within various nanoconfined systems. However, much of the attention has been focused on charged nanopores, leaving many questions unanswered regarding neutral nanopores. For instance: would salt thermodynamically reside inside neutral nanopores? How does surface chemistry influence ion retention in such pores? And how does ion pairing affect the equilibrium of ions between neutral nanopores and the bulk aqueous solution? In this study, we investigate the behavior of Na⁺ and Cl^- ions in neutral hydrophilic gibbsite and mixed hydrophobic-hydrophilic kaolinite nanopores using molecular dynamics simulations. Our results indicate that Na⁺ ions are thermodynamically unfavorable in the gibbsite nanopore containing two water layers, whereas Cl^- ions are slightly favorable. Ion pairing promotes the partitioning of both ions. A Na-Cl pair behaves like a dipole, and this dipole aligns with the water orientation in the gibbsite pore, leading to a more favorable energy compared to the pair in the bulk solution. In contrast, the kaolinite nanopore, which is more hydrophobic, discourages the partitioning of all species, including Na⁺, Cl^-, and the ion pair. This finding challenges the common practice of placing ions in neutral nanopores and studying ion pairing without considering their equilibrium with the bulk solution. These findings also offer new insights into the role of surface chemistry, nanoconfinement, and hydrogen bonding in modulating ion pairing and its impact on ion partition in neutral nanoporous materials.