Molecular insights on Ca²⁺/Na⁺ separation via graphene-based nanopores: The role of electrostatic interactions to ionic dehydration

Ca²⁺/Na⁺ separation is a common problem in industrial applications, biological and medical fields. However, Ca²⁺ and Na⁺ have similar ionic radii and hydration radii, thus Ca²⁺/Na⁺ separation is challenging. Inspired by biological channels, group modification is one of the effective methods to improve the separation performance. In this work, molecular dynamics simulations were performed to investigate the effects of different functional groups (COO⁻, NH₃⁺) on the separation performance of Ca²⁺ and Na⁺ through graphene nanopores under an electric field. The pristine graphene nanopore was used for comparison. Results showed that three types of nanopores preferred Ca²⁺ to Na⁺, and Ca²⁺/Na⁺ selectivity followed the order of GE-COO⁻ (4.06) > GE (1.85) > GE-NH₃⁺ (1.63). Detailed analysis of ionic hydration microstructure shows that different nanopores result in different hydration factors for the second hydration layer of Ca²⁺ and the first layer of Na⁺. Such different hydration factors corresponding to the dehydration ability can effectively evaluate the separation performance. In addition, the breaking of hydrogen bonds between water molecules due to electrostatic effects can directly affect the dehydration ability. Therefore, the electrostatic effect generated by group modification will affect the ionic hydration microstructure, thus reflecting the differences in dehydration ability. This in turn affects the permeable and separation performance of cations. The results of this work provide perceptive guidelines for the application of graphene-based membranes in ion separation.