Ionic Selectivity of NaCl Solutions in Graphene-Based Single-Digit Nanopores

This study provides insights into molecular mechanisms of capacitive deionization (CDI), which has emerged as a promising technique for efficient and sustainable desalination. Through molecular dynamics simulations, we investigate the structure, bonding, and energetics of NaCl aqueous solution in parallel graphene sheets of separation 0.7 or 0.9 nm and lengths 1, 2, or 6 nm. The accelerated weight histogram method implemented in GROMACS was used to compute axial free energy profiles from the reservoir solution to the pore's center, informing not only about the energetics in the pore's interior, but also about energy barriers in the transition zones at the pore's edges, which affect the flow of ions to/through the pores and the charge balance in the pore. We elucidate the roles of surface charge, pore height, and pore length on the ion adsorption, selectivity, and free energy profiles. These findings provide guidance for designing and optimizing CDI systems, thus contributing to the advancement of water treatment technologies and addressing the global water scarcity challenge.