Ion-Specific Nanoconfinement Effect in Multilayered Graphene Membranes: A Combined Nuclear Magnetic Resonance and Computational Study.

Ion adsorption within nanopores is involved in numerous applications. However, a comprehensive understanding of the fundamental relationship between in-pore ion concentration and pore size, particularly in the sub-2 nm range, is scarce. This study investigates the ion-species-dependent concentration in multilayered graphene membranes (MGMs) with tunable nanoslit sizes (0.5-1.6 nm) using nuclear magnetic resonance and computational simulations. For Na-based electrolytes in MGMs, the concentration of anions in graphene nanoslits increases in correlation with their chaotropic properties. As the nanoslit size decreases, the concentration of chaotropic ion (BF) increases, whereas the concentration of kosmotropic ions (Cit, PO) and other ions (Ac, F) decreases or changes slightly. Notably, anions remain more concentrated than counter Na ions, leading to electroneutrality breakdown and unipolar anion packing in MGMs. A continuum modeling approach, integrating molecular dynamic simulation with the Poisson-Boltzmann model, elucidates these observations by considering water-mediated ion-graphene non-electrostatic interactions and charge screening from graphene walls.