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

Ion adsorption within nanopores is involved in numerousapplications.However, a comprehensive understanding of the fundamental relationshipbetween in-pore ion concentration and pore size, particularly in thesub-2 nm range, is scarce. This study investigates the ion-species-dependentconcentration in multilayered graphene membranes (MGMs) with tunablenanoslit sizes (0.5-1.6 nm) using nuclear magnetic resonanceand computational simulations. For Na+-based electrolytesin MGMs, the concentration of anions in graphene nanoslits increasesin correlation with their chaotropic properties. As the nanoslit sizedecreases, the concentration of chaotropic ion (BF4 (-)) increases, whereas the concentration of kosmotropicions (Cit(3-), PO4 (3-))and other ions (Ac-, F-) decreasesor changes slightly. Notably, anions remain more concentrated thancounter Na+ ions, leading to electroneutrality breakdownand unipolar anion packing in MGMs. A continuum modeling approach,integrating molecular dynamic simulation with the Poisson-Boltzmannmodel, elucidates these observations by considering water-mediatedion-graphene non-electrostatic interactions and charge screeningfrom graphene walls.

Automated summary

This study investigates the relationship between ion concentration and pore size in multilayered graphene membranes (MGMs) using nuclear magnetic resonance and computational simulations. The results show that the concentration of chaotropic ions increases with decreasing nanoslit size, while the concentration of kosmotropic ions and other ions decreases or changes slightly. In addition, anions remain more concentrated than counter ions, leading to electroneutrality breakdown and unipolar anion packing in MGMs.