Ion-specific effects under confinement: the role of interfacial water.

Abstract

All-atom molecular dynamics simulations were employed for the study of the structure and dynamics of aqueous electrolyte solutions within slit-shaped silica nanopores with a width of 10.67 A at ambient temperature. All simulations were conducted for 250 ns to capture the dynamics of ion adsorption and to obtain the equilibrium distribution of multiple ionic species (Na+, Cs+, and Cl(-)) within the pores. The results clearly support the existence of ion-specific effects under confinement, which can be explained by the properties of interfacial water. Cl(-) strongly adsorbs onto the silica surface. Although neither Na+ nor Cs+ is in contact with the solid surface, they show ion-specific behavior. The differences between the density distributions of cations within the pore are primarily due to size effects through their interaction with confined water molecules. The majority of Na+ ions appear within one water layer in close proximity to the silica surface, whereas Cs+ is excluded from well-defined water layers. As a consequence of this preferential distribution, we observe enhanced in-plane mobility for Cs+ ions, found near the center of the pore, compared to that for Na+ ions, closer to the solid substrate. These observations illustrate the key role of interfacial water in determining ion-specific effects under confinement and have practical importance in several fields, from geology to biology.