Ion-Specific Effects under Confinement: The Role of Interfacial Water

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 angstrom 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.