Structure of hydrated sodium ions and water molecules adsorbed on the mica/water interface

Solid/liquid interfaces control many physical and chemical properties such as electrophoresis, friction, and dispersion of colloid particles. Atomic distribution at solid/liquid interfaces has a strong correlation with these properties, and understanding these structures is necessary to establish the fundamental physics and chemistry of the solid/liquid interfaces. In this study, we have investigated the structure of mica/aqueous NaCl solution interfaces using surface X-ray scattering measurements and molecular dynamics simulations. The sub-Å-scale atomic distribution of the mica/aqueous NaCl solution interface is revealed as a function of the distance normal to the interface. The density of the NaCl solution oscillates to be 12 Å apart from the surface observed by the surface X-ray scattering measurements. Molecular dynamics simulations indicate that the oscillation corresponds to hydrated Na⁺ ions, adsorbed water molecules, and water molecules surrounding the hydrated ions. The structure helps to understand the origin of repulsive force and tribological properties between mica surfaces immersed in the NaCl solution and to develop electric double-layer theory.