ARTICLE pubs.acs.org/JPCC
Structure of Hydrated Sodium Ions and Water Molecules Adsorbed on the Mica/Water Interface Hiroshi Sakuma,*,† Toshihiro Kondo,‡ Hironori Nakao,§ Koichi Shiraki,|| and Katsuyuki Kawamura† †
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Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan ‡ Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Ohtsuka, Bunkyo-ku, Tokyo 112-8610, Japan § Condensed Matter Research Center and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan Nippon Sheet Glass Co., Ltd., 2-13-12 Konoike, Itami, Hyogo 664-8520, Japan ABSTRACT: 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-Åscale 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.
’ INTRODUCTION Solid/liquid interfaces are ubiquitous on the earth’s surface. They control many important physical and chemical properties such as the dispersion of colloid particles, catalytic reaction, crystal growth, dissolution, precipitation, ion exchange, electrophoresis, friction, and lubrication between solid surfaces. 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. On a nanometric scale, physical properties of liquids could differ from those in bulk due to the liquids’ interaction with solid surfaces and the effect of confinement. Recent development of a surface forces apparatus (SFA) has revealed the behavior of aqueous solutions confined between mica surfaces such as the repulsive hydration forces at a surface separation
