Variation of Ionic Strength Reveals the Interfacial Water Structure at a Charged Mineral Surface

Abstract

We have used visible-infrared sum-frequency generation spectroscopy to study the structure of water molecules that form the interface with fused silica. Spectra throughout the OH-stretching region were collected in two different polarization schemes so that the orientation of interfacial water molecules could be studied. A depth profiling experiment was performed by varying the ionic strength, thereby peeling away the outskirts of this interfacial region, leaving water molecules progressively closer to the charged interface. As the ionic strength increases, the spectral intensity drops since the interfacial region is becoming thinner. However, we notice a change in the relative contribution of tetrahedrally coordinated molecules compared to those that make less hydrogen bonds. We have the opportunity to study these two water species individually; we conclude that the higher coordination species exists some distance away from the interface and is oriented close to the plane of the surface with a mean tilt ...