Experimental Determination of Water Molecular Orientation near a Silica Surface Using NMR Spectroscopy

After nearly a century of research on water properties at interfaces it has become clear that water adopts a highly ordered structure near a solid surface. However, to date, the exact orientations and structures of water molecules adjacent to a solid surface remain within the realm of theoretical simulation. In this paper, the average orientation of water molecules near a silica surface is determined experimentally for the first time using a sample where silica surfaces are similarly oriented with respect to an external magnetic field. By allowing only water with increased ordering to remain in the closed system, the water 1 H magnetic resonance corresponds to an orientation-dependent doublet. From its orientation dependence, the average angle between the silica surface and the water 1 H– 1 H vector is assessed to 78° at ambient conditions. This orientation is found to apply to tetrahedrally coordinated water molecules not directly adjacent the surface and suggests that water in subsequent layers adopt a low-energy structure with a significantly increased lifetime compared to bulk water structures. Despite preferred spatial orientations, the water molecules exhibit a relatively high degree of motional freedom.