Water growth on metals and oxides: binding, dissociation and role of hydroxyl groups

We discuss the role of the presence of dangling H-bonds from water or form surface hydroxyl species on the wetting behaviour of surfaces. Using scanning tunneling and atomic force microscopes and photoelectron spectroscopy, we have examined a variety of surfaces, including mica, oxides and pure metals. We find that in all cases, the availability of free, dangling H-bonds at the surface is crucial for the subsequent growth of wetting water films. In the case of mica, electrostatic forces and H-bonding to surface O atoms determine the water orientation in the first layer and also in subsequent layers with a strong influence ill its wetting characteristics. In the case or Oxides like TiO2, Cu2O, SiO2 and Al2O3. Surface hydroxyls form readily on defects upon exposure to water vapour and help nucleate the subsequent growth of molecular water films. On pure metals, such as Pt, Pd and Ru, the structure of the first water layer and whether or not it exhibits dangling H-bonds is again crucial. Dangling H-bonds are provided by molecules with their plane oriented vertically. or by OH groups formed by the partial dissociation of water. By tying the two H atoms of the water molecules into strong H-bonds with pre-adsorbed O on Ru can also quench the wettability of the surface.