Water Structure at Solid Surfaces of Varying Hydrophobicity

The structure of liquid water at solid surfaces with tunable hydrophobicity has been examined by molecular dynamics simulation. Methods of analysis include water density profiles, angular distributions, tilt and twist order parameters, and hydrogen-bonding coordination. It was found that interfacial water structures could be classified according to two hydrophobic regimes, a nonwetting structure and a semi-wetting structure. A smooth transition between the two occurs at surfaces with a contact angle around 130 degrees. The nonwetting regime is characterized by water immediately adjacent to the interface oriented such that hydrogens are directed toward the surface. The semiwetting regime has water oriented in the plane of the interface. We propose that the emergence of the wetting-type order is strongly dependent on the density profile across the interfacial region. Regions of low density, flanked by high-density areas, present fewer hydrogen bonding opportunities than are found in more dense regions. Our findings are able to provide ail explanation for experimental observations that, in surface-sensitive nonlinear vibrational spectroscopy, solid surfaces must be extremely hydrophobic to display spectroscopic signatures of uncoupled OH stretching modes,