Theoretical study for water structure at highly ordered surface: Effect of surface structure

The structure of water at an ordered solid surface is investigated by the reference interaction site method (RISM). A RISM equation devised especially for a solute-solvent system in which the solute is a two-dimensional periodic array is employed to formulate the electrode-solution interface. Calculations are carried out for two types of surfaces: flatlike and Au(111)-like structures. The orientation of water molecules at the Au(111)-like surface with various surface charge densities, which are deduced from the correlation functions, is in good agreement with the results of surface-enhanced infrared absorption spectroscopy. On the other hand, the model of the flatlike surface does not give a consistent picture with the experiments. The difference is significant when the walls are negatively charged. In the case of a negatively charged wall, the Au(111)-like model gives about 108° for the angle between the surface normal vector and the two O-H vectors of the water molecules in the first layer, whereas the angles are about 72° and 180° for the flatlike surface. The results demonstrate that the solid surface structure is very important in discussing the structure and orientation of water molecules at the interface.