  We show that the formation of the wetting layer and the experimentally
observed continuous shift of the H2O-OH balance towards molecular water at
increasing coverage on a TiO2(110) surface can be rationalized on a molecular
level. The mechanism is based on the initial formation of stable hydroxyl
pairs, a repulsive interaction between these pairs and an attractive
interaction with respect to water molecules. The experimental data are obtained
by synchrotron radiation photoelectron spectroscopy and interpreted with the
aid of density functional theory calculations and Monte Carlo simulations.
