The adsorption of water on a fully hydroxylated silica surface is studied by using density-functional total-energy and molecular dynamics calculations. The (100) surface of beta(alpha)-cristobalite covered by geminal hydroxyls has been taken as the substrate. A well-ordered and stable two-dimensional ice with quadrangular and octagonal patterns of hydrogen bond (H-bond) networks-an ice tessellation-is found on the surface for the first time. With the vibrational recognition, the four water molecules in the quadrangle are found to be bonded by strong H bonds while the quadrangles are connected to each other by weak H bonds. This configuration is the most stable, because all the water molecules are fully saturated with H bonds either to each other or to the surface hydroxyl groups.

We present an ab initio investigation of water adsorption on ordered hydroxylated silica surfaces, using the density functional theory within the ultrasoft pseudopotentials and generalized-gradient approximation. The ͑100͒ and ͑111͒ surfaces of the hydroxylated cristobalite are used as substrates to adsorb water clusters and overlayers. Water adsorbs through hydrogen bonds formed between water and surface hydroxyl groups on the ␤͑␣͒-cristobalite ͑100͒ surface. A large enhancement of the hydrogen bonding in the adsorbed water dimer is observed, which can be inferred from the shortened hydrogen-bond ͑H bond͒ length, the vibrational spectra from the molecular dynamics simulation and the redistribution of electron density. At one monolayer ͑ML͒ coverage, a "tessellation ice," with characteristic quadrangular and octagonal hydrogen-bonded water rings, is formed. It has two types of H bonds and can exist on two different adsorption sites with two different OH orderings in a surface supercell. Our study is further extended to the ␤-cristobalite ͑111͒ surface. Based on these studies, we find that the water-silica bond, which comprises several H bonds, is usually stronger than other associative water-surface interactions. The H bonds between water and surface usually differ in strength-and hence, in vibrational spectra-from those between adsorbed water molecules. Because the ͑100͒ and ͑111͒ surfaces sustain different silanol groups ͑geminal and isolated silanols͒, a well-defined twodimensional tessellation ice phase can be observed only on the cristobalite ͑100͒ surface. On ␤-cristobalite ͑111͒ surface, however, isolated water molecules, hydrogen-bonded to the surface hydroxyls, are formed, even at 1 ML coverage.

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