Hydrophobicity of hydroxylated amorphous fused silica surfaces.

Understanding the mechanism of water adsorption on silica is important in many fields of science and technology, such as geo- and atmospheric chemistry. Vibrational IR-visible sum-frequency generation (SFG) spectroscopy of hydroxyls (3100-3800 cm(-1)) at the amorphous SiO2 surface in contact with air of varying relative humidity provides information about the adsorption sites and orientation of water molecules. The similar magnitudes of the resonant and nonresonant contributions to the interfacial second-order susceptibility, χ((2)), allow the phases of the various hydroxyls (SiOH; HOH), and thus their orientations with respect to the surface, to be determined. The surface silanols (SiOH) appear to interact weakly with adsorbed water as indicated by the persistence of the narrow surface silanol (SiOH) peak at ~3750 cm(-1) as the relative humidity of ambient air increases from <5% to >95%. Adsorbed water molecules are represented by two oppositely oriented hydroxyl modes, at ~3350-3400 and ~3650 cm(-1), respectively. The weakly hydrogen-bonded water hydroxyls (3650 cm(-1)) are oriented toward the silica substrate and are assigned to water molecules that aggregate over the hydrophobic silica areas with exposed siloxane bridges. We believe that this is the first experimental identification of water molecules in contact with siloxane network whose hydrophobic nature has been predicted by molecular dynamics simulations for tetrahedral (SiO4) surface of kaolinite. The SFG data suggest that, at the molecular level, hydroxylated amorphous fused silica has hydrophobic character.