Vibrational IR-Vis sum-frequency generation (vSFG) spectroscopy of silica surfaces in contact with air of varying relative humidity reveals the orientation of various hydroxyls (SiOH, HOH) with respect to the surface via the interference of the resonant and nonresonant contributions of the interfacial second-order susceptibility ?(2).   vSFG spectra indicate at least two types of adsorbed water molecules: 1) weakly bonded hydroxyls (?~OH~3600 cm-1) with hydrogen atoms oriented toward the substrate; 2) stronger bonded hydroxyls (�bulk-like� water molecules, ?~OH~3400 cm-1) preferentially oriented with hydrogen away from silica.  At low humidity, water preferentially adsorbs into weakly hydrogen-bonded states; at high humidity, water molecules adsorb in multilayers with stronger hydrogen bonding.  Simultaneously, surface-bound silanol hydroxyls (~3750 cm-1) stay practically intact over a wide humidity range (~5-95%) suggesting weak interaction between SiOH and adsorbing water molecules.

Sum-frequency vibrational spectroscopy of amorphous silica surfaces in presence of water molecules adsorbing from the vapor phase

244th National Meeting (2012)

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