Average molecular orientations in the adsorbed water layers on silicon oxide in ambient conditions

The average molecular orientation in the adsorbed water layers formed on amorphous SiO2 in ambient conditions was determined as a function of relative humidity using polarization attenuated total reflectance infrared spectroscopy (ATR-IR). The silicon oxide surface was prepared by chemically cleaning in aqueous solution, washing with water, and drying with argon. After drying, this produced a SiO2 surface with hydroxyl groups, giving rise to a water contact angle <5 degrees. Primarily two types of vibrational peaks that correspond to liquid water and solid-like water were observed in the adsorbed water layers formed on this surface at room temperature. The average orientation of the water molecules was determined from the dichroic ratio of s- to p-polarization absorbances. At low relative humidities, the highly hydrogen bonded solid-like structure exhibits a dichroic ratio as low as similar to 0.4, while the liquid water structure exhibits a dichroic ratio close to similar to 1.0. As the relative humidity increases, the dichroic ratio of both water structures approaches a dichroic ratio of 0.7 similar to 0.8, which is consistent with the random orientation of molecules of bulk water and ice.