Water Adsorption on Hydrophilic and Hydrophobic Surfaces of Silicon

The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si-OH surface, the average thickness of the strongly hydrogen-bonded water layer increases up to ∼2 molecular layers as relative humidity (RH) increases, beyond which the weakly hydrogen-bonded structure is dominant. On the hydrophobic Si-H surface, the adsorbed water layer consists predominantly of the weakly hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements provide critical insights needed for better understanding of various physical processes affected by water adsorption in ambient conditions.