Structures of water molecules at solvent/silica interfaces.

The adsorption of water at solvent/silica interfaces was studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. We discovered a water layer between toluene and silica with no detectable free OHs. The water layer without free OHs showed resistance against further adsorption of water molecules. This "water-resistant" water surface was very stable at room temperature and did not become a regular water layer with free OH over a period of 12 h in water-saturated toluene. However, this special structure of water was not observed at heptane/silica interfaces, at which free OHs were observed. The study indicates that interactions between solvents and water molecules can significantly change the interfacial water properties.