Interfacial structures of methanol:water mixtures at a hydrophobic interface probed by sum-frequency vibrational spectroscopy.

Sum-frequency vibrational spectroscopy was used to study interfacial structure of methanol:water mixtures at an octyltrichlorosilane-covered hydrophobic surface. Methanol was found to adsorb preferentially than water at the interface with its methyl group tilted from the surface normal by approximately 35 degrees for all methanol concentrations. Redshift of the methanol symmetric stretch mode, gradual disappearance of the water dangling-OH mode, and blueshifts of the dangling and liquidlike bonded-OH modes were also observed as the methanol concentration increased. They could be understood from the change of the interfacial hydrogen-bonding network associated with the change of surface composition.