Vibrational Spectroscopy of Alcohol/ Silica Interfaces

Sum-frequency vibrational spectroscopy was used to probe alcohol/silica interfaces in order to study how methanol, ethanol, and 1- and 2-propanol molecules orient at the interfaces. It was found that in the CH stretch range, the symmetric CH$_{3}$ peaks of all alcohols decreased comparing with the air/liquid interface, while the spectra of ethanol and propanol exhibited prominent antisymmetric CH$_{3}$ peaks. The results can be understood by knowing that the first interfacial monolayer of alcohol molecules must adsorb with their OH terminal on the hydrophilic silica and the CH$_{3}$ terminal toward liquid, but the adjacent less ordered alcohol monolayer should have the reverse orientation. Because the CH$_{3}$ groups of the two monolayers are oppositely oriented, their symmetric stretch modes tend to cancel each other in SFG, but the antisymmetric stretch modes generally do not. In the case of methanol, the antisymmetric stretch is intrinsically weak in the sum-frequency spectra, making the overall spectra very weak.