Unusual Structure and Dynamics at Silica/Methanol and Silica/Ethanol Interfaces--A Molecular Dynamics and Nonlinear Optical Study.

Vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulations are used to investigate ethanol-silica and methanol-silica interfaces. We describe the subtle differences in molecular organization that result in the observed differences in the VSF spectra for methanol and ethanol at the alcohol-silica interface. Alcohol molecules hydrogen-bonded to the silica surface induce orientational opposition in an adjacent low-population region, which implies VSF signal reduction. This low population region is essentially of zero density in the ethanol system, implying less signal cancelation. Simulated silica defect sites increase the population of this region in both systems. Interestingly, the induced orientation in this region influences subsequent molecular orientation only in the ethanol-silica system, preserving the interfacial anisotropy. These effects suggest a stronger VSF response from the ethanol-silica system versus the methanol-silica system, where more methanol molecules reside in the low-population region, and this region does not induce order in subsequent solvent layers.