Sum-frequency vibrational spectroscopy of a water+ethanol binary solution

Sum-frequency vibrational spectroscopy (SFVS) was used to investigate the molecular structure of ethanol at the air/liquid (binary mixture of water and ethanol) interface. The sum-frequency signal from the terminal methyl group (CH3) of the ethanol molecule rapidly increased with increasing bulk ethanol concentration, reached a maximum at ∼0.2 mole fraction of ethanol, and gradually decreased to one half of the maximum value as a pure alcohol surface was reached. For a quantitative understanding of the surface conformation, the contribution to the sum-frequency signal was separated into the orientation factor and the surface density. Polarization-dependent SFVS measurements showed that the orientation of the terminal methyl group of the ethanol did not vary appreciably with changing bulk-ethanol concentration, suggesting that the origin of the signal enhancement is mainly due to the increase in the surface concentration of ethanol in the ethanol/water mixture as compared to the bulk ethanol.