Sum Frequency Vibrational Spectroscopy of the Liquid-Air Interface of Aqueous Solutions of Ethanol in the OH Region

The surface vibrational spectroscopy of ethanol (C2H5OH, ethyl alcohol): water binary mixture, or the aqueous solution of ethanol, is investigated via sum frequency generation (SFG) in the OH region (3000 to 3900 cm−1). The bulk molar ratio of the ethanol is varied from x = 0.0024 to 0.20. Our concentration dependence results support the statement that the surface density of water molecules with a free dangling OH bond is about 25% in the air/water interface. It also indicates that the ethyl group of the surface ethanol molecules sticks out above the nominal surface, while the hydroxyl group alone is immersed in the mixture solution. And the surface layer is a mixed monolayer. We have demonstrated here that a LiNbO3 crystal containing -OH can be sufficient for the SFG spectroscopy measurement in the -OH region regardless of its absorption dip.