Investigations of the Structure and Hydrogen Bonding of Water Molecules at Liquid Surfaces by Vibrational Sum Frequency Spectroscopy

The structure and hydrogen bonding of water molecules provides this unique solvent with properties essential to many physical, chemical, and biological processes. The intermolecular hydrogen bonding between water molecules in the bulk medium is disrupted at the surface, imparting the surface with unique structural and thermodynamic properties. We provide an overview of a range of experimental studies from this laboratory in which the structure, orientation, and hydrogen bonding of interfacial water molecules at liquid interfaces are directly probed by resonant vibrational sum frequency spectroscopy. The studies provide insight into the difference in water structure and hydrogen bonding at an air/water interface relative to the interface between two bulk immiscible liquids, namely the CCl4/H2O interface. Also described are studies aimed at understanding how the presence of a charged alkyl surfactant alters the structure of water at these two interfaces. In both cases field-induced alignment of water molecules in the double layer region is prevalent. This induced alignment has been examined under a variety of experimental conditions. A series of isotopic dilution studies conducted for the first time at liquid surfaces are also described. In these studies the intermolecular and intramolecular coupling of vibrational modes in the water molecules are diminished. The results of these and above-mentioned studies provide valuable information for those interested in developing theoretical descriptions of water at surfaces and interfaces.