Orientation, hydrogen bonding, and penetration of water at the organic/water interface

Vibrational sum frequency spectroscopy is used to measure the vibrational spectroscopy of water molecules at the interface between water and several hydrophobic liquids. The studies probe the bonding interactions in the interfacial region between water molecules and molecules in the organic phase. Complementary FTIR and isotopic exchange studies have been used to confirm spectral assignments in the VSF measurements. These studies which largely focus on the CCl4H2O system show that hydrogen bonding interactions weaken and the water coordination number decreases at the interface, relative to the bulk aqueous phase. Although the CCl4/H2O system exhibits weak H2O-H2O interactions, the interaction of water with the CCl4 phase results in a substantial orientational ordering of these weakly interacting interfacial water molecules. The orientation of these water molecules displays a dependence on the aqueous phase pH, which can be attributed to the screening of the neat interfacial potential, and is displayed through a series of VSF spectroscopic pH experiments. These results are in contrast to the strengthening of hydrogen bonding found for smaller nonpolar solutes in water. The molecular picture emerging from these studies of water structure at hydrophobic liquid surfaces has important relevance to many areas of science including protein folding, membrane structure and function, and the penetration of water into macroscopic structures of a hydrophobic nature.