Vibrational sum frequency spectroscopic investigations of the structure, hydrogen bonding, and orientation of water molecules at the liquid/liquid interface

The hydrogen bonding environment, orientation, and structure of water at the neat CCl₄/H₂O interface have been investigated utilizing vibrational sum frequency spectroscopy. Interfacial water molecules solvating cationic, anionic, and neutral surface-active molecules adsorbed at the liquid/liquid interface have also been investigated. The inherent surface and molecular specific nature of vibrational sum frequency spectroscopy combined with the sensitivity of the OH vibrational modes of water to molecular environment compose a unique view of water behavior at an organic/water interface.; The broad vibrational spectrum attained for water at the neat CCl ₄/H₂O interface has been resolved into spectral peaks assigned to discrete interfacial water environments by employing complimentary FTIR and isotopic exchange experiments. Interfacial water structure is characterized by weak hydrogen bonding interactions and incomplete tetrahedral water coordination, relative to the bulk aqueous phase. pH dependent investigations demonstrate that the preferential orientation of water at the neat interface due to weak organic-water interactions can be altered.; Hydrogen bonding interactions of water at the neat CCl₄/H ₂O interface are compared to that observed at the hexane/H₂O and vapor/H₂O interfaces. Surface water spectra demonstrate that organic/aqueous interfaces exhibit generally weak hydrogen bonding interactions in contrast to water at the vapor/H₂O surface.; Spectroscopic investigations of charged surfactants at the CCl₄ /H₂O interface have allowed characterization of the structure, orientation, and hydrogen bonding environment of water molecules solvating the head group of isolated interfacial surfactant molecules. Near an isolated surfactant, solvating water molecules experience interactions with the charged head group that result in spectral features attributed to water molecules having weaker H₂O-H₂O interactions than those observed in bulk water. Solvating water molecules subject to interactions with the charged surfactants exhibit a preferential orientation as a result of strong charge-dipole interactions. As surfactant surface concentrations approach monolayer coverage, solvating water molecules experience increasing electric fields that results in strengthened H₂O-H₂O interactions. Perturbation of the surface water structure with the adsorption of small polar surfactants at the CCl₄/H₂O interface is significantly different that observed with a charged surfactant.