Understanding how organic solvent polarity affects water structure and bonding at halocarbon–water interfaces

Vibrational sum-frequency spectroscopy (VSFS) is employed here to study the bonding and structure of water molecules at the mixed halocarbon–water interface. Mixtures of non-polar CCl4 with more polar solvents CHCl3, dichloromethane (DCM), and 1,2-dichloroethane (DCE) are used to vary the polarity of the organic phase. The OH stretching modes of interfacial water molecules are used to characterize and compare the interactions between water and the different halocarbons in the organic phase. The spectroscopic results show differing degrees of interaction between water and the various halocarbon molecules in the interfacial region as manifested in spectral shifts and intensity changes in the OH stretching modes. The spectral observations indicate that as the organic phase is systematically increased in polarity with the addition of polar halocarbons, the interfacial water molecules display increased bonding interactions with the organic phase. This increased interaction between interfacial water and the interfacial halocarbons leads to a reduction in the strong orientation of interfacial water molecules that is so prevalent for water adjacent to non-polar organic liquids. The results provide important insights into water adjacent to a hydrophobic surface as that surface takes on increasingly polar characteristics.