Isolated Molecular Ion Solvation at an Oil/Water Interface Investigated by Vibrational Sum-Frequency Spectroscopy

The structure, orientation, and intermolecular hydrogen bonding environment of H 2 O molecules solvating molecular ions of surfactants adsorbed at the organic/water interface has been examined by vibrational sum-frequency spectroscopy. Vibrational spectra in the 3000−3800 cm -1 region are shown to be highly sensitive to the OH stretching modes of interfacial solvating H 2 O molecules. Results demonstrate that the hydrogen bonding of water molecules at the CCl 4 /H 2 O interface in the presence of an isolated surfactant is significantly weaker than that observed near a uniform distribution of interfacial charge (saturated monolayer). Near an isolated surfactant, solvating water molecules that reside in the aqueous and organic-rich phases produce spectral features between 3500 and 3620 cm -1, an indication that these solvating water molecules have weaker H 2 O−H 2 O interactions than those observed in bulk water. These water molecules are preferentially oriented in response to strong charge−dipole interactions. As the surface density of charged surfactants is systematically augmented the stretching modes of solvating water molecules shift to longer wavelengths, an indication that the accumulation of surface charge induces structural changes to the solvating water.