Probing the Structure of Water Molecules at an Oil/Water Interface in the Presence of a Charged Soluble Surfactant through Isotopic Dilution Studies

We have employed vibrational sum frequency generation (VSFG) to obtain the first vibrational spectra of water molecules at an oil/water interface in the presence of a charged, soluble surfactant. By examining OH stretching modes that are highly sensitive to the local hydrogen-bonding environment, we have been able to compare the structure of interfacial water molecules with the structure of bulk water molecules determined from previous studies. From the VSFG spectra we infer that there is more extensive hydrogen bonding between neighboring water molecules at the CCl 4 /water interface as compared to water molecules in the bulk aqueous phase. The presence of a charged surfactant enhances the SF response in the OH stretching spectral region in a manner similar to what we have previously observed at the air/water interface. To further probe the hydrogen bonding of water molecules at the oil/water interface, we have employed VSFG to study mixed samples of H 2 O and D 2 O. As the mole fraction of H 2 O is decreased, the peak position of the ice-like OH stretching mode is blue-shifted by approximately 120 cm -1 and converges on the peak position of the uncoupled OH symmetric stretch observed in bulk ice studies. This shift in energy is discussed within the context of hydrogen bonding of the interfacial H 2 O and D 2 O molecules and the intermolecular uncoupling of the OH symmetric stretching vibration. Finally, we have obtained the first vibrational spectra of the OH stretching mode from uncoupled HOD molecules at the oil/water interface located at approximately 3460 cm -1 .