Interfacial water in the vicinity of a positively charged interface studied by steady-state and time-resolved heterodyne-detected vibrational sum frequency generation

To investigate the properties of water in the close vicinity of a positively charged surfactant/water interface, steady-state and femtosecond time-resolved interfacial vibrational spectra were measured in the presence of excess alkali halide salts. The steady-state Imχ((2)) spectra show a drastic intensity decrease with excess salts, indicating that the thickness of the probed water layer is substantially reduced. Fluoride salts do not noticeably affect spectral features in the OH stretch region whereas the chloride and bromide salts induce significant blue shifts of the OH stretch frequency. Femtosecond time-resolved ΔImχ((2)) spectra obtained with fluoride salts exhibit a very broad bleach even at 0 fs as observed without excess salts, while chloride and bromide salts give rise to a narrow spectral hole burning. These results indicate that the excess chloride and bromide ions strongly interact with interfacial water in the vicinity of the charged interface and it suppresses intramolecular coupling (i.e., Fermi resonance) that causes spectral broadening.