Vibrational Spectroscopy of Aqueous Sodium Halide Solutions and Air−Liquid Interfaces:  Observation of Increased Interfacial Depth

Air−aqueous sodium halide solution interfaces are examined using vibrational sum frequency generation spectroscopy. Raman and ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopies are also used to compare the effects of halide anions on the water structure of the bulk solution to that of the interface. The interfacial water structures for the sodium fluoride and chloride aqueous solutions are found to be similar to the air−water interface, whereas sodium bromide and iodide aqueous solutions cause significant distortion of the hydrogen-bonding network. Analysis of the spectra indicates higher concentrations of bromide and iodide anions in the interfacial region with an increase in interfacial depth.