Abstract
The air−liquid interface of aqueous hydrogen-halide solutions is examined using vibrational sum frequency generation spectroscopy. Infrared and Raman spectroscopies are used to compare the effects of the ions on the water structure in the bulk solutions to that of the interface. The addition of HCl, HBr, and HI to water is found to cause a significant disruption in the hydrogen-bonding network at the air−liquid interface, similar to that which is observed for sodium halides. However, a convolution of additional effects are observed for acids at the air−aqueous interface: interfacial H3O+ (and H5O2+) which gives rise to surface potential changes, an increase in interfacial depth from interfacial concentration gradients, and a decrease in the number of dangling OH bonds relative to the neat water surface. Additionally, and moreover, sum frequency spectra reveal a surface proton continuum at frequencies below 3000 cm-1, indicating that hydronium ions (and Zundel ions) exist at the air−aqueous interface.
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