Investigations of the air-liquid interface of aqueous salt solutions containing ammonium 
(NH4^+) and sulfate (SO4^2-)
ions were carried out using molecular dynamics simulations and vibrational sum
frequency generation spectroscopy. The molecular dynamics simulations show that the predominant
effect of SO4^2- ions,
which are strongly repelled from the surface, is to increase the thickness of the
interfacial region. The vibrational spectra reported are in the O-H stretching region of liquid water.
Isotropic Raman and ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopies
were used to study the effect of ammonium and sulfate ions on the bulk structure of water; whereas
surface sum frequency generation (SFG) spectroscopy was used to study the effect of these ions on the
interfacial structure of water. Analysis of the interfacial and bulk vibrational spectra reveal that aqueous
solutions containing SO4^2-
perturb the interfacial water structure differently than the bulk, and consistent
with the molecular dynamics simulations, reveal an increase in the thickness of the interfacial region.