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Sum frequency generation spectra indicate that the surface water of liquid sulfuric acid solutions varies as a function of concentration, but not with temperature. At 0.1 x H 2 SO 4 (where x =mole fraction, 38 wt%), subsurface ionic species orient surface water molecules. The surface of 0.2 x H 2 SO 4 (58 wt%) solutions, however, features H 2 SO 4 /H 2 O complexes both at 273 K and supercooled at 216 K. The results support a picture of stratospheric chemistry in which sulfuric acid aerosols are coated with hydrogen-bonded water/sulfuric acid complexes.

The influence of dissolved HCl on the surface of water has been investigated with sum frequency generation (SFG) spectroscopy. Ions in solution cause water on the surface to be reoriented relative to pure water, with hydrogen atoms directed toward the bulk solution. There is no signal due to molecular HCl suggesting that oriented water molecules, not molecular HCl, dominate the surface. A model is proposed to account for the reported SFG results as well as surface tension and surface potential measurements. The result suggests that application of the Gibbs equation to determine surface excess may need to be reevaluated.

Sum frequency generation (SFG) is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region (3000–3450 cm−1) has interference in the presence of ammonia molecules at the interface. In dilute solution, ν1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquid/vapor interface.

The vibrational structure of water at the air/solution interface of an ionic solution has been obtained for the first time. Using vibrational sum frequency generation it is determined that ions in solution have a large orientational effect on the structure of the surface water. Electrolytic solutions, ionic in nature, cause water to be oriented into a more regular hydrogen-bonded network through an electric double layer at the interface. In electrolytic solutions where molecular or associated H 2 SO 4 or Cs 2 SO 4 species dominate, the surface water molecules are bound into hydrate complexes. These effects are explained using hard soft acid base (HSAB) theory.