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An important issue for developing a molecular-level mechanism of heterogeneous interactions at the aqueous interface is determining changes in the interface with changes in the bulk composition. Development of the nonlinear spectroscopy, sum frequency generation (SFG) provides a technique to probe these changes. Several molecular and ionic solutes have been used to investigate changes in the structure of the aqueous interface. Molecular solutes include glycerol and ammonia. Ionic and associated ion complexes include sulfuric acid as well as alkali sulfate and bisulfate salts. Molecular solutes and associated ion complexes penetrate to the top monolayer of the aqueous-air interface displacing water from the interface. Specifically, the conjectured ammonia−water complex is observed with ammonia tilted, on average, 25−38° from the normal. Ionic solutes generate a double layer in the interfacial region due to the differential distribution of anions and cations near the interface. The strength of the double layer is dependent on ion size and charge. Due to the extreme size of the proton, the strongest field is generated by acidic solutes. As the ionic solute concentration increases, associated ion pairs form and these penetrate to the top monolayer. These results have wide implications because the aqueous interface is ubiquitous in atmospheric and biological systems.

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Articles you may be interested in "Up" versus "down" alignment and hydration structures of solutes at the air/water interface revealed by heterodyne-detected electronic sum frequency generation with classical molecular dynamics simulation Sum frequency generation surface spectra of ice, water, and acid solution investigated by an exciton model Sum and difference frequency generation as diagnostics for leaky eigenmodes in two-dimensional photonic crystal waveguides J. Appl. Phys. 99, 023105 (2006); 10.1063/1.2161415Time-resolved vibrational optical activity measurement by the infrared-visible sum-frequency-generation with circularly polarized infrared light Ammonia-water complexes have been detected with sum frequency generation ͑SFG͒ at the liquid/ vapor interface of concentrated ammonia solutions ͑0.3x NH 3 , xϭbulk mole fraction͒. SFG spectra taken with the ssp polarization combination ͑s-polarized sum frequency signal, s-polarized visible light, p-polarized infrared beam͒ are dominated by the N-H symmetric stretch ( 1 ) at 3312 cm Ϫ1 and a weaker deformation mode (2 4 ) at 3200 cm Ϫ1 . The dangling ͑free͒ OH peak due to water at 3700 cm Ϫ1 is suppressed at this concentration, indicating that water molecules are complexed through hydrogen bonds to ammonia at the interface. The polarization dependence of 1 indicates that the NH 3 C 3 molecular axis is tilted with respect to the surface normal by р38°. Variation of the antisymmetric stretch, 3 , sum frequency signal intensity with polarization restricts the lower limit for surface ammonia tilt angles such that 25°рр38°. Based on this orientation, the structure of the ammonia-water complex is discussed.