No abstract

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.

The vapor−liquid interface of aqueous inorganic acid and salt solutions are examined using sum frequency generation (SFG). The results show that the SFG intensity of hydrogen-bonded water on 0.01x acid (HCl, HNO3, and H2SO4) solutions, where x = mole fraction, is greatly enhanced compared to the 0.01x solution of the corresponding salts (NaCl, NaNO3, and KHSO4). This suggests that either surface water molecules on the acid solutions orient with the dipole in a more vertically aligned fashion than those on the salt solutions or that more layers of water are ordered. These results are interpreted with an electric double layer model, in which the double layer is composed of subsurface anions and cations. The weak association of protons, as opposed to Na+ or K+, with anions results in a greater electric field at the surface on acid solutions. The perturbation of surface water on ionic solutions is discussed in terms of electrostatic and displacement mechanisms.

The surfaces of aqueous HNO3 solutions and liquid HNO3 are examined using sum frequency generation (SFG). A molecular-level picture of these atmospherically relevant systems is developed. Consistent with previous interpretations, an electric double layer comprised of subsurface anions and cations develops in 0.005x and 0.01x HNO3 solutions, where x = mole fraction. Compared to pure water, these solutions generate more SFG signal in the hydrogen-bonded region as water molecules respond to the subsurface electric field by aligning with the surface normal. At higher concentrations, 0.05x and 0.4x HNO3, ionic complexes or molecules sufficiently approach the surface to disrupt the hydrogen-bonding network and perturb the first water layer. Neither liquid nitric acid nor its solutions show a clear O−H.