Sum frequency generation of water on NaCl, NaNO3, KHSO4, HCl, HNO3, and H2SO4 aqueous solutions

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.