Sum frequency generation investigation of water at the surface of H2O/H2SO4 binary systems

Structural data about water at the air/water interface of sulfuric acid solutions have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a ''free OH'', decreases in concentration as the sulfuric acid concentration is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solutions from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solutions change from ionic in nature at low concentrations (<0.01x) to ion pair complexes/hydrates at high concentrations (>0.4x).