SUM-FREQUENCY GENERATION INVESTIGATION OF WATER AT THE SURFACE OF H2OH2SO4 BINARY-SYSTEMS/

Structural data about water at the air/water interface of sulfuric aci d solutions have been obtained with sum frequency generation spectrosc opy. Sulfuric acid significantly affects the orientation of water at t he interface. With as little as 0.01x (mole fraction) sulfuric acid, w ater is more highly oriented compared with the pure water surface. Sur face water with one hydrogen free of hydrogen bonding, a ''free OH'', decreases in concentration as the sulfuric acid concentration is incre ased. Finally, there are no sulfuric acid free OH groups projecting ou t of the surface for solutions from 0.01x to 0.9x sulfuric acid. Obser vations of the structure of water are consistent with the fact that su lfuric acid/water solutions change from ionic in nature at low concent rations (<0.01x) to ion pair complexes/hydrates at high concentrations (>0.4x).