Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO Binary Systems
Steve Baldelli, Cheryl Schnitzer, and Mary Shultz
Department of Chemistry, Tufts University, Medford, Massachusetts 02155
D.J. Campbell
Department of chemistry, College of the Holy Cross, Worcester, Massachusetts 01610
Journal of Physical Chemistry B, 101, 10435, 1997.
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).
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