Water's unexpected acidic surface

If there's one thing everyone remembers from their introductory chemistry course, it's that the pH of a glass of pure water is 7. But a new study led by Pavel Jungwirth at the Academy of Sciences of the Czech Republic, in Prague, suggests that the surface layer of water molecules in that glass is considerably more acidic (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0611285104). The findings could have "a significant impact" on understanding aqueous surface chemistry, particularly in the atmosphere, Jungwirth and colleagues say. On the basis of both computational and experimental studies, they conclude that the pH of such surface layers of pure water is less than 4.8. Ab initio and classical molecular dynamics calculations indicate that the lower pH arises from proton stabilization stemming from hydronium ions at the surface layer. To observe the phenomenon experimentally, the researchers followed proton exchange with D₂O in nanocrystalline ice, a material with a large surface area.