Water autoionization reaction 2H2O . H3O+ +OH- is a textbook process of basic
importance, resulting in pH = 7 for pure water. However, pH of pure water surface is
shown to be significantly lower, the reduction being due to proton stabilization at the
surface. The evidence presented here includes ab initio and classical molecular dynamics
simulations of water slabs with solvated H3O+ and OH- ions, density functional studies of
(H2O)48H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional
impurities at the surface and in the interior of ice nanocrystals. Since H3O+ does but OH- does
not display preference for surface sites, the H2O surface is predicted to be acidic
with pH lower than 4.8. For similar reasons, the strength of some weak acids such as
carbonic acid is expected to increase at the surface. Enhanced surface acidity can have a
significant impact on aqueous surface chemistry, e.g., in the atmosphere.