Water surface is acidic

Water autoionization reaction 2H 2 O → H 3 O − + 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 caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H 3 O + and OH − ions, density functional studies of (H 2 O) 48 H + clusters, and spectroscopic isotopic-exchange data for D 2 O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H 3 O + does, but OH − does not, display preference for surface sites, the H 2 O surface is predicted to be acidic with pH < 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.