title: Water surface is acidic.
abstract: 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 caused by 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)(48)H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O+ does, but OH- does not, display preference for surface sites, the H2O 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.
authors: Fritz Haber Institute for Molecular Dynamics, Hebrew University, Jerusalem 91904, Israel.
authors: Victoria
authors: V
authors: Buch
authors: Anne
authors: A
authors: Milet
authors: Robert
authors: R
authors: Vácha
authors: Pavel
authors: P
authors: Jungwirth
authors: J Paul
authors: JP
authors: Devlin
mesh_terms: D000143:Acids / Q000737:chemistry*; D003198:Computer Simulation; D041961:Deuterium Exchange Measurement; D006863:Hydrogen-Ion Concentration; D014867:Water / Q000737:chemistry*
chemical_list: D000143:Acids; D014867:Water