Département de chimie moléculaire

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 redn. being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical mol. dynamics simulations of water slabs with solvated H3O+ and OH- ions, d. 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. Because H3O+ does, but OH- does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH {\textless} 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 aq. surface chem., e.g., in the atm. [on SciFinder(R)]