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.
journal_name
Proc Natl Acad Sci U S Aauthors
Buch V,Milet A,Vácha R,Jungwirth P,Devlin JPdoi
10.1073/pnas.0611285104subject
Has Abstractpub_date
2007-05-01 00:00:00pages
7342-7issue
18eissn
0027-8424issn
1091-6490pii
0611285104journal_volume
104pub_type
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