Water surface is acidic
- *Fritz Haber Institute for Molecular Dynamics, Hebrew University, Jerusalem 91904, Israel;
- †Département de Chimie Moléculaire, Unité Mixte de Recherche 5250, Centre National de la Recherche Scientifique, Université Joseph Fourier, BP53, 38041 Grenoble, France;
- ‡Institute of Organic Chemistry and Biochemistry, Academy of Sciences Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, 16610 6 Prague, Czech Republic; and
- ¶Department of Chemistry, Oklahoma State University, Stillwater, OK 74078
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Edited by Mark A. Ratner, Northwestern University, Evanston, IL, and approved March 8, 2007 (received for review December 19, 2006)
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)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 < 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.
Footnotes
- §To whom correspondence should be addressed. E-mail: pavel.jungwirth{at}uochb.cas.cz
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Author contributions: V.B., A.M., P.J., and J.P.D. designed research; V.B., A.M., R.V., P.J., and J.P.D. performed research; and V.B., P.J., and J.P.D. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
- Abbreviations:
- NVT,
- constant volume and temperature;
- DFT,
- density functional theory;
- BLYP,
- Becke-Lee-Yang-Parr;
- B3LYP,
- Becke three-parameter hybrid functional combined with Lee–Yang–Parr correlation functional;
- ML,
- monolayer;
- DZVP,
- double-zeta polarization basis set.
- © 2007 by The National Academy of Sciences of the USA
