Issue Date: June 25, 2007
A recent Science & Technology Concentrate (C&EN, April 30, page 28) cited a PNAS paper on water's unexpected acidic surface (Proc. Natl. Acad. Sci. USA 2007, 104, 7342). Although the acidification of the water surface is clearly important, the contents of this paper were not unexpected because the same conclusion was already published three years earlier (J. Phys. Chem. B 2004, 108, 14804).
The abstract of the latter paper clearly states, "It was found in our model that the hydrated excess proton displays a marked preference for water liquid/vapor interfaces." In the conclusion, it is again stated, "Our simulations would also seem to suggest the existence of a negative pH gradient toward the water liquid/vapor interface." The distribution function of the excess proton probability relative to the water liquid/vapor interface was calculated and plotted in the Journal of Physical Chemistry B paper. When inverted through the standard Boltzmann probability formula, the distribution function yields a quantitative free-energy estimate for the surface proton preference of 3???4 kcal/mole, essentially the same value as estimated by the PNAS article.
The J. Phys. Chem. B finding published in 2004 is well-known and widely recognized. It is now even described in review articles by other authors (Ann. Rev. Phys. Chem. 2006, 57, 333) and in at least one other PNAS paper published last year (Proc. Natl. Acad. Sci. USA 2006, 103, 14278). While C&EN is not responsible for a lapse in the peer review system, an accurate accounting of the history of this scientific finding seems warranted.
Gregory A. Voth
Matthew K. Petersen
Salt Lake City, Utah
Srinivasan S. Iyengar
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