Squeezing the Water Out of HCl(aq)

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Science  19 Jun 2009:
Vol. 324, Issue 5934, pp. 1522-1523
DOI: 10.1126/science.1175387

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Our earliest encounters with chemistry usually involve measuring the extent to which simple acids dissociate to ions in aqueous solution. For strong acids, such as HCl, dissociation is nearly complete, with the H+(aq) and Cl(aq) ions moving free of one another in a large excess of water. One of the current driving forces in modern science is our interest in studying chemical and physical phenomena in ever-smaller size regimes, seeking to characterize nature strained to its smallest size limits. The fields of molecular electronics (1, 2), single-molecule spectroscopy (3), and atomic-scale microscopy (4) all operate at or near this limit. On page 1545 of this issue, Gutberlet et al. (5) consider acid dissociation at its smallest scale, probing the dissociation of a single HCl molecule into its ions under conditions where no more than a handful of water molecules are present. The issue at hand is determining the minimum number of water molecules at which HCl dissociation into H3O+(H2O)n−1Cl occurs spontaneously.