Sacrifices at the Surface

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Science  03 Feb 2012:
Vol. 335, Issue 6068, pp. 505
DOI: 10.1126/science.335.6068.505-b

There has been a thriving research discussion over the past decade about the tendency of certain dissolved ions to accumulate at the air/water interface of aerosols, and the associated implications for atmospheric chemistry. The driving framework of this sort of partitioning also has some bearing on how various salts affect aqueous protein solubility and conformational equilibria. A chemically intuitive explanation for the effect might suggest that surface migration is enthalpically unfavorable (as the ions remain only partially solvated) but is ultimately driven by the entropic enhancement associated with liberating the bulk water molecules that previously surrounded the ion completely. Otten et al. now present experimental studies of thiocyanate partitioning by temperature-dependent second harmonic generation spectroscopy (a surface-sensitive technique), together with theoretical simulations, that reverse the reasoning in this framework. The data point to an enthalpic gain and accompanying entropic sacrifice as the ions move from bulk to surface. The enthalpic gain is essentially attributed to full hydrogen bonding at water molecules driven away from the surface (where they previously were half exposed to uncoordinating air). The entropic loss, somewhat harder to account for, is attributed to suppression of collective capillary wave motions by the ions, engendering a more ordered surface.

Proc. Natl. Acad. Sci. U.S.A. 109, 701 (2012).

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