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Science 28 January 2005:
Vol. 307. no. 5709, pp. 563 - 566
DOI: 10.1126/science.1106525
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Reports

Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides

Sutapa Ghosal,1,2 John C. Hemminger,1* Hendrik Bluhm,3 Bongjin Simon Mun,4 Eleonore L. D. Hebenstreit,2 Guido Ketteler,2 D. Frank Ogletree,2 Felix G. Requejo,2,5 Miquel Salmeron2

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.

1 Department of Chemistry, University of California, Irvine, CA 92697, USA.
2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3 Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5 Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900 La Plata, Argentina.

* To whom correspondence should be addressed. E-mail: jchemmin{at}uci.edu

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