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Reversibly Physisorbed and Chemisorbed Water on Carboxylic Salt Surfaces under Atmospheric Conditions

Journal article
Authors Xiangrui Kong
Dimitri Castarède
Anthony Boucly
Luca Artiglia
Markus Ammann
Thorsten Bartels-Rausch
Erik S Thomson
Jan B. C. Pettersson
Published in Journal of Physical Chemistry C
Volume 124
Issue 9
Pages 5263-5269
ISSN 1932-7447
Publication year 2020
Published at Department of Chemistry and Molecular Biology
Pages 5263-5269
Language en
Links https://doi.org/10.1021/acs.jpcc.0c...
Subject categories Surface and colloid chemistry, Meteorology and Atmospheric Sciences

Abstract

Copyright © 2020 American Chemical Society. The particle-gas interface in aerosol systems is of essential importance because it is here that many key atmospheric processes occur. In this study, we employ ambient pressure X-ray photoelectron spectroscopy (APXPS) to investigate the surface properties and processes of an atmospherically relevant carboxylic salt, sodium acetate, at subdeliquescence conditions. From the depth profiles of the elemental ratios of sodium, oxygen, and carbon, we find that after deliquescence-efflorescence cycles the salt surface is sodium-depleted. The mechanism of the observed depletion is proposed to be (i) the formation of neutral acetic acid in the solution due to the nature of the basic salt; (ii) the selective surface enhancement of neutral molecules under aqueous condition; and (iii) a hypothetical kinetic barrier to re-homogenization due to spatial separation and special local conditions on the surface, resulting in varied local surface composition. When the relative humidity gradually increases and approaches the deliquescence point, both reversible water uptake and reversible surface dissociation are confirmed by near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge and sodium K-edge, respectively. The solvation of sodium requires a higher relative humidity than needed for water adsorption, which suggests that water molecules are taken up by the surface, but the solvation of the salt surface begins only when sufficient water molecules are present, to facilitate the process. The sodium-depleted surface requires additional adsorbed water to affect and dissolve the sodium ions in deeper regions.

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