University of Oulu

Werner, J., Persson, I., Björneholm, O., Kawecki, D., Saak, C., Walz, M., Ekholm, V., Unger, I., Valtl, C., Caleman, C., Öhrwall, G., Prisle, N. (2018) Shifted equilibria of organic acids and bases in the aqueous surface region. Physical Chemistry Chemical Physics, 20 (36), 23281-23293. doi:10.1039/C8CP01898G

Shifted equilibria of organic acids and bases in the aqueous surface region

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Author: Werner, Josephina1,2; Persson, Ingmar2; Björneholm, Olle1;
Organizations: 1Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
2Department of Molecular Sciences, Swedish University of Agricultural Sciences, Box 7015, SE-750 07 Uppsala, Sweden
3Present address: Technology and Society Laboratory, Empa – Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland
4Present address: Department of Cell and Molecular Biology, Uppsala University, Box 596, SE-75124 Uppsala, Sweden
5Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Notkestraße 85, DE-22607 Hamburg, Germany
6MAX IV Laboratory, Lund University, Box 118, SE-22100 Lund, Sweden
7Faculty of Science, Nano and Molecular Systems Research Unit, University of Oulu, Box 3000, FI-90014, Finland
8Department of Physics, Division of Atmospheric Science, University of Helsinki, Box 64, FI-00014, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 8.6 MB)
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Language: English
Published: Royal Society of Chemistry, 2018
Publish Date: 2018-10-29


Acid–base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid–base pairs are strongly enriched in the aqueous surface region. Moreover, we show that for species with at least four carbon atoms in their alkyl-chain, their charged forms are also found to be abundant in the surface region. Using a combination of XPS and MD results, a model is proposed that effectively describes the surface composition. Resulting absolute surface concentration estimations show clearly that the total organic mole fractions in the surface region change drastically as a function of solution pH. The origin of the observed surface phenomena, hydronium/hydroxide concentrations in the aqueous surface region and why standard chemical equations, used to describe equilibria in dilute bulk solution are not valid in the aqueous surface region, are discussed in detail. The reported results are of considerable importance especially for the detailed understanding of properties of small aqueous droplets that can be found in the atmosphere.

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Series: PCCP. Physical chemistry chemical physics
ISSN: 1463-9076
ISSN-E: 1463-9084
ISSN-L: 1463-9076
Volume: 20
Pages: 23281 - 23293
DOI: 10.1039/C8CP01898G
OADOI: 10.1039/C8CP01898G
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
116 Chemical sciences
Funding: We thank the Swedish Research Council (2013-3940) and the Helmholtz Association through the Center for Free-Electron Laser Science at DESY. MAX IV Laboratory, Lund University, Sweden, is acknowledged for the allocation of beamtime and laboratory facilities. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Project SURFACE, Grant Agreement No. 717022). N. L. Prisle also gratefully acknowledges funding from the Academy of Finland (Grants 257411 and 308238).
EU Grant Number: (717022) SURFACE - The unexplored world of aerosol surfaces and their impacts.
Academy of Finland Grant Number: 308238
Detailed Information: 308238 (Academy of Finland Funding decision)
Copyright information: This journal is © the Owner Societies 2018. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.