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

Werner, Josephina; Persson, Ingmar; Björneholm, Olle LU ; Kawecki, Delphine; Saak, Clara Magdalena; Walz, Marie Madeleine; Ekholm, Victor; Unger, Isaak; Valtl, Corina and Caleman, Carl, et al. (2018) In Physical Chemistry Chemical Physics 20(36). p.23281-23293
Abstract

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... (More)

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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publication status
published
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Physical Chemistry Chemical Physics
volume
20
issue
36
pages
13 pages
publisher
Royal Society of Chemistry
external identifiers
  • scopus:85053827874
ISSN
1463-9076
DOI
10.1039/c8cp01898g
language
English
LU publication?
yes
id
7e5076c2-b2b3-4284-9e22-4c1c41464909
date added to LUP
2018-10-22 14:46:09
date last changed
2019-10-15 06:48:40
@article{7e5076c2-b2b3-4284-9e22-4c1c41464909,
  abstract     = {<p>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.</p>},
  author       = {Werner, Josephina and Persson, Ingmar and Björneholm, Olle and Kawecki, Delphine and Saak, Clara Magdalena and Walz, Marie Madeleine and Ekholm, Victor and Unger, Isaak and Valtl, Corina and Caleman, Carl and Öhrwall, Gunnar and Prisle, Nønne L.},
  issn         = {1463-9076},
  language     = {eng},
  month        = {01},
  number       = {36},
  pages        = {23281--23293},
  publisher    = {Royal Society of Chemistry},
  series       = {Physical Chemistry Chemical Physics},
  title        = {Shifted equilibria of organic acids and bases in the aqueous surface region},
  url          = {http://dx.doi.org/10.1039/c8cp01898g},
  volume       = {20},
  year         = {2018},
}