Issue 26, 2011
From the journal:

Physical Chemistry Chemical Physics

The protonation state of small carboxylic acids at the water surface from photoelectron spectroscopy

Niklas Ottosson,*a   Erik Wernersson,b   Johan Söderström,a   Wandared Pokapanich,a   Susanna Kaufmann,c   Svante Svensson,a   Ingmar Persson,d   Gunnar Öhrwalle  and  Olle Björneholm*a  

* Corresponding authors

a Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
E-mail: niklas.ottosson@fysik.uu.se, olle.bjorneholm@fysik.uu.se

b Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, and Center for Biomolecules and Complex Molecular Systems, Flemingovo nám. 2, 16610 Prague 6, Czech Republic

c Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05 Uppsala, Sweden

d Department of Chemistry, SLU, Box 7015, SE-750 07 Uppsala, Sweden

e MAX-lab, Lund University, Box 118, SE-221 00 Lund, Sweden

Abstract

We report highly surface sensitive core-level photoelectron spectra of small carboxylic acids (formic, acetic and butyric acid) and their respective carboxylate conjugate base forms (formate, acetate and butyrate) in aqueous solution. The relative surface propensity of the carboxylic acids and carboxylates is obtained by monitoring their respective C1s signal intensities from a solution in which their bulk concentrations are equal. All the acids are found to be enriched at the surface relative to the corresponding carboxylates. By monitoring the PE signals of acetic acid and acetate as a function of total concentration, we find that the protonation of acetic acid is nearly complete in the interface layer. This is in agreement with literature surface tension data, from which it is inferred that the acids are enriched at the surface while (sodium) formate and acetate, but not butyrate, are depleted. For butyric acid, we conclude that the carboxylate form co-exists with the acid in the interface layer. The free energy cost of replacing an adsorbed butyric acid molecule with a butyrate ion at 1.0 M concentration is estimated to be >5 kJ mol−1. By comparing concentration dependent surface excess data with the evolution of the corresponding photoemission signals it is furthermore possible to draw conclusions about how the distribution of molecules that contribute to the excess is altered with bulk concentration.

Graphical abstract: The protonation state of small carboxylic acids at the water surface from photoelectron spectroscopy

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Article information

DOI
https://doi.org/10.1039/C1CP20245F
Article type
Paper
Submitted
28 Jan 2011
Accepted
06 May 2011
First published
01 Jun 2011

Phys. Chem. Chem. Phys., 2011,13, 12261-12267

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The protonation state of small carboxylic acids at the water surface from photoelectron spectroscopy

N. Ottosson, E. Wernersson, J. Söderström, W. Pokapanich, S. Kaufmann, S. Svensson, I. Persson, G. Öhrwall and O. Björneholm, Phys. Chem. Chem. Phys., 2011, 13, 12261 DOI: 10.1039/C1CP20245F

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