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Research article - Peer-reviewed, 2016

Alcohols at the aqueous surface: chain length and isomer effects

Walz, Marie-Madeleine; Werner, Josephina; Ekholm, Victor; Prisle, Nønne; Öhrwall, Gunnar; Björneholm, Olle


Surface-active organic molecules at the liquid-vapor interface are of great importance in atmospheric science. Therefore, we studied the surface behavior of alcohol isomers with different chain lengths (C4-C6) in aqueous solution with surface-and chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. Gibbs free energies of adsorption and surface concentrations are determined from the XPS results using a standard Langmuir adsorption isotherm model. The free energies of adsorption, ranging from around -15 to -19 kJ mol(-1) (C4-C6), scale linearly with the number of carbon atoms within the alcohols with Delta G(Ads) per-CH2-approximate to -2 kJ mol(-1.) While for the linear alcohols, surface concentrations lie around 2.4 x 10(14) molecules per cm(2) at the bulk concentrations where monolayers are formed, the studied branched alcohols show lower surface concentrations of around 1.6 x 10(14) molecules per cm(2), both of which are in line with the molecular structure and their orientation at the interface. Interestingly, we find that there is a maximum in the surface enrichment factor for linear alcohols at low concentrations, which is not observed for the shorter branched alcohols. This is interpreted in terms of a cooperative effect, which we suggest to be the result of more effective van der Waals interactions between the linear alcohol alkyl chains at the aqueous surface, making it energetically even more favorable to reside at the liquid-vapor interface.

Published in

Physical Chemistry Chemical Physics
2016, volume: 18, number: 9, pages: 6648-6656

Authors' information

Walz, Marie-Madeleine
Uppsala University
Werner, Josephina
Uppsala University
Werner, Josephina
Swedish University of Agricultural Sciences, The Department of Chemistry and Biotechnology
Ekholm, Victor
Uppsala University
Prisle, Nønne
University of Helsinki
Öhrwall, Gunnar
Lund University
Björneholm, Olle
Uppsala University

UKÄ Subject classification

Meteorology and Atmospheric Sciences
Physical Chemistry
Inorganic Chemistry
Atom and Molecular Physics and Optics
Organic Chemistry

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