Issue 9, 2016

Alcohols at the aqueous surface: chain length and isomer effects

Abstract

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 ΔGAds per –CH2– ≈ −2 kJ mol−1. While for the linear alcohols, surface concentrations lie around 2.4 × 1014 molecules per cm2 at the bulk concentrations where monolayers are formed, the studied branched alcohols show lower surface concentrations of around 1.6 × 1014 molecules per cm2, 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.

Graphical abstract: Alcohols at the aqueous surface: chain length and isomer effects

Supplementary files

Article information

Article type
Paper
Submitted
23 Oct 2015
Accepted
26 Jan 2016
First published
12 Feb 2016

Phys. Chem. Chem. Phys., 2016,18, 6648-6656

Author version available

Alcohols at the aqueous surface: chain length and isomer effects

M.-M. Walz, J. Werner, V. Ekholm, N. L. Prisle, G. Öhrwall and O. Björneholm, Phys. Chem. Chem. Phys., 2016, 18, 6648 DOI: 10.1039/C5CP06463E

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