Molecular Sinkers: X-ray Photoemission and Atomistic Simulations of Benzoic Acid and Benzoate at the Aqueous Solution/Vapor Interface
2012 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 116, no 43, 13017-13023 p.Article in journal (Refereed) Published
In this work we, for the first time, provide a detailed microscopic picture of the behavior of benzoic acid at the aqueous solution/vapor interface in its neutral as well as in its dissociated form (benzoate). This is achieved through a combination of highly surface-sensitive X-ray photoelectron spectroscopy experiments and fully atomistic molecular simulations. We show that drastic changes occur in the interface behavior of the neutral acid upon release of the proton. The benzoic acid molecules are found to be strongly adsorbed in the interface layer with the planes of the aromatic rings oriented almost parallel to the water surface. In contrast, in the benzoate form the carboxylate group shows a sinker-like behavior while the aromatic ring acts as a buoy, oriented nearly perpendicular to the surface. Furthermore, a significant fraction of the molecular ions move from the interface layer into the bulk of the solution. We rationalize these findings in terms of the very different hydration properties of benzoic acid's carboxylic group in the two charge states. The molecule has an amphiphilic nature and the deprotonation thus changes the hydrophobic/hydrophilic balance between the nonpolar aromatic and the polar carboxylic parts of the molecule. That, consequently, leads to a pronounced reorientation of the molecule at the interface.
Place, publisher, year, edition, pages
2012. Vol. 116, no 43, 13017-13023 p.
Condensed Matter Physics
Research subject Physics
IdentifiersURN: urn:nbn:se:uu:diva-151434DOI: 10.1021/jp300956jISI: 000310482800014OAI: oai:DiVA.org:uu-151434DiVA: diva2:409927