Molecular sinkers: x-ray photoemission and atomistic simulations of benzoic acid and benzoate at the aqueous solution/vapor interface

Niklas Ottosson, Anastasia O. Romanova, Johan Soderstrom, Olle Bjorneholm, Gunnar Ohrwall, Maxim V. Fedorov

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


In this work, we 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 significant 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 at 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 the 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 and depletion of the molecules at the interface.

Original languageEnglish
Pages (from-to)13017-13023
Number of pages7
JournalJournal of Physical Chemistry B
Issue number43
Publication statusPublished - 1 Nov 2012


  • molecular sinkers
  • x-ray photoemission
  • atomistic simulations
  • benzoate
  • aqueous solution
  • vapor interface

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