Brønsted basicity of the air-water interface

Himanshu Mishra, Shinichi Enami, Robert J. Nielsen, Logan A. Stewart, Michael R. Hoffmann, William A. Goddard, Agustín J. Colussi*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

118 Scopus citations

Abstract

Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H2O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH- ions are present therein. Thus, by detecting RCOO- we demonstrate the presence of OH - on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me)3N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH- densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH- gradients in the interfacial region. This fact could account for the weak OH- signals detected by surface-specific spectroscopies.

Original languageEnglish (US)
Pages (from-to)18679-18683
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Volume109
Issue number46
DOIs
StatePublished - Nov 13 2012

Keywords

  • Gas-liquid reactions
  • Interfacial proton transfer
  • Surface potential
  • Water surface acidity

ASJC Scopus subject areas

  • General

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