Issue 39, 2017

Surface pKa of octanoic, nonanoic, and decanoic fatty acids at the air–water interface: applications to atmospheric aerosol chemistry

Abstract

There exists large uncertainty in the literature as to the pKa of medium-chain fatty acids at the air–water interface. Via surface tension titration, the surface-pKa values of octanoic (C8), nonanoic (C9), and decanoic (C10) fatty acids are determined to be 4.9, 5.8, and 6.4, respectively. The surface-pKa determined with surface tension differs from the bulk value obtained during a standard acid–base titration. Near the surface-pKa of the C8 and C9 systems, surface tension minima are observed and are attributed to the formation of surface-active acid–soap complexes. The direction of the titration is shown to affect the surface-pKa of the C9 system, as the value shifts to 5.2 with NaOH titrant due to a higher concentration of Na+ ions at pH values close to the surface-pKa. As the reactivity and climate-relevant properties of sea spray aerosols (SSA) are partially dictated by the charge and surface activity of the organics at the aerosol–atmosphere interface, the results presented here on SSA-identified C8–C10 fatty acids can be used to better predict the health and climate impact of particles with significant concentrations of medium-chain fatty acids.

Graphical abstract: Surface pKa of octanoic, nonanoic, and decanoic fatty acids at the air–water interface: applications to atmospheric aerosol chemistry

Supplementary files

Article information

Article type
Paper
Submitted
05 Jul 2017
Accepted
08 Aug 2017
First published
08 Aug 2017

Phys. Chem. Chem. Phys., 2017,19, 26551-26558

Surface pKa of octanoic, nonanoic, and decanoic fatty acids at the air–water interface: applications to atmospheric aerosol chemistry

B. A. Wellen, E. A. Lach and H. C. Allen, Phys. Chem. Chem. Phys., 2017, 19, 26551 DOI: 10.1039/C7CP04527A

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