Surface pKa of octanoic, nonanoic, and decanoic fatty acids at the air-water interface: applications to atmospheric aerosol chemistryElectronic supplementary information (ESI) available: This material includes (1, 2) adsorption isotherms of nonanoic acid at pH 2 and pH 12, (3) weak acid-strong base bulk titration of 1 mM nonanoic acid, (4) derivation of the surface activity model bounds, (5) IRRAS spectra comparing the CH stretching modes of a stearic acid Langmuir monolayer to a 1 mM nonanoic a

There exists large uncertainty in the literature as to the p K a of medium-chain fatty acids at the air-water interface. Via surface tension titration, the surface-p K a values of octanoic (C 8 ), nonanoic (C 9 ), and decanoic (C 10 ) fatty acids are determined to be 4.9, 5.8, and 6.4, respectively. The surface-p K a determined with surface tension differs from the bulk value obtained during a standard acid-base titration. Near the surface-p K a of the C 8 and C 9 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-p K a of the C 9 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-p K a . 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 C 8 -C 10 fatty acids can be used to better predict the health and climate impact of particles with significant concentrations of medium-chain fatty acids. Surface-p K a of medium-chain fatty acids, determined by surface tension titration, is different from the bulk value.