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

There exists large uncertainty in the literature as to the pK_a of medium-chain fatty acids at the air–water interface. Via surface tension titration, the surface-pK_a 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-pK_a determined with surface tension differs from the bulk value obtained during a standard acid–base titration. Near the surface-pK_a 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-pK_a 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-pK_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 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.