Understanding the acid–base behavior of carboxylic acids on aqueous interfaces is a fundamental issue in nature. Surface processes involving carboxylic acids such as acetic and pyruvic acids play roles in (1) the transport of nutrients through cell membranes, (2) the cycling of metabolites relevant to the origin of life, and (3) the photooxidative processing of biogenic and anthropogenic emissions in aerosols and atmospheric waters. Here, we report that 50% of gaseous acetic acid and pyruvic acid molecules transfer a proton to the surface of water at pH 2.8 and 1.8 units lower than their respective acidity constants pKa = 4.6 and 2.4 in bulk water. These findings provide key insights into the relative Bronsted acidities of common carboxylic acids versus interfacial water. In addition, the work estimates the reactive uptake coefficient of gaseous pyruvic acid by water to be γPA = 0.06. This work is useful to interpret the interfacial behavior of pyruvic acid under low water activity conditions, typically found in haze aerosols, clouds, and fog waters.
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M.I.G. thanks research funding from the National Science foundation under NSF CAREER award CHE-1255290. A.J.E. acknowledges the support by the NASA Earth and Space Science Fellowship (NESSF) Program. A.J.C. acknowledges funding from the National Science Foundation under grant AGS-1744353.
Eugene, Alexis J.; Pillar, Elizabeth A.; Colussi, Agustín J.; and Guzman, Marcelo I., "Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water" (2018). Chemistry Faculty Publications. 105.