Surface/bulk partitioning and acid/base speciation of aqueous decanoate: direct observations and atmospheric implications
1University of Helsinki, Department of Physics, P.O. Box 48, 00014, University of Helsinki, Helsinki, Finland
2University of Uppsala, Department of Physics and Astronomy, Box 516, 75120, Uppsala, Sweden
3University of Lund, MAX-lab, Box 118, 22100, Lund, Sweden
*now at: FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Abstract. Dilute aqueous solutions of the atmospheric organic surfactant sodium decanoate have been studied using surface sensitive X-ray photoelectron spectroscopy combined with synchrotron radiation. We studied the decanoate/decanoic acid speciation and preferential adsorption at the vapor–liquid interface, and the responses to mixing in solution with some of the most common atmospheric inorganic ions, Na+, NH4+, Cl−, and O42−. We observe no influence of Na+, Cl−, or SO42− ions on neither the relative speciation nor the individual adsorption properties of decanoate and decanoic acid. In particular, the anticipated salting-out effect due to common Na+ cations of the organic and inorganic salts was not observed. On the other hand, mixing with NH4+ cations resulted in a pronounced surface enhancement of decanoic acid, which is attributed to surface specific acid–base chemistry. These changes in surface/bulk partitioning and speciation may significantly affect properties of aqueous droplets containing decanoate/decanoic acid, and potential implications for several processes critical to the climate effects of atmospheric aerosols are discussed.