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 SO42−. We observe little or 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, no significant salting-out effect due to common Na+ cations of the organic and inorganic salts was observed for these solutions. 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 surface 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.