Articles | Volume 12, issue 24
Research article
21 Dec 2012
Research article |  | 21 Dec 2012

Surface/bulk partitioning and acid/base speciation of aqueous decanoate: direct observations and atmospheric implications

N. L. Prisle, N. Ottosson, G. Öhrwall, J. Söderström, M. Dal Maso, and O. Björneholm

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.

Final-revised paper