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

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 SO₄^2−. We observe little or no influence of Na⁺, Cl^−, or SO₄^2− 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.