Oxidation Kinetics of Alkyl Sulfates and Sulfonates by Sulfate Radical (SO₄^•–) in the Aqueous Phase: Deactivating Role of Sulfur Functional Groups

The sulfate radical (SO₄^•–) is a potent oxidant known to efficiently oxidize many organic compounds in the aqueous phase. To date, reactions of SO₄^•– with alkyl sulfates and sulfonates, which are common organosulfur compounds found in atmospheric aerosols and cloud droplets, are not well understood. Here, we employed a laser flash photolysis-long path absorption (LFP-LPA) technique to measure the temperature-dependent oxidation kinetics of organosulfur compounds initiated by SO₄^•– in the aqueous phase. These compounds included five alkyl sulfates, namely, methyl sulfate (MS), ethyl sulfate (ES), octyl sulfate (OS), decyl sulfate (DS), and dodecyl sulfate (SDS), as well as three sulfonates, namely, methanesulfonate (MSA), hydroxymethanesulfonate (HMS), and 2-hydroxyethylsulfonate (HES). Our kinetic data revealed that the second-order rate constants of these organosulfur compounds were in the range of 10³ to 10⁸ L mol^–1 s^–1 and exhibited a positive temperature-dependency across the range of 278 to 318 K. Upon oxidation, the hydrogen abstraction is likely the dominant pathway. Moreover, alkyl sulfates and sulfonates generally exhibit smaller reactivities compared to other organic compounds with the same carbon number, such as alcohols. This reduced reactivity could be explained by the strong electron-withdrawing nature of sulfur functional groups (i.e., −OSO₃^– in alkyl sulfates and −SO₃^– in sulfonates). Among the two sulfur functional groups, −SO₃^– shows a stronger deactivating effect than −OSO₃^–, likely due to the higher charge density of the former. Overall, the findings of this work fill a gap in the understanding of the SO₄^•– oxidation kinetics in organosulfur compounds.