Impact of Submicellar Aggregation on Reduction Kinetics of Perfluorooctanoate by the Hydrated Electron

Disagreements persist regarding the rate constants for the initial reduction of perfluorooctanoate (PFOA) by the hydrated electron, eaq–, with published values differing by nearly 2 orders of magnitude (between ∼1 × 107 and ∼1 × 109 M–1 s–1). In this letter, we demonstrate a nonlinear dependence of the eaq– decay rate on the PFOA which we attribute as being a result of aggregation of PFOA at concentrations below the critical micellar concentration. We invoke a kinetic model for the reaction between PFOA and eaq– at concentrations below 1 mM involving dimerization of the PFOA; higher-order aggregates are acknowledged but not modeled. Our kinetic model adequately fits the nanosecond transient absorption data at PFOA ≤ 1 mM providing apparent rate constants for the reduction of PFOA monomers (k1 = 1.4 × 109 M–1 s–1) and dimers (k2 = 1.1 × 107 M–1 s–1) by eaq–. For reasons that are only partially understood, the aggregation of PFOA decreases the probability of the reduction of PFOA by eaq–. These results reconcile the reports of differing rate constants for the initial reduction of PFOA by eaq– suggesting its origin to be related to the aggregation of PFOA even under dilute conditions.