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, e_aq^-, with published values differing by nearly 2 orders of magnitude (between ∼1 × 10⁷and ∼1 × 10⁹M^-1s^-1). In this letter, we demonstrate a nonlinear dependence of the e_aq^-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 e_aq^-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 × 10⁹M^-1s^-1) and dimers (k₂= 1.1 × 10⁷M^-1s^-1) by e_aq^-. For reasons that are only partially understood, the aggregation of PFOA decreases the probability of the reduction of PFOA by e_aq^-. These results reconcile the reports of differing rate constants for the initial reduction of PFOA by e_aq^-suggesting its origin to be related to the aggregation of PFOA even under dilute conditions.