Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar⁺/e^- Plasma

The reactivities of three perfluoroalkyl carboxylic acids (PFCAs) (perfluoropropanoic acid (C₂F₅COOH, PFPA), perfluorobutanoic acid (C₃F₇COOH, PFBA), and perfluorooctanoic acid (C₇F₁₅COOH, PFOA)) in a thermal, weakly ionized, argon/electron plasma were investigated from 300 to 600 K using a Langmuir probe-flowing afterglow apparatus. The results are supported by density functional theory calculations of the energetics of PFCA, CF₂COOH, from n = 1 to 7. PFPA and PFBA attach electrons at a substantial fraction of the calculated capture rate; PFOA likely attaches electrons with similarly high efficiency, but the low vapor pressure of PFOA resulted in only qualitative results. All three compounds attach electrons dissociatively via HF elimination. The "acidity channel" (i.e., formation of H + CF₂COO^-), calculated to be slightly endothermic, is never observed even at higher temperatures where this channel would be energetically allowed. Attachment to perfluorooctanesulfonic acid (n-C₈F₁₇SO₃H) is rapid, yielding the conjugate base n-C₈F₁₇SO₃^- as proton transfer to the electron is exothermic. At temperatures near 450 K (PFOA), 550 K (PFBA), or 600 K (PFPA), the parent neutrals thermally decompose as evidenced by abrupt changes in rate constants and branching ratios. PFPA and PFBA react with Ar⁺ close to the calculated capture rate, and PFOA likely does as well. The reaction mechanism starts via charge transfer, which can then lead to a range of product ions. Reactions with Ar⁺ yield fluorocarbon radicals, clarifying and supporting a previously proposed mechanism of PFCA degradation in an argon plasma.