Perfluorinated Surfactant Chain-Length Effects on Sonochemical Kinetics

Abstract

The sonochemical degradation kinetics of the aqueous perfluorochemicals (PFCs) perfluorobutanoate (PFBA), perfluorobutanesulfonate (PFBS), perfluorohexanoate (PFHA), and perfluorohexanesulfonate (PFHS) have been investigated. Surface tension measurements were used to evaluate chain-length effects on equilibrium air−water interface partitioning. The PFC air−water interface partitioning coefficients, K_(eq)^(PF), and maximum surface concentrations, Γ_(max)^(PF), were determined from the surface pressure equation of state for PFBA, PFBS, PFHA, and PFHS. Relative K_(eq)^(PF) values were dependent upon chain length K_(eq)^(PFHS) ≅ 2.1K_(eq)^(PFHA) ≅ 3.9K_(eq)^(PFBS) ≅ 5.0K_(eq)^(PFBA), whereas relative Γ_(max)^(PF) values had minimal chain length dependence Γ_(max)^(PFHS) ≅ Γ_(max)^(PFHA) ≅ Γ_(max)^(PFBS) ≅ 2.2Γ_(max)^(PFBA). The rates of sonolytic degradation were determined over a range of frequencies from 202 to 1060 kHz at dilute (<1 μM) initial PFC concentrations and are compared to previously reported results for their C8 analogs: perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA). Under all conditions, the time-dependent PFC sonolytic degradation was observed to follow pseudo-first-order kinetics, i.e., below kinetic saturation, suggesting bubble−water interface populations were significantly below the adsorption maximum. The PFHX (where X = A or S) sonolysis rate constant was observed to peak at an ultrasonic frequency of 358 kHz, similar to that for PFOX. In contrast, the PFBX degradation rate constants had an apparent maximum at 610 kHz. Degradation rates observed for PFHX are similar to previously determined PFOX rates, k_(app,358)^(PFOX) ≅ k(app,358)^(PFHX). PFOX is sonolytically pyrolyzed at the transiently cavitating bubble−water interface, suggesting that rates should be proportional to equilibrium interfacial partitioning. However, relative equilibrium air−water interfacial partitioning predicts that K_(eq)^(PFOX) ≅ 5K_(eq)^(PFHX). This suggests that at dilute PFC concentrations, adsorption to the bubble−water interface is ultrasonically enhanced due to high-velocity radial bubble oscillations. PFC sonochemical kinetics are slower for PFBS and further diminished for PFBA as compared to longer analogs, suggesting that PFBX surface films are of lower stability due to their greater water solubility.

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