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Acid dissociation versus molecular association of perfluoroalkyl oxoacids: environmental implications

Psyllaki Eleftheria, Cheng Jie, Colussi Arthur, Gabriella, 1957-, Hoffmann M. R.

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Year 2009
Type of Item Peer-Reviewed Journal Publication
Bibliographic Citation J .Cheng, E. Psillakis, M. R. Hoffmann and A. J. Colussi , "Acid dissociation versus molecular association of perfluoroalkyl oxoacids: environmental implications", J. Phys. Chem. A, vol.113, no. 29, pp.8152–8156, Jul. 2009. doi: 10.1021/jp9051352
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Perfluorooctanoate (PFO) and perfluorooctanesulfonate (PFOS) surfactant anions, once released, may rapidlyreach remote regions. This phenomenon is puzzling because the water-bound anions of strong F-alkyl acidsshould be largely transported by slow oceanic currents. Herein, we investigate whether these hydrophobicF-alkyl oxoanions would behave anomalously under environmental conditions, as suggested elsewhere. Negativeelectrospray ionization mass spectra of micromolar aqueous PFO or PFOS solutions from pH 1.0 to 6.0 show(1) m/z ) 499 (PFOS) signals that are independent of pH and (2) m/z ) 413 (PFO) and 369 (PFO - CO2)signals, plus m/z ) 213 (C F CO -) and 169 (C F -) signals at higher collision energies, and, below pH ∼ 372 374, m/z ) 827 signals from a remarkably stable (PFO)2H- cluster that increase with decreasing pH. Since the sum of the m/z ) 369, 413, and 827 signal intensities is independent of pH, that is, effectively encompasses all major species, we infer that pKa(PFOSA) < 1.0 and pKa(PFOA) < 1.0. We also derive K2 e 4 × 107 M-2 for the clustering equilibrium 2PFO + H+ h (PFO)2H. Thus, although (PFO)2H is held together by an exceptionally strong homonuclear covalent hydrogen bond, neither PFOS nor PFO will associate or protonate significantly at environmentally relevant subnanomolar concentrations above pH ∼ 1.