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Published July 23, 2009 | Supplemental Material
Journal Article Open

Acid dissociation versus molecular association of perfluoroalkyl oxoacids: environmental implications


Perfluorooctanoate (PFO) and perfluorooctanesulfonate (PFOS) surfactant anions, once released, may rapidly reach remote regions. This phenomenon is puzzling because the water-bound anions of strong F-alkyl acids should be largely transported by slow oceanic currents. Herein, we investigate whether these hydrophobic F-alkyl oxoanions would behave anomalously under environmental conditions, as suggested elsewhere. Negative electrospray 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 - CO_2) signals, plus m/z ) 213 (C_3F_7CO_2 -) and 169 (C_3F_7 -) signals at higher collision energies, and, below pH ~ 4, 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 pK_a(PFOSA) < 1.0 and pK_a(PFOA) < 1.0. We also derive K_2 ≤ 4 × 10^7 M^(-2) for the clustering equilibrium 2PFO + 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.

Additional Information

© 2009 American Chemical Society. Received: June 1, 2009; Revised Manuscript Received: June 16, 2009. Publication Date (Web): July 1, 2009. This project was financially supported by the National Science Foundation (ATM-0714329). E.P. is grateful to the Fullbright Foundation for financial support.


Acid Dissociation versus Molecular Association of Perfluoroalkyl Oxoacids: Environmental Implications Jie Cheng, Elefteria Psillakis, M. R. Hoffmann, and A. J. Colussi The Journal of Physical Chemistry A 2009 113 (34), 9578 DOI: 10.1021/jp906650d

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