Acid Dissociation versus Molecular Association of Perfluoroalkyl Oxoacids: Environmental Implications

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 113; no. 29; pp. 8152 - 8156
• Main Authors: Cheng, Jie, Psillakis, Elefteria, Hoffmann, M. R, Colussi, A. J
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 23.07.2009
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp9051352

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 − CO2) signals, plus m/z = 213 (C3F7CO2 −) and 169 (C3F7 −) 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 × 107 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.