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 − 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 K₂ ≤ 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.