Accurate Determination of Perfluorooctanoate Aqueous Solubility, Critical Micelle Concentration, and Acid Dissociation Constant

Abstract

Although physical-chemical properties are critical for understanding the behavior of per- and polyfluoroalkyl substances (PFAS) in the environment, data reported in the literature can vary by orders of magnitude. The goal of this research was to accurately determine the aqueous solubility (Saq), critical micelle concentration (CMC), and acid dissociation constant (pK_a) of perfluorooctanoic acid (HPFO) and ammonium perfluoro-n-octanoate (APFO). The aqueous solubilities of HPFO in deionized (DI) water and 100 mM NaCl were ∼4500 and ∼12,500 mg/L, respectively, while APFO yielded aqueous solubilities of ∼47,000 and ∼45,000 mg/L in DI water and 100 mM NH₄Cl, respectively. Based on surface tension measurements, no CMC value was obtained for HPFO because the solubility limit was reached before micelle formation occurred, while in 100 mM NaCl the CMC was ∼3000 mg/L. The CMCs of APFO in DI water and 100 mM NH4Cl were ∼12,700 and ∼4,900 mg/L, respectively. These findings demonstrate that PFOA (1) is unlikely to form micelles in solution at environmentally relevant concentrations, (2) reported CMC values may actually correspond to the solubility limit rather than micelle formation, and (3) aqueous solubility values can vary by orders of magnitude depending upon the counterion species and solution properties.