Per- and polyfluoroalkyl substances (PFAS) exhibit a wide range of acid dissociation constants (pKa), influencing their environmental fate and behavior. However, experimental pKa values remain scarce, and computational predictions often show inconsistencies. The acid dissociation constants (pKa) of 10 per- and polyfluoroalkyl substances (PFAS), monofluoroacetic acid (MFA), difluoroacetic acid (DFA), and trifluoroacetic acid (TFA) were experimentally determined using ¹⁹F-NMR and ¹H-NMR chemical shift perturbation measurements. Results were compared to previously reported experimental values and computational predictions from EPA CompTox and COSMO-RS calculations. Measured pKa values for PFAS are: PFPrA (-0.49), PFOA (-0.26), PFBS (-1.04), PFOS (-0.96), 6:2 FTS (-0.03), HFPO-DA (GenX) (-0.20), 5:3 FTCA (3.97), NFDHA (-0.28), PFMPA (-0.25), and 6:2 FTUCA (2.51). Trends across perfluorocarboxylic acids (PFCAs) suggest a general pKa of ~ -0.3 for this class. Findings classify PFAS into three acidity groups: very strong acids (pKa ≤ -1), strong acids (-0.5 < pKa < 0.5), and weak acids (pKa > 0.5). PFCAs, perfluoroether carboxylic acids, TFA, and 6:2 FTS act as strong acids, while perfluorosulfonic acids (PFBS, PFOS) are very strong acids. Telomerization significantly reduces acidity, with 6:2 FTCA (pKa 3.97) showing a 10⁴-fold reduction relative to PFOA, while 6:2 FTS (pKa -0.03) exhibits a smaller attenuation (~10¹) compared to PFOS. Fluorine-induced acidity in PFAS is primarily driven by -CF₂ groups adjacent to CO₂H and SO₃H, with inductive effects diminishing beyond two perfluorocarbons. Non-concentration-based pKa measurements yield systematically lower values than prior reports because NMR chemical shifts respond to electron density changes around nuclei and are unaffected by sorption and micellization events. COSMO-RS predictions align within 0.6 pKa units for carboxylic acids, while CompTox pKa values show greater deviations (0.5 < |ΔpKa| < 2.2). In contrast, CompTox sulfonic acid pKa values (0.6 < |ΔpKa| < 1.2) align more closely with experimental data than COSMO-RS (2.7 < |ΔpKa| < 3.4).

Tristan Vick

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