Accurate measuring n-octanol/water partition coefficients (log Kow) of perfluoroalkyl carboxylic acids (PFCAs) using experimental approach has been proven to be very difficult due to their special properties. The ionizable carboxyl groups in PFCAs make their log Kow dependent on pH. In this study, the log Kow values of neutral species of PFCAs (C4≤n≤14) were measured based on reversed-phase high-performance liquid chromatography (RP-HPLC) with the mobile phase pH varying in the range of 1.09-5.00. The relationship between log Kow and retention times was established using some reference compounds (including agrochemicals, polycyclic aromatic hydrocarbons) with known log Kow values, and then validated with alkyl fatty acids, which have similar chemical structures as PFCAs. The apparent log Kow (i.e., log Dow) of the C4-14 PFCAs were calculated based on their retention times using the established model, and they displayed a negative linear relationship with the mobile phase pH in the range of 1.09-4.00. Consequently, the log Dow values were converted to the corresponding log Kow values (1.05-7.19) based on the relationship of log Dow = log Kow + pKa - pH. The log Kow increased with perfluorinated carbon chain length with a greater rate for C4 to C5 PFCAs than for C5-14 PFCAs.
Keywords: N-octanol/water partition coefficient; Perfluoroalkyl carboxylic acids; Reversed-phase high-performance liquid chromatography; pH; pK(a)-pH conversion.
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