Measuring log Kow coefficients of neutral species of perfluoroalkyl carboxylic acids using reversed-phase high-performance liquid chromatography

Accurate measuring n -octanol/water partition coefficients (log K ow ) 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 K ow dependent on pH. In this study, the log K ow values of neutral species of PFCAs (C 4≤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 K ow and retention times was established using some reference compounds (including agrochemicals , polycyclic aromatic hydrocarbons) with known log K ow values, and then validated with alkyl fatty acids, which have similar chemical structures as PFCAs. The apparent log K ow (i.e., log D ow ) of the C 4–14 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 D ow values were converted to the corresponding log K ow values (1.05–7.19) based on the relationship of log D ow  = log K ow  + p K a – pH. The log K ow increased with perfluorinated carbon chain length with a greater rate for C 4 to C 5 PFCAs than for C 5–14 PFCAs.