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 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 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 + pK(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 C5-14 PFCAs. (C) 2018 Elsevier Ltd. All rights reserved.