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

Highlights: • RP-HPLC with strong mobile phase pH was applied to measure log K_ow of C₄–₁₄ PFCAs. • Relationship of log K_ow and retention times was developed using reference compounds. • SFAs were used to further validate the calculation model. • Log D_ow of PFCAs decreased linearly with mobile phase pH of 1.09–4.00 • The log K_ow of neutral PFCAs were obtained using the sigmoidal curve. 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 + pK_a – pH. The log K_ow increased with perfluorinated carbon chain length with a greater rate for C₄ to C₅ PFCAs than for C_5–14 PFCAs.