Modeling the Partitioning of Anionic Carboxylic and Perfluoroalkyl Carboxylic and Sulfonic Acids to Octanol and Membrane Lipid.

Perfluoroalkyl carboxylic and sulfonic acids (PFCAs and PFSAs) have low pK a 's and are, therefore, deprotonated under most experimental and environmental conditions. Hence, the anionic species dominate their partitioning between water and organic phases, including, octanol and phospholipid bilayers which are often used as model systems for environmental and biological matrices. However, data for solvent-water and membrane-water partition coefficients of the anion species are only available for a few per- and polyfluoroalkyl substances (PFAS). In this paper, an equation is derived using a Born-Haber cycle that relates the partition coefficients of the anions to those of the corresponding neutral species. It is shown via a thermodynamic analysis that for carboxylic acids (CAs), PFCAs, and PFSAs, the log of the solvent-water partition coefficient of the anion, log K SW (A - ), is linearly related to the log of the solvent-water partition coefficient of the neutral acid, log K SW (HA), with a unity slope and a solvent-dependent but solute independent intercept within a PFAS (or CA) family. This finding provides a method for estimating the partition coefficient of PFCAs and PFSAs anions using the partition coefficients of the neutral species, which can be reliably predicted using quantum chemical methods. In addition, we have found that the neutral octanol-water partition coefficient, log K OW , is linearly correlated to the neutral membrane-water partition coefficient, log K MW , and, therefore, log K OW being a much easier property to estimate and/or measure, can be used to predict the neutral log K MW . Application of this approach to K OW and K MW for PFCAs and PFSAs demonstrates the utility of this methodology for evaluating reported experimental data and extending anions property data for chain lengths that are unavailable.