The log k0 values were plotted against indicators of molecular size. The Goss-modified McGowan molar volume (VGoss) showed an excellent linear relationship with log k0 for both PFAS and non-PFAS combined (Figure ). VGoss has been successfully used as a descriptor of the polyparameter linear free energy relationship model for neutral compounds. The linear relationship with VGoss suggests that the difference in cavity formation energy between the C18 phase and water is the main driver of PFAS partitioning and that VGoss may also be useful in describing partition coefficients of anionic substances, irrespective of the degree of fluorination. The data for 6:2 and 8:2 monoPAPs deviate somewhat from the trend, likely because these compounds are the only divalent anions in the data set. The original McGowan molar volume, which uses a slightly different atomic incremental volume for F, 27 also showed a high correlation with log k0 for PFAS, but the non-PFAS compounds are slightly off the regression line for PFAS (Figure ). Molecular weight also exhibited a linear relationship with log k0 for PFAS, although non-PFAS are far from the PFAS regression line. Comparison of k0, k25, and k50 to relevant partition coefficients Phospholipid membranes are among the most important phases for the bioaccumulation of anionic chemicals including PFAS. The log phospholipid membrane/water partition coefficients (Kmem/w) from Droge et al., consisting of 10 PFAS and 9 nonfluorinated anionic surfactants, correlate well with the log k0 obtained in this study (R 2 , 0.92; Figure ). It is encouraging that both per-and nonfluorinated surfactants fall into a single correlation. The log Kmem/w data set for 12 PFAS from Ebert et al. includes ether-containing PFAS and also correlates well with log k0 (R 2 , 0.84; Figure ). This R 2 value is higher than would be obtained by correlating log Kmem/w with nCF2 (R 2 ,0.62), because nCF2 cannot account for the influences of ether oxygen atoms of PFECAs and PFESAs, -CHF-of ADONA, or -CClF2 of 9Cl-PF3ONS on log Kmem/w. Adsorption at the air/water interface is a relevant process for surfactants. Increased retention during transport in unsaturated porous media and significant transport from seawater to the atmosphere via sea spray have been reported for PFAS, and these interesting phenomena occur due to the strong interfacial adsorption properties of PFAS relative to bulk partitioning. The log of air/water interfacial adsorption coefficients at infinite dilution (Kiw-0 [cm]; i.e., the partition coefficient between the air/water interface and bulk water) collected by Le et al. is linearly related to log k0 (Figure ). This data set includes both anionic perfluoroalkyl and nonfluoroalkyl substances. Note that only Kiw-0 data for Na-or K-salt are considered here, as the Kiw-0 data for acids show considerable differences from the corresponding salts, as reported in the original article. Soil organic carbon/water partition coefficients (Koc) also correlate well with k0, as shown in Figure . The log Koc data considered were derived from data for the most organic-rich soil reported by Nguyen et al. to minimize the influence of other soil components on sorption. Nevertheless, short-chain PFAS (i.e., PFBA, PFPeA, PFBS) show similar log Koc values independent of log k0, likely because nonhydrophobic factors control their sorption, as suggested by the authors. The slopes of the regression lines in the four panels of Figure are all less than 1, again indicating that the C18 retention time is more sensitive to nCF2 and nCH2 than any of the partition phases considered here.