The values of log k0 are linearly related to the number of CF2 (nCF2) for PFAS and CH2 (nCH2) for non-PFAS (Figure ). The slope of log k0 against nCF2 was 0.94 (mean of PFCAs and PFSAs) and that for nCH2 was 0.66 (mean of SOs, SAs, and CAs). A higher slope for PFAS than non-PFAS indicates a stronger hydrophobic effect on CF2 than CH2. These results agree qualitatively with those of Hammer et al., who reported incremental values of 0.56/CF2 and 0.47/CH2 for log k0 on C18. The difference from the result of the present study may be due to the different experimental setups or, in part, to the long-range linear extrapolations from high methanol concentrations made by Hammer et al. In the literature, the log of the hexadecane/water partition coefficient has been reported to increase by 0.74/CF2 and 0.64/CH2 for neutral compounds. Similarly, log Kow increases by ~0.6/CF2 and 0.54/CH2 23 for neutral compounds. The log k0 derived in the current study appears to be relatively sensitive to the number of perfluorocarbon atoms, which may indicate that the cavity formation energy required for a perfluoroalkyl compound to partition into the C18 phase is low compared to the other phases mentioned above.