log k = nCF2 (a1 φMeOH 2 + b1 φMeOH + c1) + a2 φMeOH 2 + b2 φMeOH + c2 (5) Transforming eq 5, we obtain, log k = (a1 nCF2 + a2)φMeOH 2 + (b1 nCF2 + b2)φMeOH + (c1 nCF2 + c2) (6) The six fitting parameters (a1, a2, b1, b2, c1, c2) were obtained by nonlinear least-squares fitting using the nls() function of the statistical software R. Using eq 6, k0 for a given compound can be extrapolated as a cross section of the two conditions represented by eqs 3 and 4, based on all data available for the respective class of compounds and is thus expected to be more accurate than k0 determined on an individual compound basis using eq 3. As an example, the two panels of Figure show the same experimental data but with different fitted models (eq 3 and eq 6). Fitting eq 3 to individual compounds resulted in broad 95% CI bands for long-chain compounds in the low φMeOH range, demonstrating the problem of extrapolation. In contrast, the CI bands were much narrower when eq 6 was used, due to the additional model constraint represented by eq 4 as well as the large amount of the data. Note that perfluoroalkyl ethers (PFECAs and PFESAs) are not a group of compounds with just different perfluoroalkyl chain lengths, but with more variation in terms of, e.g., the number of ether oxygens and the presence of a partially fluorinated carbon atom. For these compounds, eq 6 cannot be used, and thus k0 had to be estimated compound by compound using eq 3. The values of k25 and k50 were also obtained from eq 6 or eq 3. Apparently, there must be at least 3 data points for eq 3 to fit. We were not able to obtain k0, k25, and k50 values for any of diPAPs because only a few data at high φMeOH were available for these strongly sorbing PFAS.