This study provided a large set of C18 retention factors as an empirical hydrophobicity index for anionic PFAS. The k0 value, which is proportional to the C18-bonded silica/water partition coefficient, shows a correlation with other partition coefficients, demonstrating its potential as a predictive descriptor for anionic Ebert et al., Le et al. (Na and K-salt) and Nguyen et al. (Soil S6, pH 7.2). PFAS partitioning. It is of interest whether the correlations shown in Figure hold when more diverse anionic compounds are included. The actual intermolecular interactions that occur between PFAS and the phospholipid membrane, air/water interface, or soil organic matter should be different from those between PFAS and the C18-bonded silica. The simple correlations presented can be considered a first step in model development toward a more comprehensive multiparameter model for partitioning of anionic substances in general.