Experimental Determination of Air/Water Partition Coefficients for 21 Per- and Polyfluoroalkyl Substances Reveals Variable Performance of Property Prediction Models.

Per- and polyfluoroalkyl substances (PFAS) are a group of chemicals of high environmental concern. However, reliable data for the air/water partition coefficients ( K aw ), which are required for fate, exposure, and risk analysis, are available for only a few PFAS. In this study, K aw values at 25 °C were determined for 21 neutral PFAS by using the hexadecane/air/water thermodynamic cycle. Hexadecane/water partition coefficients ( K Hxd/w ) were measured with batch partition, shared-headspace, and/or modified variable phase ratio headspace methods and were divided by hexadecane/air partition coefficients ( K Hxd/air ) to obtain K aw values over 7 orders of magnitude (10 -4.9 to 10 2.3 ). Comparison to predicted K aw values by four models showed that the quantum chemically based COSMO therm model stood out for accuracy with a root-mean-squared error (RMSE) of 0.42 log units, as compared to HenryWin, OPERA, and the linear solvation energy relationship with predicted descriptors (RMSE, 1.28-2.23). The results indicate the advantage of a theoretical model over empirical models for a data-poor class like PFAS and the importance of experimentally filling data gaps in the chemical domain of environmental interest. K aw values for 222 neutral (or neutral species of) PFAS were predicted using COSMO therm as current best estimates for practical and regulatory use.