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 groupof chemicalsof high environmental concern. However, reliable data for the air/waterpartition coefficients (K (aw)), which arerequired for fate, exposure, and risk analysis, are available foronly a few PFAS. In this study, K (aw) valuesat 25 degrees C were determined for 21 neutral PFAS by using the hexadecane/air/waterthermodynamic 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 dividedby hexadecane/air partition coefficients (K (Hxd/air)) to obtain K (aw) values over 7 ordersof magnitude (10(-4.9) to 10(2.3)). Comparisonto predicted K (aw) values by four modelsshowed that the quantum chemically based COSMOtherm model stood out for accuracy with a root-mean-squared error (RMSE)of 0.42 log units, as compared to HenryWin, OPERA, and the linearsolvation energy relationship with predicted descriptors (RMSE, 1.28-2.23).The results indicate the advantage of a theoretical model over empiricalmodels for a data-poor class like PFAS and the importance of experimentallyfilling data gaps in the chemical domain of environmental interest. K (aw) values for 222 neutral (or neutral speciesof) PFAS were predicted using COSMOtherm as currentbest estimates for practical and regulatory use. New data for air/water partition coefficientsfor 21 PFASdemonstrated accurate prediction by quantum chemically based COSMOtherm and lower performance of empirical fit models.