Determining the Temperature-Dependent Air–Water Partitioning of Ether- and Thioether-Alcohol Perfluoroalkyl and Polyfluoroalkyl Substances Using a Modified Static Headspace Method

The temperature-dependent air–water partitioning behavior of a novel class of perfluoroalkyl and polyfluoroalkyl substances (PFAS) was assessed both experimentally and via in silico prediction. These PFAS contain ether or thioether linkages and are transformation products of an alternative PFAS surfactant. A modified version of the static headspace method with variable headspace/solution ratios was used to determine the dimensionless air/water partition coefficients (K_aw) over a wide range of temperatures (25–80 °C). The samples were analyzed through the aqueous phase instead of the headspace because of their relatively low volatility. The obtained log K_awvalues of the tested chemicals ranged from −2.6 to −1.0 at 25 °C. No differences in K_awwere observed between ether and thioether congeners with the same perfluorinated carbon chain length. Increasing the length of the perfluorinated carbon chain from CF₃- to C₃F₇- increased K_aw by about 1.5 log units. The obtained K_aw of a well-studied fluorotelomer alcohol, 4:2 FTOH, matched those of previous studies, indicating the appropriateness of the method used. The temperature dependence of K_aw, as quantified by the molar internal energy change of air–water partitioning, ΔU, ranged from 20 to 37 kJ/mol and was not substantially influenced by the structure of the chemicals. Among five in silico tools to predict air–water partitioning, the quantum chemistry-based COSMO therm ensured the most reliable and accurate prediction as compared to the experimental results.