Unification of surface tension isotherms of PFOA or GenX salts in electrolyte solutions by mean ionic activity

The surface tension isotherms of soluble salts of per- and polyfluoroalkyl substances (PFAS) in electrolyte solutions are typically reported as functions of the PFAS concentration. However, for univalent salts and electrolytes, the Langmuir-Szyszkowski equation is a function of the mean ionic activity a*. Using previously reported data, we show that for salts of perfluorooctanoic acid (PFOA) or hexafluoropropylene oxide dimer acid (GenX™), use of a* rather than concentration provides a unified surface tension isotherm, independent of the electrolyte concentration. This suggests that the electrolyte dependence of the isotherm arises purely from its effect on PFAS activity, rather than an intrinsic surface property. This finding has important implications for the understanding of PFAS retention in saline unsaturated soils, and for PFAS extraction from saline waters by foam fractionation. [Display omitted] •Reported PFAS surface tension isotherms replotted as functions of mean ionic activity.•One master curve found for various PFOA or GenX surface tension data in electrolyte solutions.•Electrolyte dependence of PFAS surface tension isotherm arises purely from its effect on PFAS activity.•Unified isotherms contribute to simulating PFAS transport and retention in unsaturated zone.