The influence of molecular structure on the adsorption of PFAS to fluid-fluid interfaces: Using QSPR to predict interfacial adsorption coefficients

Per- and poly-fluoroalkyl substances (PFAS) are emerging contaminants of critical concern for human health risk. Assessing exposure risk requires a thorough understanding of the transport and fate behavior of PFAS in the environment. Adsorption to fluid-fluid interfaces, which include air-water, OIL-water, and air-OIL interfaces (where OIL represents organic immiscible liquid), is a potentially significant retention process for PFAS transport. Fluid-fluid interfacial adsorption coefficients (K_i) are required for use in transport modeling and risk characterization, yet these data are currently not available for the vast majority of PFAS. Surface-tension and interfacial-tension data sets collected from the literature were used to determine interfacial adsorption coefficients for 42 individual PFAS. The PFAS evaluated comprise homologous series of perfluorocarboxylates and perfluorosulfonates, branched perfluoroalkyls, polyfluoroalkyls, alcohol PFAS, and nonionic PFAS. The K_i values vary across eight orders of magnitude, and are a function of molecular structure. The results of quantitative-structure/property-relationship (QSPR) analysis demonstrate that a model employing molar volume (V_m) as a descriptor provides robust predictions of log K_i values for air-water interfacial adsorption of the wide range of PFAS. The model also produced good predictions for a limited set of data for OIL-water interfacial adsorption. The predictive capability of the QSPR model for a wide range of PFAS with greatly varying structures reflects the fact that molar volume provides a reasonable representation of the influence of molecular size on cavity formation/destruction in solution, and thus the hydrophobic-interaction driving force for interfacial adsorption. The QSPR model presented herein provides a means to incorporate the fluid-fluid interfacial adsorption process into transport characterization and risk assessment of PFAS in the environment. This will be particularly relevant for determining PFAS mass flux in the atmosphere, in the vadose zone, in source zones containing organic immiscible liquids, and in water/wastewater treatment systems.