Measurement of Aqueous Diffusivities for Perfluoroalkyl Acids

Aqueous diffusivities were experimentally determined for perfluoroalkyl acids (PFAAs) with perfluorinated carbon chain lengths ranging from 3 to 8, including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). The capillary method was employed at room temperature. Measured aqueous diffusivities ranged from 2.5x10-5 cm2/s (perfluorobutanoic acid) to 0.45x10-5 cm2/s (perfluorohexane sulfonic acid). The experimental results were in good agreement with aqueous diffusivities determined from previously performed experiments that measured PFAA uptake into activated carbon. Although molecular simulations for PFOA and PFOS were in reasonable agreement with experimental measurements, the Wilke-Chang model was not able to accurately predict the values nor describe the trend for the PFAAs used in this study. The nonuniform trend in the PFAA diffusivities with respect to the molar volumes likely is reflective of unique molecular interactions associated with highly fluorinated compounds.