Aggregation,\nNot Micellization: Perfluorooctanoic\nAcid, Perfluorobutanesulfonic Acid, and Potassium Perfluorooctanesulfonate\nBehavior in Aqueous Solution

Surface tension, conductivity, and dynamic light scattering\n(DLS)\nmeasurements were used to examine the surface and bulk solution behaviors\nof three members of the PFAS family, perfluorooctanoic acid (PFOA),\nperfluorobutanesulfonic acid (PFBS), and the potassium salt of perfluorooctanesulfonic\nacid (PFOS). Measurements were carried out in solutions having variable\n(acidic) pH and in solutions buffered to pH = 8.0. Surface tension\ndata show traditional soluble surfactant behavior, and results illustrate\nthat PFOA, PFBS, and PFOS surface activity depends sensitively on\nsolution phase pH. The tightly packed monolayers formed by PFOA in\nmildly acidic solutions imply that the surface pH of PFOA solutions\nis several units lower than bulk. Results from conductivity experiments\ngenerally show increasing conductivity with increasing bulk solution\nsurfactant concentration. In pH = 8.0 solutions, changes in conductivity\nslope with surfactant concentration suggest the onset of micelle formation\nat concentrations <1 mM, markedly lower than reported in literature.\nIn general, apparent critical micelle concentrations (CMCs) determined\nfrom conductivity data agree with similar predictions made from surface\ntension results. DLS measurements show that at concentrations close\nto the predicted PFAS CMCs, objects with diameters ≤10 nm start\nto form. However, unlike micelles, these objects continue to grow\nwith increasing bulk solute concentration. These aggregates form structures\nhaving diameters of 50–150 nm. Aggregate size shrinks modestly\nas solution phase temperature increases, and this behavior is reversible.\nCryo-EM images of PFOA solutions confirm a broad distribution of particles,\nsupporting the DLS measurements. Findings reported in this work represent\nthe first evidence that these three EPA-regulated PFAS surfactants\nform aggregates rather than micelles in solution. Findings also begin\nto reconcile differences in reported surface behaviors that have led\nto CMC predictions in the literature varying by more than an order\nof magnitude.