Molecular-Scale Insights into the Interactions between Perfluoroalkyl Substances and Polyethylene

Microplastics (MPs) and per- and polyfluoroalkyl substances (PFAS) are two classes of highly persistent contaminants that frequently co-occur in the environment, raising concern about potential synergistic effects. To better understand their interactions, we investigated the adsorption of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) on polyethylene (PE) through molecular dynamics (MD) simulations. The potential of mean force (PMF) at infinite dilution was calculated for both the semicrystalline and crystalline PE models. For semicrystalline PE systems, the PMF minima were -26.5 ± 4.8 kJ mol^-1 for PFOA and -43.9 ± 4.3 kJ mol^-1 for PFOS, whereas, for crystalline PE, the values were -26.6 ± 5.2 and -42.0 ± 7.7 kJ mol^-1, respectively. These results indicate that, within statistical uncertainty, no significant differences are observed between the two PE morphologies for either PFAS when considering the depth of the free-energy minimum. Moreover, PFOS exhibited stronger interactions with PE than PFOA. This behavior reflects not only differences in fluoroalkyl chain length but also the distinct chemical nature of the functional groups, with the larger and more hydrophobic sulfonate headgroup of PFOS compared to the carboxylate group of PFOA. In addition to adsorption strength, molecular orientation at the PE-water interface was characterized. PFAS tails showed a general tendency to align parallel to PE chains within the polymer slab, but this alignment was disrupted upon the transition into water. Notably, PFOS interacting with semicrystalline PE exhibited orientation changes with transitions between parallel and perpendicular alignment associated with local PMF barriers. These orientation-dependent interactions highlight the importance of both chain packing and functional group chemistry in driving PFAS-polymer affinity. Taken together, these findings provide molecular-scale evidence that microplastics can act as reservoirs for PFAS, potentially enhancing their environmental persistence and transport.