Free-Energy Profiles of perfluoroalkyl substances Interactions on Polyethylene Surfaces from Molecular Dynamics

Microplastics (MPs) and per- and polyfluoroalkyl substances (PFAS) may coexist in environmental systems at low concentrations. In this context, we investigated how two widely detected PFAS, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), interact with virgin polyethylene (PE) immersed in water under infinite-dilution conditions. Given the pronounced size difference between PFAS molecules and PE particles, the polymer was represented as a slab from the molecular perspective of the contaminants. To characterize the systems, density profiles of PE and water were computed, revealing regions of increased polymer packing associated with chain alignment in both crystalline and semicrystalline models. Molecular orientation was examined using a cylindrical sampling region embedded within the polymer matrix. In most models, the fluoroalkyl tails of PFOA and PFOS tended to align with PE chains, while their hydrophilic heads remained oriented toward the aqueous phase. The potential of mean force (PMF) profiles exhibited similar profile shape behavior across systems but showed distinct free-energy minima. For PFOA, the minima were approximately –30 kJ mol⁻¹ in the semicrystalline model and –24 kJ mol⁻¹ in the crystalline model. For PFOS, the corresponding values were –46 kJ mol⁻¹ and –37 kJ mol⁻¹, respectively, indicating a stronger affinity for the semicrystalline structure. These results demonstrate that polymer microstructure, combined with the chemical characteristics of PFAS functional groups, governs the interaction strength at the PE–water interface. Overall, the findings suggest that MPs can act as reservoirs for PFAS, contributing to their persistence and transport in aquatic environments.