Adsorption behavior of long-chain perfluoroalkyl substances on hydrophobic surface: A combined molecular characterization and simulation study

Hydrophobic interaction is a prevalent sorption mechanism of poly- and perfluoroalkyl substances (PFAS) in natural and engineered environments. In this study, we combined quartz crystal microbalance with dissipation (QCM-D), atomic force microscope (AFM) with force mapping, and molecular dynamics (MD) simulation to probe the molecular behavior of PFAS at the hydrophobic interface. On a CH₃-terminated self-assembled monolayer (SAM), perfluorononanoic acid (PFNA) showed ∼2-fold higher adsorption than perfluorooctane sulfonate (PFOS) that has the same fluorocarbon tail length but a different head group. Kinetic modeling using the linearized Avrami model suggests that the PFNA/PFOS-surface interaction mechanisms can evolve over time. This is confirmed by AFM force-distance measurements, which shows that while the adsorbed PFNA/PFOS molecules mostly lay flat, a portion of them formed aggregates/hierarchical structures of 1–10 nm in size after lateral diffusion on surface. PFOS showed a higher affinity to aggregate than PFNA. Association with air nanobubbles is observed for PFOS but not PFNA. MD simulations further showed that PFNA has a greater tendency than PFOS to have its tail inserted into the hydrophobic SAM, which can enhance adsorption but limit lateral diffusion, consistent with the relative behavior of PFNA/PFOS in QCM and AFM experiments. This integrative QCM-AFM-MD study reveals that the interfacial behavior of PFAS molecules can be heterogeneous even on a relatively homogeneous surface.