An integrated molecular characterization and simulation study of chain length effects on PFAS adsorption at hydrophobic interfaces

The molecular behavior of per- and polyfluoroalkyl substances (PFAS) at solid-liquid interfaces is key to understanding their environmental fate and removal by sorption-based treatment. This study investigates the impact of PFAS chain length on adsorption at a model hydrophobic surface formed by a methyl-terminated self-assembled monolayer (CH₃-SAM). Sorption of perfluorohexanoic acid (PFHxA, C6) and perfluorododecanoic acid (PFDoDA, C12) was measured using quartz crystal microbalance with dissipation (QCM-D), and the interaction mechanisms were examined by atomic force microscopy (AFM) and all-atom molecular dynamics (MD) simulation. QCM-D isotherms show that the longer-chain PFDoDA exhibits four times higher sorption capacity and more spatially heterogeneous adsorption than PFHxA, whereas PFHxA adsorbs more rapidly, as captured by the Boltzmann-sigmoidal kinetics. Explicit-solvent MD simulations reproduced the PFHxA isotherm observed in QCM-D at comparable concentrations and revealed PFAS aggregation already at early stages of adsorption on CH₃-SAM with carboxyl head groups oriented toward the aqueous phase. At equilibrium, PFDoDA presented more aggregated adsorption than PFHxA, maintaining a predominantly head-up configuration and a higher probability of tail insertion into the CH₃-SAM. AFM force spectroscopy showed that PFDoDA-coated surfaces present higher long-range repulsion (80–100 nm) than those coated by PFHxA (<10 nm), consistent with MD simulation showing a higher density of exposed PFDoDA head groups at the interface. These complementary methodologies provide a molecular-scale picture of PFAS adsorption and interfacial structuring, revealing how chain length governs not only adsorption capacity but also aggregation, orientation, and insertion at hydrophobic surfaces.