Influence of Polar Hydrophobicity of PFAS Tails on Adsorption at Electrostatic-Hydrophobic Interfaces

The adsorption of perfluoroalkyl acids (PFAAs) onto surfaces is conventionally attributed to electrostatic interactions from anionic headgroups and hydrophobic effects from fluorocarbon tails. Fluorocarbon chains may also exhibit “polar hydrophobicity”, a dual character arising from their local C–F bond polarity and overall hydrophobicity. However, its role in PFAAs surface adsorption remains poorly understood. Here, we investigated the contribution of fluorocarbon-chain polar hydrophobicity to PFAAs adsorption at a quaternary ammonium-modified graphene interface, focusing on chain-length dependence and conformational diversity. Using dispersion-corrected density functional theory and molecular dynamics simulations, we show that long-chain PFAAs interact with these interfaces primarily through van der Waals interactions together with weak C–F polar interactions, alongside electrostatic contributions from the anionic headgroup. Solvent-driven hydrophobic effects orient fluorocarbon tails toward the interface, where stabilization occurs through increased contact areas, enhanced van der Waals interactions, and C–F dipole-associated contributions. Short-chain PFAAs, by contrast, exhibit relatively stronger polar contributions from both their anionic headgroups and fluorocarbon chains. Overall, our results suggest that quaternary ammonium-modified carbon enhances PFAAs adsorption not only through direct electrostatic attraction, but also by amplifying interfacial van der Waals interactions. These findings offer molecular-level insights for designing rational adsorbents and for understanding PFAA fate and transport in complex environments.