Conformation-dependent Adsorption of Long-chain PFAS via Polar Hydrophobic Interactions

The adsorption of perfluoroalkyl acids (PFAAs) is typically attributed to electrostatic interactions of their anionic headgroups and hydrophobicity of their fluorocarbon tails. However, the fluorocarbon chain also exhibits a unique “polar hydrophobicity”, a dual character that could significantly influence surface adsorption chemistry. In this study, we show that conformational variations of PFAAs enable polar interactions between fluorocarbon tails and polarized substrates, thereby manifesting this polar hydrophobicity. Using dispersion-corrected density functional theory (DFT-D) and molecular dynamics (MD) simulations, we found that bent conformers of long-chain PFAAs (also detected by 19F-NMR) interact with electrostatic-hydrophobic interfaces through combined electrostatic (including hydrogen bonding) and van der Waals forces. In contrast, short-chain PFAAs exhibited predominantly headgroup-driven electrostatic adsorption consistent with conventional concepts. These findings demonstrate that conformational dynamics govern the polar hydrophobicity of long-chain PFAAs and their selective adsorption at electrostatic-hydrophobic interfaces, providing molecular-level insights for advanced adsorbent design and improved understanding of PFAS fate and transport in complex environments.