Adsorption Mechanism of Perfluorooctanoate on Cyclodextrin-Based Polymers: Probing the Synergy of Electrostatic and Hydrophobic Interactions with Molecular Dynamics Simulations

Contamination of natural water resources by per- and polyfluorinatedalkyl substances (PFAS) has affected millions of people around the world andemphasized the need for development of novel and effective adsorbent materials. Wedemonstrate how atomistic molecular dynamics (MD) simulations can be used toprovide molecular scale insight into the role of electrostatic and hydrophobicinteractions on the adsorption of the perfluorooctanoate (PFOA) surfactant, aprominent longer-chain PFAS, on a polymer-based network in water. Specifically, theadsorption of ammonium perfluorooctanoate salt has been investigated on the beta-cyclodextrin (CD) network cross-linked with decafluorobiphenyl linkers as anexample of an absorbent material that has already demonstrated efficient PFASadsorption. Examination of pairwise interactions reveals the importance of the dualpronged adsorption mechanism involving both electrostatic and hydrophobicinteractions. The adsorption of ammonium counterions on the CD segmentsfacilitates attraction of the anionic headgroup of the PFOA surfactant, whilefluorinated linkers provide an additionalhydrophobic attraction for the PFOA tail as well as higher affinity of the network toward PFOA in comparison withhydrocarbons. These competing interactions result in PFOA adsorption primarily outside of the CD cavity with the PFOA tailmostly interacting withfluorinated linkers. We demonstrate that simulations using"what if"scenarios are a powerful approachto infer the role of different interactions in the adsorption of PFAS