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 polyfluorinated alkyl substances (PFAS) has affected millions of people around the world and emphasized the need for development of novel and effective adsorbent materials. We demonstrate how atomistic molecular dynamics (MD) simulations can be used to provide molecular scale insight into the role of electrostatic and hydrophobic interactions on the adsorption of the perfluorooctanoate (PFOA) surfactant, a prominent longer-chain PFAS, on a polymer-based network in water. Specifically, the adsorption of ammonium perfluorooctanoate salt has been investigated on the β-cyclodextrin (CD) network cross-linked with decafluorobiphenyl linkers as an example of an absorbent material that has already demonstrated efficient PFAS adsorption. Examination of pairwise interactions reveals the importance of the dual pronged adsorption mechanism involving both electrostatic and hydrophobic interactions. The adsorption of ammonium counterions on the CD segments facilitates attraction of the anionic headgroup of the PFOA surfactant, while fluorinated linkers provide an additional hydrophobic attraction for the PFOA tail as well as higher affinity of the network toward PFOA in comparison with hydrocarbons. These competing interactions result in PFOA adsorption primarily outside of the CD cavity with the PFOA tail mostly interacting with fluorinated linkers. We demonstrate that simulations using "what if" scenarios are a powerful approach to infer the role of different interactions in the adsorption of PFAS.