A computational investigation of PFAS dimer interaction energies

Starting from the late 1930s, per- and polyfluoroalkyl substances (PFASs) have had a long history of use. Long thought to be safe for use in commercial and industrial applications, they have recently been recognized as a contaminant of concern with an urgent need for remediation technologies. However, this is a challenging problem as the molecular forces that govern PFAS interactions are poorly understood. Thus, in this study we seek to understand these forces and how they influence PFAS-PFAS aggregation in the presence of an aqueous environment. Using a combination of molecular dynamics simulations and detailed quantum mechanical calculations allows us to demonstrate that the nature of the PFAS-PFAS interaction lies in the PFAS chain length as well as the nature of the head-group (carboxylic acid vs. sulfonated). Utilization of the symmetry-adapted perturbation theory (SAPT) method allows us to determine that the PFAS-PFAS interaction in the presence of water is driven by electrostatic and inductive factors, which is also supported by an analysis of the molecular dynamics simulations. Moreover, we find that carboxylic acid PFAS aggregates are likely to favor parallel orientation of the head-groups, whereas for sulfonated PFAS aggregates are likely to orient head groups away from each other. Solvation in an aqueous environment serves to mitigate the effects of anionic repulsion from the head groups, which favors conformations maximizing the hydrophobic consolidation between the tail groups with potential of mean force barriers on the order of 1 kcal mol^-1, indicating rapid dynamics and exchange of the aggregation process.