Molecular Interactions of Perfluorinated and Branched\nFluorine-Free Surfactants at Interfaces: Insights from a New Reliable\nForce Field

Per-\nand polyfluoroalkyl substances (PFAS) constitute a class of\nsynthetic compounds with exceptional interfacial properties. Their\nwidespread use in many industrial applications and consumer products,\ncombined with their remarkable chemical and thermal stability, has\nled to their ubiquitous presence in environmental matrices, including\nsurface water and groundwater. To replace PFAS with fluorine-free\nsurfactants, it is necessary first to develop a deep molecular-level\nunderstanding of the mechanisms responsible for the exceptional properties\nof PFAS. For instance, it has been shown that fluorine-free surfactants\nwith highly branched or methylated chains can achieve low surface\ntensions at air–water interfaces and can provide highly hydrophobic\nsurface coatings. Although molecular simulations combined with experiments\nare promising for uncovering these mechanisms, the reliability of\nsimulation results depends strongly on the accuracy of the force fields\nimplemented. At the moment, atomistic force fields are not available\nto describe PFAS in a variety of environments. Ab initio methods could\nhelp fill this knowledge gap, but they are computationally demanding.\nAs an alternative, ab initio calculations could be used to develop\naccurate force fields for atomistic simulations. In this work, a new\nalgorithm is proposed, which, built from accurate ab initio calculations,\nyields force fields for perfluorinated sulfonic and perfluoroalkyl\nacids. The accuracy of the new force field was benchmarked against\nsolvation free energy and interfacial tension data. The new force\nfields were then used to probe the interfacial behavior of the PFAS\nsurfactants. The interfacial properties observed in our simulations\nwere compared with those manifested by two branched fluorine-free\nsurfactants. The good agreement achieved with experiments and ab initio\ncalculations suggests that the proposed protocol could be implemented\nto study other perfluorinated substances and help in the design of\nfluorine-free surfactants for targeted applications.