Dual H–O Bond Relaxation Reveals Hydration–Aggregation Dynamics of PFAS at Aqueous Interfaces: Many-Body Cooperativity

Understanding interfacial interactions between water and per- and polyfluoroalkyl substances (PFAS) is crucial for developing effective removal strategies. However, the dynamic evolution of the interfacial states during PFAS hydration and aggregation remains unclear. Here, we investigate how varying concentrations of trifluoroacetic acid, heptafluorobutyric acid, and 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy) propanoate modulate H─O bond relaxation. Perturbative resolved spectrometric refinements confirmed two distinct modes: bond contraction induced by dipolar polarization and bond elongation driven by fluoroalkyl-water lone pair repulsion, F:⇔:O. Combined with molecular dynamics simulations, the spectral evolution demonstrated that the populations of two components vary with PFAS species and dosages. Solute clustering enhances polarization while diminishing the F:⇔:O interaction, consequently reducing the elongated-to-contracted bond ratios (<i>R</i>_e/c). This ratio serves as a quantitative descriptor for differentiating hydration and clustering of PFAS solutes. These findings advance the understanding of PFAS-water interactions and establish a spectroscopic framework resolving the structural dynamics of fluorinated compounds.