¹⁹F NMR Characterization of the Encapsulation of Emerging Perfluoroethercarboxylic Acids by Cyclodextrins

Legacy perfluoroalkyl substances (PFASs) are known environmental pollutants with serious adverse health effects. Perfluoroethercarboxylic acids (PFECAs), emerging PFASs now being substituted for legacy PFASs, have recently been detected in the environment. Cyclodextrins (CDs) have been proposed as agents for the remediation of problematic pollutants, including legacy PFASs. The current study uses ¹⁹F NMR spectroscopy to measure the complexation of mono-, di-, and triether PFECAs by CDs for eventual environmental applications. Eight PFECAs were characterized by ¹⁹F and ¹³C NMR. The change in chemical shift of individual fluorines upon complexation of CDs at various stoichiometric ratios was used to determine the host-guest association constants. All studied PFECAs were most strongly encapsulated by β-CD, with association constants from 10²-10⁵ M^-1 depending on chain length and number of ether functionalities. ¹⁹F-¹H heteronuclear Overhauser effect spectroscopy (HOESY) NMR experiments were performed for the β-CD complexes of two branched monoethers, PFPrOPrA ("GenX") and PFDMMOBA, to elucidate the structural details of the complexes, determine the specific orientation, and position of β-CD along the PFAS chain, and assess the roles of hydrogen-bonding and PFECA branching on the host-guest interactions. The results give new understanding into the fundamental nature of the host-guest complex between cyclodextrins and perfluorinated surfactants.