Hydrophobic Ion Pairing with Cationic Surfactant Enhances the Octanol–Water Distribution Coefficient of PFAS

Cationic additives are increasingly used to enhance per- and polyfluoroalkyl substances (PFAS) removal, particularly for short-chain PFAS; however, the mechanisms underlying this enhancement remain poorly understood. Here, we quantitatively evaluate how hydrophobic ion pairing (HIP) with cationic additives alters PFAS hydrophobicity by measuring apparent octanol–water distribution coefficients (DOW). Seven PFAS (C4–C8), including perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, and hexafluoropropylene oxide dimer acid (GenX), were evaluated with cetyltrimethylammonium chloride (CTAC). Additive structure was further evaluated using C4–C18 quaternary ammonium compounds (QACs) and biodegradable ethyl lauroyl arginate (LAE), all below their critical micelle concentrations. At a 1:1 PFAS ratio (1 μM each), apparent log DOW increased by 0.28–0.79 log units across all PFAS, including 0.79 for perfluorobutanesulfonate (PFBS), 0.47 for perfluorobutanoate (PFBA), and 0.53 for GenX, with further increases at higher CTAC concentrations. For PFBS, HIP effects were greatest near neutral pH and decreased at ionic strengths ≥0.01 M, consistent with electrostatic screening and competition from inorganic ions. CTAC also increased PFBS apparent log DOW in surface water and groundwater, demonstrating that HIP can increase PFAS hydrophobicity under environmentally relevant conditions and supporting its use as a pretreatment to enhance adsorption and interfacial separation of less hydrophobic PFAS.