Hydrophobic Ion Pairing with Cationic Surfactant Modifies Surface Activity and Colloidal Behavior of PFAS

Quaternary ammonium compounds (QACs) are increasingly used to enhance the removal of per- and polyfluoroalkyl substances (PFAS), particularly short-chain compounds, through hydrophobic ion pairing (HIP). Here, we investigated how HIP with cetyltrimethylammonium chloride (CTAC) influences the interfacial and colloidal behavior of selected PFAS at low-micromolar concentrations below reported critical micelle concentrations (CMCs) and across varying solution chemistries (pH, ionic strength, and natural organic matter) using surface tension measurements, dynamic light scattering (DLS), ζ-potential analysis, and transmission electron microscopy (TEM). Relative to the corresponding PFAS-only controls, CTAC (6.66 μM) reduced the surface tension of PFBA, PFBS, and PFOA solutions (3.33 μM each) by 24.64, 45.71, and 70.17%, respectively. PFOS exhibited little additional surface-tension response to CTAC but responded strongly to a shorter-chain QAC (C12), indicating that interfacial behavior depends on fluorocarbon–hydrocarbon chain-length compatibility. Targeted DLS, ζ-potential, and TEM characterization identified detectable bulk coassembly in selected PFAS–QAC systems at concentrations of 3.33–33.30 μM. Electrolyte addition produced PFAS-dependent changes in interfacial behavior, while NOM reduced the interfacial activity of several mixtures. These findings provide mechanistic insight into PFAS–QAC behavior relevant to the development of QAC-assisted PFAS separation processes.