Mechanistic insights into electrolyte–PFAS interactions at the air–water interface

Abstract

Understanding how electrolytes modulate the interfacial behavior of per- and polyfluoroalkyl substances (PFAS) is critical for predicting their environmental fate and guiding remediation strategies. To this end, we have measured and modeled the surface tension and interfacial partitioning of four PFAS regulated under US and European drinking water guidelines, as well as a C4 PFAS, in the presence of eight inorganic salts designed to capture a broad range of cationic and anionic identities. The surface tension data were fitted with the Szyszkowski equation, extended with the mean ionic activity coefficient a* to account for electrolyte influence. For the long-chain species, estimation of parameters yielded very diverse values, which can be attributed to ion-specific effects and limitations of the model. Multivariate analysis of the data revealed clear trends dominated by the PFAS chain length and ion type. Density functional theory simulations provide a molecular-level understanding that cations influence interfacial adsorption via electrostatic binding and, in some cases, polarize orbitals or co-localize at the interface as a function of the cation's hydration characteristics and electronic structure. These results illustrate the failure of traditional activity-based models for long-chain PFAS and highlight the need to revisit the role of specific ion effects in interfacial transport. Our combined experimental–theoretical approach paves way for a mechanistic understanding of PFAS–electrolyte interactions and provides fresh insights into their behavior at the air–water interface.