Adsorption Mechanism of a Perfluorinated Carboxylic Acid on Charged Polystyrene Surfaces in Aquatic Environment: A Combined Experimental and Computer Simulation Investigation

Interactions between per- and polyfluoroalkyl substances (PFASs) and small plastic particles (PPs) in the micro- and nanosize ranges raise serious concerns due to the possible joint harmful physiological effects of these emerging contaminants on the ecosystem. In this study, the adsorption of undecafluorohexanoic acid (UFHA), a C6 representative of PFASs, on amidine-functionalized polystyrene PP is investigated by using experimental and computer simulation methods. Electrophoretic light scattering measurements reveal that UFHA adsorbs strongly on oppositely charged PP leading to charge neutralization and overcharging at appropriate concentrations. Charged UFHA–PP adducts form stable colloids, while rapid aggregation was observed at the isoelectric point. Simulation results indicate that, at least at isoelectric conditions, a H-bond is formed between the oppositely charged groups of the PP and UFHA, providing an additional driving force, besides charge pairing, for this adsorption. This strong interaction results in an association of the PP chains and the majority of UFHA molecules, although some of the UFHA molecules can still stay isolated from these associates. It is found that the ionic strength applied in the solutions influences the adsorption process through charge screening of the PP surface, while it does not affect the charge neutralization conditions. Accordingly, the spatial distribution of the Na⁺ and Cl^– ions is found to be largely uncorrelated with that of the polymeric chains and UFHA molecules at every salt concentration studied. The findings obtained provide new insights into the adsorption mechanism of UFHA onto a charged polystyrene surface and the features of the new interfaces formed, which play a key role in the joint migration and toxicity of these emerging contaminants.