Surface functional group dependent enthalpic and entropic contributions to molecular adsorption on colloidal microplastics

Molecular interaction with micro- and nano-plastics (MNPs) is an important chemical process that dictates the fate and transport of organic contaminants, and that of MNPs, within the aquatic environment. In this study, adsorption of cationic, anionic, and neutrally charged organic molecules from aqueous solution to model microplastics (MPs) is presented. Second harmonic generation, an interfacial selective laser-based technique that allows <i>in situ</i> measurements of adsorption isotherms, has been used. Polystyrene (PS) and polymethyl methacrylate (PMMA) microplastic particles with distinct charge characteristics were chosen. PS MPs investigated include distinct surface functional groups such as -OSO₃, -COOH, mixed -NH₂/-COOH, and amidine. Thus, the effect of different types of electrostatic interactions on adsorption have been analyzed. Comparison of adsorption Gibbs free energy for these MPs reveals that Coulombic attraction is important for spontaneous adsorption. However, distribution of water at the interfacial region, and functional group dependent interactions determine the magnitude of binding strengths. Entropic (Δ<i>S</i>_ads) and enthalpic (Δ<i>H</i>_ads) contributions to spontaneous adsorption have been determined by analyzing temperature-dependent adsorption isotherms. We show that Δ<i>H</i>_ads and Δ<i>S</i>_ads are dependent on the interfacial chemical composition of MPs and are not constant with respect to temperature. Although of varying degrees, MPs studied show an increase in entropy upon adsorption of organic molecules. These findings hint at a plausible influence of hair-like structure, common in polymeric soft matter, on the adsorption mechanism. This systematic study thus underscores unique colloidal features of the plastic/aqueous interface that are critical in adsorption of organic molecules by microplastics.