Polyelectrolyte Nature of Model Colloidal Plastics and Their Impact on Molecular Adsorption

Abstract Micro- and nanoplastics (MNPs) are pervasive environmental pollutants, yet their interfacial structures and molecular adsorption behaviors remain poorly understood. This study investigates the polyelectrolyte nature of model colloidal micro- and nanoplastics (CMNPs)─specifically polystyrene particles featuring plain, sulfate, or carboxylate surface functional groups─and examines how these features dictate molecular adsorption and electrokinetic properties. Using a combination of in situ dynamic light scattering (DLS), electrophoretic mobility measurements, surface-selective second harmonic generation (SHG) spectroscopy, and by relying on previous theoretical and experimental work, we demonstrate that CMNPs possess a soft, hydrated, polyelectrolyte-rich interfacial layer. This brush-like or “hairy” architecture persists regardless of the specific surface functional group and fundamentally alters particle behavior in aqueous media. Electrokinetic measurements reveal nonclassical trends in electrophoretic mobility under increasing electrolyte concentrations, matching theoretical frameworks for soft colloids while contradicting classical hard sphere models. Furthermore, SHG adsorption isotherms for a model cationic dye (DASPI) show that adsorption affinity is governed by a combination of electrostatic and entropic effects, with hydrogen bonding serving as a strong possibility for carboxylated particles. Notably, the adsorption affinity is less sensitive to electrolyte concentration than predicted by classical models, indicating that volumetric charge density and interfacial hydration are likely the primary determinants governing organic molecule adsorption at the CMNP/aqueous interface. By overturning the conventional view of microplastics as rigid, inert particles, these findings highlight the critical importance of accounting for soft, hydrated interfaces when predicting the transport, environmental fate, and contaminant interactions of plastic pollution in natural waters.