Lignin-stabilized polystyrene dispersions as nanoplastic reference materials

In natural aquatic systems, nanoplastics rapidly acquire organic coatings or so-called eco-coronas that govern their environmental behavior. In contrast, laboratory-made reference nanoplastics rely on artificial surfactants, forming non-natural coatings that enable small size and colloidal stability while failing to replicate these environmentally relevant interfaces. To address this limitation, we developed environmentally relevant polystyrene (PS) nanoplastic models by incorporating three lignin biopolymers, including softwood kraft lignin (SKL), hardwood kraft lignin (HKL), and acetylated softwood kraft lignin (ACL), through anti-solvent precipitation. By combining gravimetric analysis, machine learning-assisted particle sizing, and high-resolution interfacial analysis, we propose mechanisms governing dispersion stability. Systems that formed thick or continuous layers around PS particles showed poor performance, as ACL induced bridging-driven aggregation (~42% recovery), while HKL promoted strong self-association (~22% recovery). In contrast, an optimal 2% SKL formulation achieved high mass recovery (~90%, compared to ~46% for the uncoated PS benchmark) and produced uniform, well-dispersed particles (median size ~110 nm). Notably, SKL formed a thin stabilizing layer together with a distinct formation of lignin nanoparticle surrounding the PS cores. This interfacial architecture enhanced electrosteric stabilization while maintaining colloidal stability. At higher SKL loadings, excess lignin disrupted this balance, leading to reduced stability likely due to depletion flocculation caused by enrichment of free lignin nanoparticles in the continuous phase. Overall, this work provides a mechanistic basis for designing environmentally relevant nanoplastics and highlights the potential of lignin as a sustainable material for stabilizing and studying such systems.