Weathering effects of secondary microplastics from expanded polystyrene foam on interactions with palmitic acid monolayers

Microplastics (MPs) and nanoplastics (NPs) accumulate at environmental interfaces such the sea surface microlayer (SSML), which contains surface active compounds and natural and anthropogenic matter. Studying interactions at environmental interfaces requires well-characterized, representative MPs and interfacial compositions containing natural surfactants. We describe a process for generating size-fractionated MPs and NPs (5–10 μm, 1–5 μm, and < 1 µm) by colloidal ball-milling using expanded polystyrene foam plates that were photochemically weathered. The chemical and physical properties of the macroplastic and each MP fraction are reported. Colloidal dispersions were prepared with the 1–5 μm size fraction and MP interactions were examined with palmitic acid monolayers and compared to commercial polystyrene microbeads. Long-chain fatty acids are abundant in nature and found in the SSML and in sea spray aerosols. Weathered MPs led to the greatest change in the surface pressure isotherms due to their colloidal stability, which inhibited aggregation prior to partitioning to the monolayer, and negative surface charge, which led to repulsive interactions within the monolayer. All MPs increased the elastic compressibility of the monolayer, indicating a disruption of palmitic acid orientation at the air/water interface. Transformations and interactions with SSMLs are expected to provide insight into the environmental fate and ecological impacts of MPs in marine ecosystems. • Size fractionated non-spherical MPs and NPs were prepared from polystyrene foam. • MP colloidal properties and interactions with palmitic acid monolayers were examined. • Weathered MPs were colloidally stable and showed greater monolayer interactions. • Interaction with MPs changed the surface pressure and elasticity of the PA monolayer.