Electrophoresis Characterization of Nanoplastic Particle Surface Charge in Dilute Aqueous Electrolytes

Aggregation, adsorption, and biofilm formation are involved in the fate and transport of nanoplastics in the aquatic environment. These interfacial processes are closely related to surface charge and electrical double layer (EDL) structure. As it is experimentally difficult to obtain surface potential, laser doppler electrophoresis is currently the most popular technique to measure zeta potential, or the potential at shear plane. However, the determination of zeta potential by laser doppler electrophoresis is not a trivial undertaking. Inaccurate zeta potential values could potentially lead to misleading conclusions. This study aims to present a comprehensive method tailored for nanoplastics to accurately measure zeta potential and convert it to surface charge and potential with classical EDL theory. The effect of particle size and number concentration on zeta potential measurement was investigated using monodisperse polystyrene (PS) latex. An optimal number concentration was between 10¹⁰ and 10¹² #/L to generate sufficient scattered light with minimal interparticle interaction. Six nanoplastics were synthesized from major polymers to acquire the zeta potential at various pH, including low-density polyethylene, high-density polyethylene, polypropylene, PS, polyvinyl chloride, and polyethylene terephthalate. Based on classical EDL theory, the zeta potential measured at various ionic strength was converted to surface potential, revealing that the shear plane was 0.4 to 2.1 nm away from the surface. Finally, the surface charge density of nanoplastics was acquired, which is essential to describe interfacial processes of nanoplastics in the environment.