Evidence for Enhanced Tracer Diffusion in Densely Packed Interfacial Assemblies of Hairy Nanoparticles

Nearly monodisperse nanoparticle (NP) spheres attached to a nonvolatile ionic liquid surface were tracked by in situ scanning electron microscopy to obtain the tracer diffusion coefficient D-tr as a function of the areal fraction phi. The in situ technique resolved both tracer (gold) and background (silica) particles for similar to 1-2 min, highlighting their mechanisms of diffusion, which were strongly dependent on phi. Structure and dynamics at low and moderate phi paralleled those reported for larger colloidal spheres, showing an increase in order and a decrease in D-tr by over 4 orders of magnitude. However, ligand interactions were more important near jamming, leading to different caging and jamming dynamics for smaller NPs. The normalized D-tr at ultrahigh phi depended on particle diameter and ligand molecular weight. Increasing the PEG molecular weight by a factor of 4 increased D-tr by 2 orders of magnitude at ultrahigh phi, indicating stronger ligand lubrication for smaller particles.