Diffusional aging at water/oil interfaces laden with charged nanoparticles studied by single-molecule tracking

The adsorption of charged nanoparticles at water-oil interfaces constitutes a fundamental phenomenon, underlying pivotal technologies spanning from emulsion stabilization to the sophisticated fabrication of foams. However, the diffusional behavior of these nanoparticles remains poorly understood. Here, we use single-molecule tracking experiments to show that the diffusion of like-charged nanoparticles at the water-oil interface not only becomes anomalous but also displays a diffusional aging phenomenon at the interfacial coverage that is not associated with the glassy state. We further develop a theoretical framework that quantitatively reproduces all experimental observations. Molecular dynamics simulations reveal the necessity of the coexistence of attraction and repulsion for the emergence of aging dynamics. The interplay between attraction and repulsion leads to nanoparticle adhesion taking place over observable timescales, which is manifested as aging dynamics. The discovery of diffusional aging demonstrates that interfacial evolution persists beyond adsorption equilibrium, suggesting that this effect must be accounted for in applications involving water-oil interfaces laden with charged nanoparticles.