Diffusion of charged nano-disks in aqueous media: Influence of competing inter-particle interactions and thermal effects

Abstract

In this work we investigate binary mass diffusion of negatively charged disk-shaped nanoparticles of Laponite JS in ultrapure water using Mach-Zehnder interferometry aided by a sensitivity analysis. We observe that binary mass diffusivity of aqueous suspension shows a consistent enhancement with concentration of Laponite at a given temperature. The dependence on temperature, however, is observed to be non-monotonic, wherein the binary mass diffusivity first increases with temperature up to 20°C while decreasing at higher temperatures. We propose that the observed non-monotonic dependence of binary diffusivity on temperature is due to competing effects such as: thermal energy of Laponite particles, that of counterions, and the magnitude of charge on the Laponite particles. These factors affect the aggregation rate of Laponite particles leading to the behaviour observed in the experiments. Interestingly the binary diffusivity of Laponite obtained in the present work is significantly higher than the self-diffusion coefficient of a corresponding hard disk. We propose that enhanced binary diffusivity is due to repulsive interaction among the Laponite particles that augments mobility of the solute in ultrapure water.