Temperature Dependence and Mechanism of Chloride-Induced Aggregation of Silver Nanoparticles

With the increasing occurrence of silver nanoparticles in commercial products and in the environment, it is important to understand the transformations that the nanoparticles undergo as a result of their interactions with other species. In this paper, we focus on interactions with the chloride ion, which is abundant in natural waters as well as biological systems. Chloride ion added to a solution of citrate-capped silver nanoparticles disturbs their stability by modifying the nanoparticle surface, enhancing dissolution of the particles, and increasing the ionic strength of the solution. Because of the surface modifications, aggregation occurs more rapidly than would be expected from the increase in ionic strength. This indicates that the nanoparticles are experiencing a decrease in surface charge. To elucidate the atomic-scale processes behind this behavior, we have studied the temperature dependence of the rate of decay of silver nanoparticles at a low NaCl concentration (10 mM), where dissolution but no aggregation occurs, and a higher concentration (40 mM), where aggregation is the dominant process. Particle dissolution was found to have a positive temperature dependence with an activation energy of 69 +/- 6 kJ/mol. Conversely, the aggregation rate was inversely dependent on temperature but exhibited a lag time that increased with temperature. We develop an empirical model for chloride-induced aggregation, which is based on a time-dependent activation energy that arises from surface changes brought about by chloride reactions on the silver nanoparticle surface. The mathematical form of the model fits the data very well and provides insight into the molecular processes involved in aggregation in aqueous chloride solutions.