Effect of adhesion on aggregation in nanoparticle dispersions

The detailed structure of a dispersed nanoparticle suspension has been studied by transmission electron microscopy and dynamic light scattering for comparison with an off-lattice gas computer model. For low adhesion between particles, the structure of the suspension is shown to be a "gas with clusters" phase of particles comprising aggregates ranging from singlets, doublets, triplets, etc. to 16-plets in a continuous distribution. Further increasing the adhesion between the particles reduces the number of singlets and causes larger aggregates to form, without precipitation, as some condensed phase appears in the gas. This phase appears as metastable clumps, which do not grow rapidly until adhesion is raised. Then, at high adhesion, flocculation occurs with aggregates growing with time and a large reduction in the number of singlets. Experiments on monosize 62-nm-diameter hematite particles in water confirm this behaviour. An off-lattice computational model based on a square well adhesion potential qualitatively described the dynamic light scattering and transmission electron microscopy results.