Model of Formation of Monodispersed Colloids

Abstract

Ample experimental evidence has been accumulated demonstrating that the formation of monodispersed colloids proceeds through a more complex mechanism than the generally excepted diffusional “burst nucleation” process. Instead, the synthesis of narrow-size-distribution colloidal dispersions involves two distinct stages. Nanosize primary particles are nucleated in a supersaturated solution. They then aggregate to form much larger uniform secondary particles. To explain the size selection in such a process, a kinetic model has been developed which couples the two growth/aggregation stages. Our earlier study has shown the burst-nucleation growth of the primary particles to depend strongly on the value of the effective surface tension entering the surface term in the free energy of the subcritical embryos. The aim of the present work has been to identify an appropriate control parameter in the process of secondary particle formation. We tried modifications of the aggregation rates to account for singlet size and aggregate diffusivity. We found that only the introduction of a “bottleneck” factor in the dimer formation rate has a profound effect on the final size distribution and suggests a possible size-control mechanism.