Model of formation of monodispersed colloids

Ample experimental evidence has been accumulated demonstrating that the for mation 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 supersat urated 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 ea rlier study has shown the burst-nucleation growth of the primary particles to depend strongly on the value of the effective surface tension entering t he surface term in the free energy of the subcritical embryos. The aim of t he present work has been to identify an appropriate control parameter in th e process of secondary particle formation. We tried modifications of the ag gregation rates to account for singlet size and aggregate diffusivity. We f ound that only the introduction of a "bottleneck" factor in the dimer forma tion rate has a profound effect on the final size distribution and suggests a possible size-control mechanism.