  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
several stages, i.e., nuclei produced in a supersaturated solution, grow to
nanosize subunits, which then aggregate to form much larger uniform secondary
particles. To explain the size selection in such a series of processes, a
kinetic model was developed which combined two growth/aggregation stages. This
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 aggregation. Modifications of the aggregation
rates to account for singlet size and aggregate diffusivity produced only small
changes. However, introduction of a "bottle-neck" factor in the dimer formation
rate had a profound effect on the final size distribution and suggested a
possible size-control mechanism.
