  Polymeric nanoparticles (NPs) have a great application potential in science
and technology. Their functionality strongly depends on their size. We present
a theory for the size of NPs formed by precipitation of polymers into a bad
solvent in the presence of a stabilizing surfactant. The analytical theory is
based upon diffusion-limited coalescence kinetics of the polymers.
  Two relevant time scales, a mixing and a coalescence time, are identified and
their ratio is shown to determine the final NP diameter. The size is found to
scale in a universal manner and is predominantly sensitive to the mixing time
and the polymer concentration if the surfactant concentration is sufficiently
high. The model predictions are in good agreement with experimental data. Hence
the theory provides a solid framework for tailoring nanoparticles with a priori
determined size.
