  We report on the use of magnetic sedimentation as a means to determine the
size distribution of dispersed magnetic particles. The particles investigated
here are i) single anionic and cationic nanoparticles of diameter D = 7 nm and
ii) nanoparticle clusters resulting from electrostatic complexation with
polyelectrolytes and polyelectrolyte-neutral copolymers. A theoretical
expression of the sedimentation concentration profiles at the steady state is
proposed and it is found to describe accurately the experimental data. When
compared to dynamic light scattering, vibrating sample magnetometry and
cryogenic transmission electron microscopy, magnetic sedimentation exhibits a
unique property : it provides the core size and core size distribution of
nanoparticle aggregates.
