  We exploit a precipitation-redispersion mechanism for complexation of short
chain polyelectrolytes with cerium oxide nanoparticles to extend their
stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of
monodisperse cationic nanocrystalline particles having a hydrodynamic diameter
of 10 nm and a molecular weight 400000 gmol-1. We show that short chain
uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads
to macroscopic precipitation. As the pH is increased, the solution
spontaneously redisperses into a clear solution of single particles with an
anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide
nanosols and their hybrid polymer-inorganic complexes in solution are
investigated by static and dynamic light scattering, X-ray scattering, and by
chemical analysis. Quantitative analysis of the redispersed sol gives rise to
an estimate of 40 - 50 polymer chains per particle for stable suspension. This
amount represents 20 % of the mass of the polymer-nanoparticle complexes. This
complexation adds utility to the otherwise unstable cerium oxide dispersions by
extending the range of stability of the sols in terms of pH, ionic strength and
concentration.
