  We report on the electrostatic complexation between oppositely charged
polymers and inorganic nanoparticles investigated by static and dynamical light
scattering. The nanoparticles put under scrutiny were citrate-coated
nanocrystals of cerium oxide (CeO2, nanoceria), of iron oxide (Fe2O3,
maghemite) and of europium-doped yttrium vanadate (Eu:YVO4) with sizes in the
10 nm range. For the polymers, we have used cationic-neutral diblock copolymers
(poly(trimethylammonium ethylacrylate)-b-poly(acrylamide), hereafter referred
to as PTEA-b-PAM) with different molecular weights. For the three colloidal
dispersions, we show that the electrostatic complexation gives rise to the
formation of stable nanoparticle clusters in the 100 nm range. The complexation
was monitored by systematic measurements of the scattering intensity versus X,
the mixing ratio between nanoparticles and polymers. For 5 nanoparticle/polymer
pairs, namely CeO2/PTEA5K-b-PAM30K, Fe2O3/PTEA5K-b-PAM30K,
Fe2O3/PTEA11K-b-PAM30K, Eu:YVO4/PTEA2K-b-PAM60K and Eu:YVO4/PTEA5K-b-PAM30K, we
found a unique behavior : the scattering intensity exhibits a sharp and
prominent peak in the intermediate X-range. To account for this behavior, we
have developed a model which assumes that regardless of X, the mixed aggregates
are formed at a fixed polymer-to-nanoparticle ratio. The agreement between the
results and the model is excellent on the 5 systems. Results at different
molecular weights suggest that the stoichiometry of the mixed aggregates is
controlled by the electrostatic interactions between the opposite charges. The
model allows to derive the molecular weight and the stoichiometry of the mixed
aggregates.
