Osmotic compression of mixtures of polymers and particles

Aqueous dispersions of nanometric ceria particles have been concentrated through osmotic stress. Mixed dispersions of ceria with small adsorbing macromolecules of poly (vinylpyrrolidone) have been prepared by the same method. The osmotic pressure of pure ceria dispersions results from electrostatic repulsions between particles. The osmotic pressure of dispersions containing a non-saturating amount of PVP is the same as that of pure dispersions, and the colloidal stability is depressed with respect to the pure dispersions. The osmotic pressure of dispersions containing an excess of PVP is increased by the free macromolecules: and the colloidal stability is enhanced. The organization of particles in these dispersions has been examined by small-angle x-ray scattering and cryotransmission electron microscopy. In pure ceria dispersions and in saturated dispersions, a liquid-like short-range order was found; when the concentration is increased, this short-range order follows a three-dimensional swelling law. In dispersions containing a non-saturating amount of PVP, the structure shows an alternance of clusters and voids, and the separations of clusters follow an unusual one-dimensional swelling law.