OSMOTIC COMPRESSION OF MIXTURES OF POLYMERS AND PARTICLES

Aqueous dispersions of nanometric ceria particles have been concentrat ed through osmotic stress. Mixed dispersions of ceria with small adsor bing macromolecules of poly (vinylpyrrolidone) have been prepared by t he 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 dispersi ons containing an excess of PVP is increased by the free macromolecule s: and the colloidal stability is enhanced. The organization of partic les in these dispersions has been examined by small-angle x-ray scatte ring and cryotransmission electron microscopy. In pure ceria dispersio ns and in saturated dispersions, a liquid-like short-range order was f ound; when the concentration is increased, this short-range order foll ows a three-dimensional swelling law. In dispersions containing a non- saturating amount of PVP, the structure shows an alternance of cluster s and voids, and the separations of clusters follow an unusual one-dim ensional swelling law.