Adhesion between Oxide Nanoparticles: Influence of Surface Complexation
Colloidal cerium oxide particles of nanometer size are irreversibly adsorbed on molecularly smooth mica sheets from bulk dispersions. The approach to equilibrium, the homogeneity and stability of the adsorbed films, and the effects of pH and solution conditions, are determined by means of the surface force apparatus and atomic force microscopy techniques. Driven by electrostatic interactions between oppositely charged substrate and particles, a dense and relatively homogeneous flat film composed of ceria nanoparticles assembled in a single layer can be obtained. Coating substrates with nanometric particles gives a surface smooth at the nanometer scale only, but which is fully representative of a colloidal oxide/water interface. Force-distance profiles between such layers and colloidal stability can thus be compared. The long-range electrostatics repulsion is consistent with the zeta-potential values measured independently in bulk dispersions. An adhesion at contact between ceria is evidenced and its relation with the intricate colloidal stability of the oxide dispersions is demonstrated. The chemical origin for the adhesion between ceria surfaces is further supported by the effect that complexant molecules play. In their presence the ceria layers are protected and adhesion is prevented, with an efficiency increasing in the order nitrate < acetate < acetyl acetone. It appears that the stability of nanometric dispersions is better achieved by controlling the adhesion at contact rather than affecting the long-range electrostatic repulsion.