A COMPARISON OF THE INTERACTION FORCES BETWEEN MODEL ALUMINA SURFACESAND THEIR COLLOIDAL PROPERTIES

Previous work has demonstrated that alumina dispersions are only desta bilized by monovalent electrolytes such as KCl at concentrations <0.1 M over a wide pH range. This unusual stability has been qualitatively attributed to a repulsive hydration force that operates at distances < 5 nm. Intermolecular forces, measured between an aluminium coated coll oidal silica sphere and a flat alumina substrate, carried out using an atomic force microscope demonstrates that the additional repulsion is due to short range forces not expected in DLVO (Derjaguin-Landau-Verw ey-Overbeek) theory. The origin of these forces is postulated to be du e to a combination of surface gel formation, probably due to polymeric A1 species, and the natural hydration of the surface. The gel layer t hickness determined at pH 8 was at least 15 nm. At pH less than or equ al to 7 (i.e. less than or equal to isoelectric point) the forces obta ined conformed to DLVO behaviour down to separation distances of ca 3- 5 nm, at smaller separations an additional repulsive force was detecte d. A thin gel-layer may be formed even at lower pH values which would contribute to this short range repulsion. These results may partially explain the difficulty encountered in dewatering aluminium hydroxide r ich sludges generated during water treatment. (C) 1997 Elsevier Scienc e B.V.