A comparison of the interaction forces between model alumina surfaces and their colloidal properties

Previous work has demonstrated that alumina dispersions are only destabilized 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 colloidal 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-Verwey-Overbeek) theory. The origin of these forces is postulated to be due to a combination of surface gel formation, probably due to polymeric A1 species, and the natural hydration of the surface. The gel layer thickness determined at pH 8 was at least 15 nm. At pH less than or equal to 7 (i.e. less than or equal to isoelectric point) the forces obtained conformed to DLVO behaviour down to separation distances of ca 3-5 nm, at smaller separations an additional repulsive force was detected. 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 rich sludges generated during water treatment. (C) 1997 Elsevier Science B.V.