INFLUENCE OF THE DISPERSANT STRUCTURE ON PROPERTIES OF ELECTROSTATICALLY STABILIZED AQUEOUS ALUMINA SUSPENSIONS

A series of molecules bused on a benzene ring substituted with (-OH) a nd (-COOH) groups was wen to derive guidelines for the selection of di spersants for a given colloidal forming process. The relation between the molecular structure of low-molecular-weight organic dispersants an d their influence on the properties of alpha-Al2O3 suspensions was stu died. In particular, the effect of the nature, number and position of the functional groups attached to the benzene ring on the adsorption b ehaviour of these molecules on alpha-Al2O3 and on the electrophoretic mobility of alumina particles in aqueous suspension has been examined. The effect of a low-molecular-weight dispersant is mainly determined by its ability to adsorb on the powder surface and by the number of fu nctional groups that can dissociate and act as surface-charge modifier s. The maximum degree of adsorption for any given dispersant was found in the pH range corresponding to the pKa value of its functional grou p. High adsorption ability, and high efficiency over a wide pH range c ould be achieved for a single dispersant by combining several function al groups of different pKa values in the same molecule. Adsorption of a dispersant was promoted strongly by chelate ring formation with the powder surface. The efficiency of the dispersants increased with incre asing number of charge carriers per molecule, provided that adsorption behaviour was otherwise similar. (C) 1996 Elsevier Science Limited.