Influence of the dispersant structure on properties of electrostatically stabilized aqueous alumina suspensions

A series of molecules based on a benzene ring substituted with (OH) and (COOH) groups was used to derive guidelines for the selection of dispersants for a given colloidal forming process. The relation between the molecular structure of low-molecular-weight organic dispersants and their influence on the properties of α-Al 2O 3 suspensions was studied. In particular, the effect of the nature, number and position of the functional groups attached to the benzene ring on the adsorption behaviour of these molecules on α-Al 2O 3 and on the electrophoretic mobility of alumina particles in aqueous suspension has been examined. The effectof a low-molecular-weight dispersant is mainly determined by its ability to adsorb on the powder surface and by the number of functional groups that can dissociate and act as surface-charge modifiers. The maximum degree of adsorption for any given dispersant was found in the pH range corresponding to the pKa value of its functional group. High adsorption ability and high efficiency over a wide pH range could be achieved for a single dispersant by combining several functional 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 increasing number of charge carriers per molecule, provided that adsorption behaviour was otherwise similar.