From powders to dispersions in water: Effect of adsorbed molecules on the redispersion of alumina particles

Aqueous dispersions of colloidal alumina particles have been submitted to p rocesses where the dispersion is aggregated, dried, and then redispersed in water. This cycle was performed with alumina particles that had different surface states: bare surfaces with monovalent counterions; surfaces that ha ve been covered with small molecule ligands and have their surface charge r eversed. In all cases, redispersion was produced by an increase in the ioni c pressure due to the counterions, after immersion of the powder in water a t the appropriate pH. In all cases, the pH that produced the appropriate io nic pressure was 5-6 pH units away from the isoelectric point of the surfac es. This was related to the fact that the surfaces were, in all cases, sepa rated by a thin water film, which had a constant thickness (8 Angstrom) set by the strength of binding of water to the alumina surfaces. However, for surfaces that had been covered with high amounts of macromolecular ligands, the minimum separation of surfaces was increased, and redispersion could b e obtained at a pH which was closer to the isoelectric point. These phenome na can be accounted for, quantitatively, through a calculation of the balan ce of surface forces, including van der Waals attractions, hydration forces , and ionic pressures.