Redispersion of alumina particles in water

This work deals with the transformation of a powder into an aqueous suspension. Powders of alpha -alumina were dispersed in water, dried under controlled conditions, and then redispersed into water. The aim was to obtain spontaneous redispersion. To obtain reproducible results, standard protocols were designed (i) for preparing the surface state of the particles, and for generating controlled interparticle contacts; and (ii) for measuring the amount of redispersed material. The results show that there are thresholds, determined by amount of surface water and by the pH of redispersion, where all the powder goes from the aggregated state to the dispersed state. According to these thresholds, the mechanism of redispersion is a shift in the balance of forces between surfaces in contact: van der Waals attractions, electrostatic repulsions, and hydration forces. A model was built that describes the state of surfaces, their hydration, and the interactions between surfaces; using a unique set of parameters, this model gave accurate predictions for the redispersion thresholds observed in widely different mechanical and chemical conditions.