Ions effects on sol-gel transition and rheological behavior in alumina slurries

Aqueous slurries made of silica or alumina particles may behave like sols or like gels depending on the particle concentration, but also the pH or the ion force of the medium. The present study aimed at investigating the different sol-gel transitions observed in systems between a diluted state and concentrated formulations. For this purpose zeta potential and G', G'' measurements, reflecting elastic and viscous behaviors, were selected as the tools of choice to get insights into the mechanisms accounting for the sol-gel transition. We discuss about the relevancy of these simple measurements and affirm that they are sufficient to explain the evolutions in an alumina based system. Indeed, particles can adopt a 'repulsive' organization at low salt concentration values (i.e. high surface potential values) or form an attractive percolating network, when the ion strength reaches a critical level. Moreover, the addition of NaOH can lead to very complex moduli variations as both the electrostatic shielding and the pH change; yet, these variations are easily predictable using our reasoning. (authors)