Rheological properties of aqueous alumina suspensions containing polyacrylic acid and divalent ions

Rheological and aggregation properties of alumina suspensions were studied in aqueous systems containing both and anionic polyelectrolyte (poly (acrylic acid), PAA) and calcium ions. All experiments were made close to the pH of zero charge of alumina (pH 9). At low calcium concentration the polyelectrolyte acts normally as a dispersing agent. Addition of calcium favors polymer adsorption and decreases slightly the viscosity, up to a molar ratio (calcium/monomer acrylic units = 0,35) where suspensions exhibit a sudden increase in yield stress and viscosity. At the same ratio both calcium complexation with PAA and polyelectrolyte adsorption on particles attain a maximum value. In these circumstances, electrokinetic measurements and surface charge calculations suggest that particle/particle interactions do not obey simple electrostatic rules, since the surface charge density is high enough to prevent aggregation. This phenomenon, which is not observed with sodium ions at comparable ionic strength, adds up to several reports showing that calcium ions (may be other multivalent ions) induce specific attractive forces between negatively charged surfaces.