Aqueous Dispersions of Positively Charged Al/Mg Mixed Metal-Hydroxide Particles:Surface Chemistry and Rheological Properties

The solids concentration, pH and NaCl were found to have a very significant effect on the rheological properties of aqueous dispersions of positively charged Al-Mg mixed metal hydroxide particles (Al-Mg MMH). At low solids concentrations the flow curves of the dispersions at natural pH followed the Newtonian model very well in the observed range of shear rate, while at high solids contents the dispersions developed a yielding type of response, with yield stress increasing rapidly as solids concentration increases. The variation in rheological properties with pH and NaCl contents correlated well with the change in surface properties of Al-Mg MMH particles. Al-Mg MMH dispersions with maximum yield stress occurred near the isoelectric point (IEP) where the Zeta potential (or electrophoretic mobility) is zero. At the IEP, the shear yield stress decreased monotonously in magnitude as a function of the increasing NaCl concentration, which is in contrast to the increment in the yield stress observed below the DEP at NaCl concentrations less than 0.1 mol dm^-3. At low NaCl concentrations, significant shear yield stress differences exist at and below the IEP. By contrast, at high NaCl concentrations, almost identical shear yield stress versus NaCl results are obtained in both cases.