Influence of aluminum doping on titania pigment structural and dispersion properties

The influence of aluminum concentration on the structural properties and rheological behavior of aqueous suspensions of aluminum-doped titania pigment from the chloride process was investigated. The variation in rheological properties correlates with the change in the pigment surface properties, determined from electrophoresis measurements and atomic surface concentrations. Pigment suspensions exhibited a maximum yield stress and viscosity at or near the isoelectric point (iep). The pH of the maximum yield value of the pigment suspension increases with increasing aluminum hydroxyl group density at the particle surface. For pigments with a high aluminum surface concentration, at pH values where the magnitude of the zeta potential was high, a low-viscosity, dispersed suspension was obtained. The pigment with the lowest aluminum concentration, however, retained high yield stresses over a large pH range even when the zeta potential was of considerable magnitude. Pigment particle interactions are chiefly dictated by van der Waals forces and electrostatic repulsive forces, likely to be influenced by heteroaggregation. The aggregate strength would therefore depend upon the proportion and distribution of aluminum and titanium surface groups of the heterogeneous pigment, which will influence both the Hamaker constant and the degree of heteroaggregation. Overall, very small additions to the total aluminum concentration translate to significant aluminum surface concentration disparities and subsequently to large particle interaction differences.