A rheological and electrokinetic investigation of the interactions between pigment particles dispersed in aqueous solutions of short-chain phosphates

The effects of short-chain phosphate adsorption on the colloid and interfacial chemistry of concentrated metal oxide pigment dispersions were investigated with rheological, electrokinetic and adsorption isotherm measurements. The phosphates were observed to specifically adsorb at the solid–aqueous solution interface. This caused the isoelectric point of the pigment particles to shift to lower pH values. The magnitude of the shift in the isoelectric point was found to be a function of both phosphate concentration and chain length. At a fixed phosphate concentration, but with changing pH, the yield stress of the dispersion (and, hence, the interaction force between particles) was seen to vary with the square of the zeta potential of the particles. This behaviour was attributed solely to the variation of electrostatic forces with pH, in line with the DLVO theory of colloid stability. However, a reduction in the magnitude of the yield stresses at the isoelectric point was observed with either increasing phosphate concentration or chain length. This was ascribed to the presence of repulsive steric forces. At low and high coverage, the range of the force was found to be equivalent to approximately twice the minimum and maximum dimension, respectively, of the phosphate molecules.