Polyphosphate interaction with aluminium-doped titania pigment particles

The interaction of a widely used Calgon™ polyphosphate dispersing reagent with aluminium-doped titania pigment particles has been investigated using electrokinetic and rheological studies combined with adsorption isotherms. The influence of pH, aluminium dopant concentration and polyphosphate concentration is reported. Polyphosphate adsorption density and affinity with the titania pigment surface is highest under acidic solution conditions. This however, does not necessarily transfer to enhanced dispersion properties at low pH values. At pH 9, the polyphosphate adsorption density correlates directly with a reduction in pigment particle interactions making polyphosphate an effective titania pigment dispersant under alkaline conditions. Conversely, at pH 4, polyphosphate adsorption densities less than 0.1 mg m-2 have no effect on the colloidal stability of the titania particles and their Newtonian flow behaviour. At adsorption densities of ∼ 0.1 mg m-2, approaching the iep (near 0.2 mg m-2), the suspension aggregates. It is not until the polyphosphate adsorption density is greater than 0.3 mg m-2 that the titania pigment suspension begins to restabilise. It is proposed that chemisorption dominates polyphosphate adsorption at pH 9 whilst at pH 4 a combination of chemisorption and electrostatic adsorption occurs. Stabilisation by the polyphosphate present at the pigment surface depends on both electrostatic and steric effects. At high pH, both are effective but at low pH, electrostatic stabilisation is partly neutralised and higher adsorption densities are required for effective stabilisation.