Polyphosphate interaction with aluminium-doped titania pigment particles

The interaction of a widely used Calgon (TM) polyphosphate dispersing reage nt with aluminium-doped titania pigment particles has been investigated usi ng electrokinetic and rheological studies combined with adsorption isotherm s. The influence of pH, aluminium dopant concentration and polyphosphate co ncentration is reported. Polyphosphate adsorption density and affinity with the titania pigment surface is highest under acidic solution conditions. T his however, does not necessarily transfer to enhanced dispersion propertie s at low pH values. At pH 9, the polyphosphate adsorption density correlate s directly with a reduction in pigment particle interactions making polypho sphate an effective titania pigment dispersant under alkaline conditions. C onversely, 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 t heir Newtonian flow behaviour. At adsorption densities of similar to 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 m g m(-2) that the titania pigment suspension begins to restabilise. It is pr oposed 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 b ut at low pH, electrostatic stabilisation is partly neutralised and higher adsorption densities are required for effective stabilisation. (C) 2001 Els evier Science B.V. All rights reserved.