Charge interactions in semi-concentrated titania suspensions at very high ionic strengths

ζ-Potentials of titania particles in aqueous suspensions containing up to 1.2 mol dm^-3 1-1 electrolyte are reported. At sufficiently high concentrations of supporting inert electrolytes (> 0.1 mol dm^-3), the isoelectric point (i.e.p.) of titania shifts toward higher pH values. At even higher ionic strength the ζ-potential of the titania particles are positive even at pH > 10 (no i.e.p.). The lowest concentration at which such an effect is observed ranges from 0.5 mol dm^-3 for NaI to 0.9 mol dm^-3 for NaNO₃ and over 0.9 mol dm^-3 for NaCl. In mixed solvents, e.g. 30% methanol-70% water, similar effects are observed at substantially lower ionic strengths than in purely aqueous systems. The acidic branch of the ζ(pH) curves is much less sensitive than the alkaline branch to the electrolyte concentration in aqueous and mixed solvent systems. The effect of the supporting electrolyte is evaluated experimentally and discussed in relation to recently published results on the properties of suspensions of relatively high solids fraction.