The influence of pH and NaCl on the zeta potential and rheology of anatase dispersions

The rheological behaviour of anatase dispersions was compared with the ζ potential, when the pH (from 3 to 10) and sodium chloride concentration (from 0 to 1 mol dm−3) was varied over a wide range. The isoelectric point, pHiep, is shifted to higher values with increasing salt concentration until a charge reversal was no longer observed at a concentration of 1 mol dm−3 NaCl. The rheological properties were analyzed in the form of the critical yield stress and the shear stress and viscosity at arbitrarily selected high and low shear rates, respectively. The maximum of the rheological properties was found to correlate with to the pHiep. The electrokinetic and rheological features of the system were related to a recent theory of Hunter et al. [J. Colloid Int. Sci. 28 (1968) 250] by plotting the yield stress against the square of the ζ potential for each salt concentration. The acidic branch, i.e. when pH<pHiep, of these curves followed the expectations of the theory quite well. For the alkaline branch (pH>pHiep), however, the curves did not show the expected linear behaviour. Also when comparing the influence of pH and sodium chloride concentration on the rheology, it was found that on the acidic yield stress branch the dependency of pH and salt concentration was very different from the one on the alkaline branch. On the acidic branch, low ionic strengths shared a common straight line. On the contrary, the alkaline branch had separate linear dependencies for each salt concentration. Moreover, the lines on the alkaline side seemed to be quite parallel for medium salt concentrations.