The influence of pH and NaCl on the zeta potential and rheology of anatasedispersions

The rheological behaviour of anatase dispersions was compared with the zeta potential, when the pH (from 3 to 10) and sodium chloride concentration (f rom 0 to 1 mol dm(-3)) was varied over a wide range. The isoelectric point, pH(iep), is shifted to higher values with increasing salt concentration un til 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 crit ical yield stress and the shear stress and viscosity at arbitrarily selecte d high and low shear rates, respectively. The maximum of the rheological pr operties was found to correlate with to the pH(iep). The electrokinetic and rheological features of the system were related to a recent theory of Hunt er et al. [J. Colloid Int. Sci. 28 (1968) 250] by plotting the yield stress against the square of the zeta potential for each salt concentration. The acidic branch, i.e. when pH < pH(iep), of these curves followed the expecta tions of the theory quite well. For the alkaline branch (pH > pH(iep)), how ever, the curves did not show the expected linear behaviour. Also when comp aring 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 bran ch. 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 t o be quite parallel for medium salt concentrations. (C) 2000 Elsevier Scien ce B.V. All rights reserved.