Ion specificity and viscosity of rutile dispersions

Abstract

The isoelectric point (IEP) of rutile is shifted to higher pH values in the presence of greater than 10−4 mol dm−3 Ba2+, Ca2+ and Mg2+, and when a critical concentration (5 × 10−4 mol dm−3 for Ba2+ and 1 × 10−3 mol dm−3 for Ca2+) is exceeded there is no IEP at all and the ζ potential is always positive. A common intersection point for the ζ-potential curves of the different concentrations of salt is found, but for the various salts the point is shifted from ζ = 0 mV for Mg2+ up to ζ = 20 mV for Ba2+. Between the IEP and the charge-reversal point a rheologically unstable region is discovered. The shear stress of rutile dispersions (2.5 g rutile + 4 g electrolyte solution) at shear rates of 116 s−1 shows the same pH dependence irrespective of the concentration of alkaline-earth metal cations up to 10−2 mol dm−3. The shear stress is less than 1 Pa below pH 3.8 and in the pH range 5–12 it assumes a value between 50 and 80 Pa at 116 s−1 with some scatter; however, no systematic trend with concentration of alkaline-earth metal cations and a rather insignificant decrease with pH at pristine conditions are observed. The acidic branch of the yield stress (pH) and low shear rate viscosity (pH) curves is insensitive to the presence of alkaline-earth metal cations, and the same behaviour is found for the ζ potential. The alkaline-earth metal cations induce an increase in viscosity in the basic region and a shift in the pH of maximum viscosity to high pH values. It was also discovered that the effect different alkaline-earth metal cations have on the rheological properties at the same concentration is different from the effect induced by indifferent electrolytes. When the ζ potential increases the viscosity at high pH is increased in a series which follows the increase in size of the cation.