Effects of nonionic and cationic dextran polymers on the formation of chromium hydroxide sols

Abstract

The effects of various dextrans (MW = 4 × 10 4, 5 × 10 5, and 2 × 10 6) and of their cationic derivatives (DEAE-Dextran 500 and 2000) on the formation of uniform spherical colloidal chromium hydroxide particles, obtained by aging of chrome alum solutions at elevated temperatures, were studied in detail. With increasing concentration and molecular weight of dextrans a decrease in the modal particle diameter, a broadening of the particle size distribution, and a roughening of the particle surface were observed. These effects were attributed to an increase in the rate of uptake and to the amount of the polymer adsorbed on the particles as the concentration of dextrans and their molecular weight became higher. Cationic dextrans had a much stronger effect on reducing the particle diameter and, when added in sufficient concentration, inhibited the particle formation altogether. These phenomena were shown to be due to the binding of sulfate ions, which are essential to the formation of the chromium hydroxide particles, to the polycation molecules.