THE KINETICS OF HETEROFLOCCULATION IN THE SYSTEM CATIONIC STARCH AND COLLOIDAL ANIONIC SILICIC-ACID

The flocculation kinetics of cationic amylopectin (CAP) with hydrodynamic radii of 200-400 nm or cationic amylose (CA) with an anionic colloidal silicic acid (CSA) in the size range 5.5-21 nm in NaCl solutions have been studied with the stopped-flow technique. Turbidity has been used for detection. Electrostatic forces seem to be of greatest importance. For maximum flocculation it is found that the two polyions involved should carry equal or nearly equal amounts of charge. This condition must be closely fulfilled for small particles of colloidal silicic acid and at low concentrations of excess salt. With increasing salt concentration and particle size this condition decreases in importance and flocculation occurs over a fairly broad interval around the charge equivalence of the polyions involved. The experimental results can be explained by assuming the formation of bridges between the CAP molecules. These bridges can contain either a single CSA particle or two (or more) CSA particles. The two-particle bridge type is important when there is a high excess salt concentration and a comparatively high degree of substitution of cationic groups on the CAP molecules simultaneously with an excess of CSA particles. In other cases an excess of CSA particles stabilizes the system against flocculation. After an initial increase in turbidity in the time range 10 ms-1.5 s the turbidity decreases again. This can be interpreted as a contraction of the flocs due to an electrostatic screening of the positive charges of the CAP molecules. This contraction phenomenon is observed only for the smallest CSA particles. © 1992.