Flocculation of latex particles of varying surface charge densities by chitosans

A series of well-characterized chitosan samples spanning a wide range of molecular weights, having been homogeneously acetylated to produce varying degrees of linear charge density,were used to flocculate model colloid polymer latices. Latices were selected to cover a wide range of charge densities and were sized between 85 nm and 2.1 mum. The optimum flocculation concentrations were identified using both residual turbidity and initial rate methods together with electrophoretic mobility measurements. A charge neutralization mechanism with an enhancement of rate in comparison to the rate of rapid coagulation indicative of a charge patch effect was confirmed, except in the case of the 2.1 mum particles. The optimum flocculation concentration of the latter fell below that of rapid coagulation, an effect ascribed to the influence of adsorbed chitosan upon the drainage between particles on close approach. Both molecular weight and degree of acetylation of the chitosans had a small effect upon the optimum flocculation concentration, with efficiency increasing with increases in both molecular weight and linear charge density. Increasing the ionic strength of the dispersion medium broadened the flocculation concentration range and diminished the enhancement of rate produced by the charge patch mechanism. With doses of chitosans in excess of the optimum far flocculation, charge reversal occurred and the cationic latices were rather more stable to electrolyte addition than their anionic precursors but were electrostatically rather than sterically stabilized.