Brownian flocculation of negatively charged latex particles with low charge density polycation at various ionic strengths

Rate of Brownian flocculation of polystyrene latex particles with low charge density polyelectrolyte (σ = 4%, Mw = 5.2 × 106 g/mol) was measured by Coulter Counter as a function of the ionic strength. As reported in the case of high charge density polyelectrolyte [L. Feng, Y. Adachi, A. Kobayashi, Colloid Surf. A. 440 (2014) 155–160], the rate of flocculation goes through a maximum against polyelectrolyte dosage, and a maximum rate of flocculation was observed near the isoelectric point. However, several important trends are different from the results obtained with high charge density polyelectrolyte. (1) The rate of flocculation is always slower than that of salt-induced rapid coagulation, even at the isoelectric point. (2) The optimum polyelectrolyte dosage corresponding to the isoelectric point appears several times higher. (3) The shift of isoelectric point was also observed with an increase of ionic strength but toward a lower polyelectrolyte dosage. (4) The thickness of adsorbed polyelectrolyte layer was found to be detectable (about 15 nm) at the optimum dosage of flocculation where the fastest rate was observed. These results suggest a picture of bridging flocculation that competes with steric effects of adsorbed polyelectrolytes.