Kinetics of Brownian flocculation of polystyrene latex by cationic polyelectrolyte as a function of ionic strength

Rate of Brownian flocculation of polystyrene latex particles with high molecular weight cationic polyelectrolyte is measured by coulter counter as a function of ionic strength. The rate of flocculation goes through a maximum against polyelectrolyte concentration. Under the condition of low ionic strength, it is confirmed that not only does the flocculation proceed faster than salt-induced rapid coagulation but also the breadth of flocculation region is relative narrow demonstrating a sharp peak. On the contrary, under the condition of high ionic strength, the flocculation proceeds like salt-induced rapid coagulation at low polyelectrolyte dosage, but the rate starts to slow down gradually with increasing polyelectrolyte dosage above a certain critical dosage. The flocculation region is confirmed to spread out in a relatively wide range. The isoelectric point, measured by electrophoretic mobility as a function of polyelectrolyte concentration, nearly keeps constant below a certain ionic strength then takes an obvious shift to the direction of a higher polyelectrolyte concentration with the increase of ionic strength. The isoelectric point corresponds to the peak point of flocculation rate for low ionic strength. The thickness of adsorbed polyelectrolyte layer was estimated on the basis of trajectory analysis of Brownian motion of a single particle. However, no remarkable thickness was observed at the isoelectric point.