Initial stage dynamics of bridging flocculation of polystyrene latex particles with low charge density polycation in a mixing flow near the isoelectric point

Dynamics of ortho-kintetic flocculation of PSL particles with long-chain polycation of low charge density was studied together with the adsorption process of polyelectrolyte near isoelectric point as a function of ionic strength. Within the range of investigation, the initial rate of flocculation was found to take the maximum value immediately after the onset of mixing, but the flocculation gradually slows down with the progress of adsorption. The initial rate of flocculation goes through the minimum as ionic strength increases. Without salt addition, the rate of flocculation is about three times faster than that of salt-induced rapid coagulation. With slightly increasing ionic strength the rate of flocculation slows down because of the reductions of the stiffness and the size of polymer coil in solution and also of the smooth rearrangement of adsorbed chains. The adsorption process monitored by the electrophoresis demonstrates that the zero mobility appears much earlier than the time predicted by collision process in the presence of salt (KCl 100 mM). This trend can be explained by the concept of electrokinetically stagnant layer and by smooth spreading of polyelectrolyte on the colloidal surface. The kinetics of adsorption is in good accordance with the change in the flocculation rate