Effect of Ionic Strength on the Initial Dynamics of Flocculation of Polystyrene Latex with Polyelectrolyte

The effect of ionic strength on the kinetics of flocculation of polystyrene latex (PSL) particles induced by addition of excess polyelectrolyte was studied. The flocculation experiment was performed in the standardized mixing flow generated by using an end-over-end rotation apparatus and by changing the ionic strength and polyelectrolyte concentration. We focused our attention on the initial process of flocculation, which is characterized by a remarkably high rate of flocculation (the first stage) followed by an abrupt decrease in the flocculation rate (the second stage). The combined measurements of the rate of flocculation and the electrophoretic mobility as a function of mixing steps revealed a transient state of polyelectrolyte adsorbing on the bare surface of PSL particles. Under the conditions of low ionic strength, the picture is consistent with that of kinetically controlled adsorption of polyelectrolyte, while, under high ionic strength, the evidence of kinetically controlled adsorption disappeared. However, even in the latter, the rate of flocculation in the first stage was a function of polyelectrolyte concentration. This is consistent with a picture of nonequilibrium flocculation. A crossover with respect to ionic strength can be ascribed to an increase in flexibility of polyelectrolytes at colloidal surfaces with an increase in ionic strength. Copyright 1998 Academic Press.