Polymer adsorption and its effect on the stability of hydrophobic colloids. III. Kinetics of the flocculation of silver iodide sols

Abstract

In a previous study on the flocculation of silver iodide sols by polyvinyl alcohol (PVA) it was demonstrated that the extent of flocculation depends critically on the way in which sol particles and polymer are mixed. Optimal flocculation was shown to occur if a two-portion method of mixing is applied in which equal numbers of polymer-covered and uncovered particles are brought together. The observations could be fully explained by a bridging model of flocculation. In this study the kinetics of the flocculation process has been investigated with a stopped flow spectrophotometer. The coagulation of AgI-sols by low molecular weight electrolytes was found to be a bimolecular process. Critical coagulation concentrations were found that matched closely those reported in the literature. The flocculation of sol by polymer brought about by mixing of polymer-covered and uncovered particles, follows also bimolecular kinetics. In this case only 50% of the collisions leads to aggregation and the fast flocculation rate is just half the rate of fast coagulation. In accordance with previous results, the critical flocculation concentration of electrolytes with univalent and bivalent counterions was shown to be much lower than the corresponding critical coagulation concentration. Comparison of the kinetics of our two-portion method with the one-portion method (in which the total amount of polymer is added to the total amount of sol) leads to the conclusion that absorbing PVA molecules need a few seconds to form a layer that can effectively protect the sol particles. Thus, the rate of flocculation can provide information on the rate of adsorption. This study underlines again that the way of mixing is a very important parameter in the flocculation of hydrophobic sols by polymers.