Dynamic aspects of bridging flocculation studied using standardized mixing.

A reproducible method of colloid mixing carried out by means of end-over-end rotation was applied to investigate dynamic aspects of bridging flocculation. Experiments consisted of adding solutions of polyethylene oxide (PEO) and salt to dilute dispersions of polystyrene latices and monitoring the number concentration of flocs as a function of mixing steps. We found a rapid initial flocculation, which stopped rather abruptly after a while. The rate of flocculation in the initial stage was several times higher than that of rapid coagulation induced only by salt. It was also found that the degree of this enhancement was dependent on the time of incubation of polymer molecules in the saline water prior to the mixing. This enhancement was ascribed to the increase of the collision radius of the colloid particles due to the attached polymer. The estimated thickness of the attached polymer at the initial stage was found to correlate with the size of the free polymer coil in the bulk solution rather than with the equilibrium thickness of adsorption; the latter was measured by dynamic light scattering. It was also found that the formation of PEO clusters in the solution has an influence on the rate of flocculation. The abrupt cessation of the flocculation was ascribed to the steric stabilization of the particles. From the time which was needed to attain the steric stabilization, it was concluded that the rate of establishment of steric stabilization is limited by transport of polymer toward the surface of the colloidal particles.