Flocculation kinetics and equilibria in sterically stabilized dispersions

Abstract

The kinetics of flocculation of an aqueous polystyrene latex dispersion, in the presence of adsorbed poly(ethylene oxide) (PEO, molecular weight: 1,500 or 20,000) and sodium nitrate, were investigated using a turbidimetric technique employing a specially constructed Couette-type apparatus. The flocculation that occurred was weak and reversible and the “effective” flocculation rate constant was found to be always less than the limiting (Smoluchowski), diffusion-controlled irreversible flocculation rate constant. With PEO 20,000 the rate constant fell to zero at a critical PEO concentration, dependent on the particle concentration and the temperature. With PEO 1500, on the other hand, the rate constant rose to a maximum and then fell to a constant nonzero level. These differences are explained in terms of a previously developed equilibrium theory of weak flocculation. In addition, from the kinetic data it was proved possible to estimate the depth of the free energy minimum in the theoretical particle pairwise interaction curve. Moreover, by combining the equilibrium and kinetic theories used to analyze the data, it is shown that the data obtained at different particle volume fractions are internally consistent.