The controlled flocculation of particulate dispersions using small particles of opposite charge. III. Investigation of floc structure using rheological techniques

Abstract

The flocculation of concentrated (25% v/v) dispersions of large, negative polystyrene latex particles (particle diameter, 1.15 μm) by the addition of small, positive polystyrene particles (diameter, 0.25 μm), where the two sets of particles each have an adsorbed layer of poly (vinyl alcohol), has been investigated as a function of electrolyte (sodium chloride) concentration and small particle volume fraction. The extent of flocculation in the concentrated dispersion was assessed using several rheological techniques. The residual viscosity, η (0), was measured from the creep cruves obtained using a “Deer” rheometer. The instantaneous shear modulus, G 0, was determined using a pulse shearometer, the plastic viscosity, η p1, and Bingham yield value, τ B, were determined from steady state shear experiments using a Haake “Rotovisko” rheometer. With the exception of η p1, all the rheological parameters showed an initial increase on increasing the ratio of the volume fraction of the positive particles to that of negative particles (φ +/φ.), reached a maximum at an optimum ratio, and then decreased at high φ +/φ. ratios. The trends in the rheological results are compared with those obtained from sediment volume experiments. The view is confirmed that heteroflocculation in those systems takes place by “bridging” of the large negative particles by the small positive ones. From the measured Bingham yield value, the energy, E, required to totally disperse the flocs into single large particles (plus their associated, adsorbed small particles) has been calculated. E, in turn, may be related to the total number, N, of “bridges,” and the energy, V b, required to break a bridge. If it is assumed that, in breaking a bridge, the small particle remains adsorbed on one of the two large particles that had formed the bridge, then V b may also be computed from a knowledge of the interparticle interactions, at least at higher electrolyte concentrations (⪢ 10 −3 mol dm −3 NaCl). In addition, two aspects of the reversibility of the flocculation of the mixed dispersions have been studied; firstly, the effect of changing the electrolyte concentration, and, secondly, of changing the number of added small positive particles. Reducing the electrolyte concentration from 10 −1 to 10 −3 mol dm −3 NaCl resulted in an increase in the various rheological parameters to values approaching those obtained with a mixed dispersion prepared at the final concentration. However, on increasing the electrolyte concentration from 10 −5 to 10 −1 mol dm −3 NaCl, the relatively strong flocculation that occurred at low electrolyte concentration was not completely reversed. On the other hand, an increase in the positive particle number concentration in the mixed dispersion, initially at the concentration corresponding to maximum flocculation, resulted in deflocculation.