Heteroflocculation of particle mixtures by a coacervation mechanism. A rheological study.

In most of the classical studies of heteroflocculation two sets of oppositely charged particles are mixed. In this current study, a somewhat different mechanism of heteroflocculation is described. Two sets of concentrated dispersions of polyacrylate latex particles (having the same surface-charge sign) have been mixed, where the surface of one set had been functionalized with methacrylic acid (MAAc) groups and the second set with poly(ethylene oxide) (PEO) chains. The resultant heteroflocculation has been investigated as a function of the number fraction (F) of MAAc-functionalized particles, the size ratio (R) of the two sets of particles (R = d(MMAc)/d(PEO)), the background electrolyte concentration (0-0.2 M KCl), the pH (3 or 9), and the order of mixing of the particles. The relative extent and strength of flocculation were assessed using two basic rheological techniques: (i) the plastic viscosity (eta(pl)) and the Bingham yield stress (sigmaB) were determined from steady-state shear experiments, (ii) the modulus (G) and the viscosity (eta) (both at a given applied stress) and the actual yield stress (sigmaY) were determined from creep-recovery experiments. Heteroflocculation was observed at a pH value of 3, where the carboxylic groups at the surface of the MAAc-functionalized particles remain largely undissociated. However, no flocculation was observed at pH 9, where the COOH groups dissociate to become COO-. The aggregation mechanism is, therefore, believed to be due to hydrogen-bonding between the hydrogens of the carboxylic acid groups and the ether oxygens present on the surface of the PEO-functionalized particles. To this extent, this mechanism of heteroflocculation resembles the coacervation of mixtures of solutions of two H-bonding polymers, for which aqueous mixtures of PMAAc and PEO (at low pH) are a well-known example. Because both sets of particles carried negative surface charge groups, arising from the polymerization initiator used in their preparation, a minimum concentration of added electrolyte (KCl) was needed before any heteroflocculation between the two sets of particles was observed. However, this minimum KCl concentration for the onset of heteroflocculation was significantly lower than the concentration of KCl required to induce homoflocculation of either set of latex particles separately. At an R value of 1.3, all the rheological parameters passed through a maximum value at F = 0.5, whereas when R was 6.2 the maximum occurred at a value of F = 0.018 or F = 0.025, depending on the order of mixing.