Equilibrium aspects of heteroflocculation in mixed sterically-stabilised dispersions

Abstract

The equilibrium adsorption–desorption behaviour of small, positive polystyrene latex particles onto much larger, negative polystyrene latex particles, in which both sets of particles carry adsorbed poly(vinyl alcohol)(PVA) molecules, has been studied as a function of the molar mass of the PVA, and the ionic strength. Two types of isotherm have been observed: at low ionic strengths, one obtains high-affinity, irreversible isotherms; at high ionic strengths, S-shaped, reversible isotherms result. There appears to be a critical electrolyte concentration (or at least a very narrow range of electrolyte concentrations) at which the transition, from one isotherm type to the other, occurs. This critical electrolyte concentration decreases with increasing molar mass of the PVA. The results are rationalised in terms of two sets of forces acting in the system: the primary, adsorbing forces normal to the interface, and the lateral interactions between adsorbed particles (which are repulsive at low ionic strengths, but attractive at high ionic strengths). Scanning electron microscopy results broadly confirm the conclusions reached from the adsorption studies.