Reversible flocculation of sterically-stabilised dispersions

Abstract

Because of the existence of a shallow minimum in the interaction energy against particle separation diagram for neutral, sterically-stabilised particles, dispersions of such particles exhibit reversible flocculation above a threshold particle number concentration. The equilibrium state achieved is one in which a “floc phase” co-exists with a “dispersed phase”. By analogy with related phase separation processes in molecular systems the critical particle number concentration for flocculation is shown to be temperature dependent. An equivalent statement is that the critical flocculation temperature is dependent on the particle number concentration. Temperature phase diagrams have been constructed for the system: neutral polystyrene latex particles carrying anchored, low molecular weight poly(ethylene oxide) chains, dispersed in various concentrations of aqueous MgSO₄ solutions. Reversible flocculation may also be achieved by adding polymer to the continuous phase, at a given temperature. A “three-component” phase diagram has been constructed for the system: polystyrene latex particles carrying anchored poly(ethylene oxide) chains as above, plus water, plus free poly(ethylene oxide) molecules of varying weight. A preliminary, thermodynamic analysis of the results is presented.