Effects of colloid polydispersity on the phase behavior of colloid-polymer mixtures

We study theoretically the equilibrium phase behaviour of a mixture ofpolydisperse hard-sphere colloids and monodisperse polymers, modelled using theAsakura-Oosawa model within the free volume approximation of Lekkerkerker etal. We compute full phase diagrams in the plane of colloid and polymer volumefractions, using the moment free energy method. The intricate features of phaseseparation in pure polydisperse colloids combine with the appearance ofpolymer-induced gas-liquid coexistence to give a rich variety of phase diagramtopologies as the polymer-colloid size ratio and the colloid polydispersity arevaried. Quantitatively, we find that polydispersity disfavours fluid-solidagainst gas-liquid separation, causing a substantial lowering of the thresholdvalue above which stable two-phase gas-liquid coexistence appears. Phase splitsinvolving two or more solids can occur already at low colloid concentration,where they may be kinetically accessible. We also analyse the strength ofcolloidal size fractionation. When a solid phase separates from a fluid, itspolydispersity is reduced most strongly if the phase separation takes place atlow colloid concentration and high polymer concentration, in agreement withexperimental observations. For fractionation in gas-liquid coexistence welikewise find good agreement with experiment, as well as with perturbativetheories for near-monodisperse systems.Comment: 14 pages, 13 figure