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

Abstract

We study the equilibrium behaviour of a mixture of monodisperse hard sphere colloids and polydisperse non-adsorbing polymers at their $\theta$-point, using the Asakura-Oosawa model treated within the free-volume approximation. Our focus is the experimentally relevant scenario where the distribution of polymer chain lengths across the system is fixed. Phase diagrams are calculated using the moment free energy method, and we show that the mean polymer size ${\xi }_{\mathrm{c}}$ at which gas-liquid phase separation first occurs decreases with increasing polymer polydispersity $\delta$. Correspondingly, at fixed mean polymer size, polydispersity favours gas-liquid coexistence but delays the onset of fluid-solid separation. On the other hand, we find that systems with different $\delta$ but the same {\em mass-averaged} polymer chain length have nearly polydispersity-independent phase diagrams. We conclude with a comparison to previous calculations for a semi-grandcanonical scenario, where the polymer chemical potentials are imposed, which predicted that fluid-solid coexistence was over gas-liquid in some areas of the phase diagram. Our results show that this somewhat counter-intuitive result arose because the actual polymer size distribution in the system is shifted to smaller sizes relative to the polymer reservoir distribution.

arXiv commentChanges in v2: sketch in Figure 1 corrected, other figures improved; added references to experimental work and discussion of mapping from polymer chain length to effective radius

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Cited by in corpus (6)

• Influence of polydispersity on the critical parameters of an effective potential model for asymmetric hard sphere mixtures
• Crowding of Polymer Coils and Demixing in Nanoparticle-Polymer Mixtures
• Phase diagram of mixtures of colloids and polymers in the thermal crossover from good to $\theta$ solvent
• Virial coefficients and demixing in the Asakura-Oosawa model
• Multiphase coexistence in polydisperse colloidal mixtures
• Fluid-fluid demixing curves for colloid-polymer mixtures in a random colloidal matrix