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Using both theory and continuum simulation, we examine a system comprising a mixture of polymer chains formed from 100 hard-sphere (HS) segments and HS colloids with a diameter which is 20 times that of the polymer segments. According to Wertheim's first-order thermodynamic perturbation theory (TPT1) this athermal system is expected to phase separate into a colloid-rich and a polymer-rich phase. Using a previously developed continuous pseudo-HS potential [J. F. Jover, A. J. Haslam, A. Galindo, G. Jackson, and E. A. Muller, J. Chem. Phys.137, 144505 (2012)], we simulate the system at a phase point indicated by the theory to be well within the two-phase binodal region. Molecular-dynamics simulations are performed from starting configurations corresponding to completely phase-separated and completely pre-mixed colloids and polymers. Clear evidence is seen of the stabilisation of two coexisting fluid phases in both cases. An analysis of the interfacial tension of the phase-separated regions is made; ultra-low tensions are observed in line with previous values determined with square-gradient theory and experiment for colloid–polymer systems. Further simulations are carried out to examine the nature of these coexisting phases, taking as input the densities and compositions calculated using TPT1 (and corresponding to the peaks in the probability distribution of the density profiles obtained in the simulations). The polymer chains are seen to be fully penetrable by other polymers. By contrast, from the point of view of the colloids, the polymers behave (on average) as almost-impenetrable spheres. It is demonstrated that, while the average interaction between the polymer molecules in the polymer-rich phase is (as expected) soft-repulsive in nature, the corresponding interaction in the colloid-rich phase is of an entirely different form, characterised by a region of effective intermolecular attraction. [ABSTRACT FROM PUBLISHER]

We study fluid–fluid phase separation in a colloid–polymer mixture adsorbed in a colloidal porous matrix close to the θ point. For this purpose we consider the Asakura–Oosawa model in the presence of a quenched matrix of colloidal hard spheres. We study the dependence of the demixing curve on the parameters that characterize the quenched matrix, fixing the polymer-to-colloid size ratio to 0.8. We find that, to a large extent, demixing curves depend only on a single parameter f, which represents the volume fraction which is unavailable to the colloids. We perform Monte Carlo simulations for volume fractions f equal to 40% and 70%, finding that the binodal curves in the polymer and colloid packing-fraction plane have a small dependence on disorder. The critical point instead changes significantly: for instance, the colloid packing fraction at criticality increases with increasing f. Finally, we observe for some values of the parameters capillary condensation of the colloids: a bulk colloid-poor phase is in chemical equilibrium with a colloid-rich phase in the matrix. [ABSTRACT FROM AUTHOR]

We analyse demixing and ordering transitions in systems of hard cylindrical particles. The second virial approximation of Onsager and a bifurcation analysis, as introduced by Koda and Kimura, are used to evaluate the free energies, pressures, and density distribution functions in mixtures of equally long but differently wide cylinders. The spatial density distribution along the one relevant coordinate is of particular importance as it provides more detailed information concerning the nature of the phase transition than the bare bifurcation diagnosis. Detailed results are given for the nematic-nematic spinodal and the nematic-smectic transitions. Allowing for the absence of an isotropic phase, our results are in good qualitative agreement with those for freely orienting rods reported previously, and indicate a complex sequence of phase diagrams as the diameter dissimilarity of the two components is increased, with upper and lower critical points bounding nematic and smectic demixing regions. However, experimental results on colloidal rods show that nematic demixing occurs at a diameter ratio much smaller than ours or those for freely rotating fluids, indicating that Onsager-type theories may be insufficient to reproduce this phenomenon in a quantitative manner and, consequently, that more sophisticated approaches, presumably incorporating particle flexibility and additional interactions, are required. [ABSTRACT FROM AUTHOR]