Phase behavior of mixtures of colloidal platelets and nonadsorbing polymers

The phase behavior of a model system of colloidal platelets and nonadsorbing polymers is studied using computer simulations and perturbation theory. The equation of state for the pure platelet reference system is obtained by Monte Carlo simulations, and the free volume fraction accessible to polymers is measured by a trial insertion method. The free volume fraction is also calculated using scaled particle theory. Subsequently, the phase diagram for platelet–polymer mixtures is calculated. For a platelet aspect ratio $\text{L/D=0.1}$ and a polymer to platelet size ratio $\text{d/D>0.2,}$ we observe coexistence between two isotropic phases with different densities. For smaller polymers $\text{d/D<0.2,}$ only one isotropic phase is present. At higher platelet concentrations nematic and columnar phases are found. Where possible, direct simulations of plate–polymer mixtures, namely Gibbs ensemble simulation and Gibbs–Duhem integration, are used to check the validity of the perturbation approach. Qualitatively similar results are obtained for platelets of $\text{L/D=0.05.}$ The results are compared with existing theoretical data as well as with experimental observations.