Model colloid-polymer mixtures in porous matrices:: density functional versus integral equations

We test the accuracy of a recently proposed density functional (DF) for a fluid in contact with a porous matrix. The DF was constructed in the spirit of Rosenfeld's fundamental measure concept and was,derived for general mixtures of hard core and,ideal particles. The required double,average over fluid and matrix configurations is performed explicitly. As an application we consider a model mixture where colloids and matrix particles are represented by hard spheres and polymers by ideal spheres. Integrating over the degrees of freedom of the polymers leads to a binary colloid-matrix system with effective Asakura-Oosawa pair potentials, which we, treat with an integral-equation theory. We find that partial pair correlation functions from both theories are in good agreement with our computer simulation results, and that the theoretical results for the demixing binodals compare well, provided the polymer-to-colloid size ratio, and hence the effect of many-body interactions neglected in the effective model, is not too large. Consistently, we find that hard (ideal) matrix-polymer interactions induce capillary condensation (evaporation) of the colloidal liquid phase.