Model colloid–polymer mixtures in porous matrices: density functional versusintegral equations

Abstract

We test the accuracy of a recently proposed density functional (DF) for afluid in contact with a porous matrix. The DF was constructed in thespirit of Rosenfeld’s fundamental measure concept and was derived forgeneral mixtures of hard core and ideal particles. The required doubleaverage over fluid and matrix configurations is performed explicitly. As anapplication we consider a model mixture where colloids and matrix particles arerepresented by hard spheres and polymers by ideal spheres. Integrating overthe degrees of freedom of the polymers leads to a binary colloid–matrixsystem with effective Asakura–Oosawa pair potentials, which we treatwith an integral-equation theory. We find that partial pair correlationfunctions from both theories are in good agreement with our computersimulation results, and that the theoretical results for the demixing binodalscompare well, provided the polymer-to-colloid size ratio, and hence the effectof many-body interactions neglected in the effective model, is not toolarge. Consistently, we find that hard (ideal) matrix–polymer interactionsinduce capillary condensation (evaporation) of the colloidal liquid phase.