Penetrability in model colloid-polymer mixtures

In order to study the effects of penetrability in mixtures of dissimilar particles we consider hard (colloidal) spheres and penetrable spheres. The latter may be taken to represent ideal, noninteracting polymer coils. Polymers and colloids interact by means of a repulsive step-function pair potential, which allows for insertion of colloids into the polymer coil. The potential strength is obtained from scaling arguments for the cross virial coefficient of true colloid-polymer systems. For this model we construct a geometry-based density functional and apply it to bulk fluid demixing. We find that taking into account penetrability leads to a significant stabilization of the mixed phase for large polymer-to-colloid size ratio. (C) 2002 American Institute of Physics.