Interfacial properties of model colloid-polymer mixtures

We summarize the main results of our recent investigations of the interfacial properties of the simplest model of a colloid-polymer mixture, namely that introduced by Asakura & Oosawa and by Vrij, in which colloid-colloid and colloid-polymer interactions are treated as hard sphere-like, while the polymer-polymer interaction is ideal (perfectly interpenetrating coils). In spite of its simplicity, we find that the model exhibits rich interfacial behaviour which depends on the size ratio q equivalent to sigma (p)/sigma (c), where sigma (p) and sigma (c) denote the diameters of polymer and colloid, respectively. For highly asymmetric mixtures, q < 0.1547, an explicit and exact mapping of the binary mixture to an effective one-component Hamiltonian for the colloids allows one to perform computer simulations for inhomogeneous mixtures. We investigate a mixture with q = 0.1 and find that small amounts of polymer give rise to strong depletion effects at a hard wall. The colloid density at contact with the wall is several times greater than that for pure hard spheres at a hard wall. However, for states removed from the bulk fluid-solid coexistence curve we find no evidence of wall-induced crystallization. In order to treat less asymmetric cases, where stable fluid-fluid demixing occurs in bulk, we have designed a density functional theory specifically for this model mixture. For q = 0.6 we find a first order wetting transition from partial to complete wetting by the colloid-rich phase at the hard-wall-colloid-poor interface as the packing fraction eta (r)(p) of polymer in the reservoir is decreased. At a slightly higher value of eta (r)(p), there is a novel single layering transition, characterized by a jump in the densities in the first two adsorbed layers, as the bulk colloid packing fraction eta (c) is increased. The same density functional has been used to investigate the surface tension and colloid and polymer density profiles at the free interface between the demixed fluid phases.