Interfacial properties of model colloid-polymer mixtures

We summarize the main results of our recent investigations of the interfaci al properties of the simplest model of a colloid-polymer mixture, namely th at introduced by Asakura & Oosawa and by Vrij, in which colloid-colloid and colloid-polymer interactions are treated as hard sphere-like, while the po lymer-polymer interaction is ideal (perfectly interpenetrating coils). In s pite of its simplicity, we find that the model exhibits rich interfacial be haviour 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 Hamilton ian for the colloids allows one to perform computer simulations for inhomog eneous 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. Th e colloid density at contact with the wall is several times greater than th at for pure hard spheres at a hard wall. However, for states removed from t he bulk fluid-solid coexistence curve we find no evidence of wall-induced c rystallization. In order to treat less asymmetric cases, where stable fluid -fluid demixing occurs in bulk, we have designed a density functional theor y specifically for this model mixture. For q = 0.6 we find a first order we tting 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 et a (r)(p), there is a novel single layering transition, characterized by a j ump 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 pr ofiles at the free interface between the demixed fluid phases.