Phase diagrams of nearly-hard-sphere binary colloids

We have investigated the equilibrium compositions and structures of the phases of nearly-hard-sphere binary colloids and obtained experimental phase diagrams. Aqueous dispersions of charge-stabilized polystyrene spheres were studied in the hard-sphere limit, with sphere-diameter ratios ranging from 2 to 12 and total volume fractions less than 0.4. At sufficiently high volume fractions, the samples separated into two phases. One phase, consisting primarily of small spheres, is a disordered fluid. In the other phase, the large spheres form an ordered crystalline solid which is permeated by a disordered fluid of small spheres. Previously reported crystallites on the surface of the sample cell were demonstrated to have the same structure as the bulk crystals and are assumed to be a wetting of the bulk phase. A simple model of the bulk phases is described and free energies calculated. The predicted phase diagrams agree closely with the results of computer simulations and with our experimental results.