PHASE-DIAGRAMS OF NEARLY HARD-SPHERE BINARY COLLOIDS

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