The equilibrium properties and microstructure of mixtures of colloidal particles with long-range, soft repulsions

Osmotic pressure and elastic moduli of bimodal suspensions of particles exp eriencing long-range, soft repulsions were measured. At fixed osmotic press ure, the total suspension volume fraction, phi, varies linearly as the mixi ng ratio phi(l)/phi is increased from 0 to 1. Here phi(l) is the volume fra ction of large particles based on total suspension volume. This result sugg ests the suspensions studied here are phase separated into domains containi ng primarily small and primarily large particles and is not expected for ha rd sphere suspensions where, at fixed osmotic pressure, phi passes through a maximum as phi(l)/phi is increased. Elastic moduli are well described by a model based on a composite microstructure where the domains of pure large and small particles must have the same osmotic pressure which fixes the lo cal particle volume fraction and hence the elastic modulus in each phase. T he existence of phase separation is supported by electron micrographs taken on samples prepared by rapidly drying suspensions with volume fractions ne ar 0.6. (C) 1999 Academic Press.