Phase behaviour of a model colloid–polymer mixture at low colloid concentration

Abstract

The phase behaviour of a colloid–polymer mixture is studied at very low colloid concentrations (below 0.5%). The size ratio between the polymer and the colloidal particles is around 0.09, so that the colloids experience short-range attractions. At these low volume fractions, fluid–crystal coexistence is found at moderate polymer concentrations and the kinetics of crystallization are analyzed by turbidimetry. At higher polymer concentrations, clustering of the particles occurs, but some of them remain in a diluted, gas, phase composed mainly of single particles. These states do not correspond to vapor–liquid coexistence, as shown by studying the density of the gas phase. Strong interactions induce flocculation of the system, producing fractal aggregates with dimension d_f ≈ 1.8, compatible with diffusion limited cluster aggregation (DLCA). These results are discussed in connection with the phase diagram of colloid–polymer mixtures, obtained at much higher colloid concentrations. For low colloid volume fractions, below ∼0.05%, no clustering of the particles is observed for any polymer concentration.