An experimental study of a model colloid-polymer mixture exhibiting colloidal gas, liquid and crystal phases

Abstract

We report an experimental study of the phase behaviour of a model hard sphere colloid + non-adsorbing polymer mixture — sterically stabilised PMMA and polystyrene in cis-decalin. The addition of non-adsorbing polymer to an otherwise stable colloidal suspension can induce phase instability. This is due to an entropic effect whereby the polymer induces an effective “depletion” attraction between the particles. The topology of the resultant phase diagram (colloid volume fraction versus polymer concentration) depends on the ratio of polymer size (δ to particle size (a), ξ=δ/a. For ξ<0.20, moderate concentrations of polymer cause the suspension to separate into coexisting colloidal fluid and colloidal crystalline phases whereas more polymer leads to “gel” states in which crystallization is supressed. For ξ > 0.20 three phase (colloidal gas, liquid, crystal) coexistence is observed for the first time, and is reported here for two systems of different polymer size (ξ=0.24, and ξ=0.57 at 23°C). These results are in qualitative agreement with statistical mechanical predictions.