Phase behavior of a model colloid-polymer mixture

We report a detailed experimental study of a model hard-sphere colloid plus nonadsorbing polymer mixture in which the depletion effect causes phase separation. The key parameter that determines the topology of the phase diagram is the ratio ξ=${\mathit{r}}_{\mathit{g}}$/a of the radius of gyration (${\mathit{r}}_{\mathit{g}}$) of the polymer molecules to the radius (a) of the colloidal particles. At small ξ, the addition of the polymer simply expands the colloidal fluid-crystal coexistence region of the pure hard-sphere system. For larger values of ξ, however, a three-phase coexistence of colloidal gas, liquid, and crystal phases is observed. The crossover between the two topologies is found at ξ∼0.25. These experimental observations are compared with the predictions of a recent statistical mechanical theory for such mixtures [Lekkerkerker et al., Europhys. Lett. 20, 559 (1992)]. The relevance of our results to the current debate about the conditions necessary for the existence of a ‘‘liquid’’ state is discussed.