Phase behavior of a model colloid-polymer mixture

Abstract

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 \ensuremath{\xi}=${\mathit{r}}_{\mathit{g}}$/a of the radius of gyration (${\mathit{r}}_{\mathit{g}}$) of the polymer molecules to the radius (a) of the colloidal particles. At small \ensuremath{\xi}, the addition of the polymer simply expands the colloidal fluid-crystal coexistence region of the pure hard-sphere system. For larger values of \ensuremath{\xi}, however, a three-phase coexistence of colloidal gas, liquid, and crystal phases is observed. The crossover between the two topologies is found at \ensuremath{\xi}\ensuremath{\sim}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.