The role of long-range forces in the phase behavior of colloids and proteins

Abstract

The phase behavior of colloid-polymer mixtures, and of solutionsof globular proteins, is often interpreted in terms of a simple model of hard spheres with short-ranged attraction. While such a model yields a qualitative understanding of thegeneric phase diagrams of both colloids and proteins, it fails to captureone important difference: the model predicts fluid-fluid phase separation in the metastable regime below the freezing curve. Such demixing has been observed for globular proteins, but for colloids it appears to be pre-empted by the appearance of a gel. In this paper, we study the effect of additional long-range attractions on the phase behavior of spheres with short-ranged attraction. We find that such attractions can shift the (metastable) fluid-fluidcritical point out of the gel region. As this metastablecritical point may be important for crystal nucleation, our results suggestthat long-ranged attractive forces may play an important role in thecrystallization of globular proteins. However, in colloids, where refractiveindex matching is often used to switch off long-ranged dispersion forces,gelation is likely to inhibit phase separation.