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

The phase behavior of colloid-polymer mixtures, and of solutions of globula r proteins, is often interpreted in terms of a simple model of hard spheres with short-ranged attraction. While such a model yields a qualitative unde rstanding of the generic phase diagrams of both colloids and proteins, it f ails to capture one important difference:: the model predicts fluid-fluid p hase separation in the metastable regime below the freezing curve. Such dem ising 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 ef fect 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-fluid critical point out of the gel region. As this metas table critical point may be important for crystal nucleation, our results s uggest that long-ranged attractive fortes may play an important role in the crystallization of globular proteins. However, in colloids, where refracti ve index matching is often used to switch off long-ranged dispersion forces , gelation is likely to inhibit phase separation.