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

TL;DR

This paper investigates how long-range attractive forces influence the phase behavior of colloids and proteins, revealing that such forces can promote fluid-fluid phase separation and potentially aid protein crystallization, contrasting with colloids where gelation inhibits phase separation.

Contribution

The study demonstrates that incorporating long-range attractions shifts the fluid-fluid critical point out of the gel region, highlighting their role in protein crystallization and phase behavior.

Findings

Long-range attractions can move the critical point out of the gel region.

In proteins, long-range forces may facilitate crystallization.

In colloids, gelation often prevents phase separation.

Abstract

The phase behavior of colloid-polymer mixtures, and of solutions of 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 the generic phase diagrams of both colloids and proteins, it fails to capture one 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-fluid critical point out of the gel region. As this metastable critical point may be important for crystal nucleation, our results suggest that long-ranged attractive forces may play an important role in the crystallization of globular proteins. However, in colloids, where refractive index matching is often used to switch off long-ranged dispersion forces, gelation is likely to inhibit phase separation.