Europhys. Lett, 48 (3), pp. 332-338 (1999)
The role of long-range forces in the phase behavior of colloids and proteins
M. G. Noro, N. Kern and D. Frenkel
FOM Institute for Atomic and Molecular Physics
Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
(received 21 April 1999; accepted in final form 3 September 1999)
PACS. 82.70Dd - Colloids.
PACS. 87.15Nn - Properties of solutions; aggregation and crystallization of macromolecules.
PACS. 64.75 - Solubility, segregation and mixing.
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.
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