Analytic calculation of phase diagrams for solutions containing colloids or globular proteins

Abstract

Second-order Barker-Henderson perturbation theory gives phase diagrams for colloid and protein solutions that include stable and metastable fluid-fluid, solid-fluid, and solid-solid phases. The potential of mean force is described by a hardsphere interacting with a Yukawa potential. Calculations for different ranges of attraction show that, as expected, fluid-fluid coexistence becomes metastable when the potential becomes short-ranged. For a very short-ranged Yukawa potential, the phase diagram shows isostructural solid-solid equilibria with a critical point. To test more simplified models, phase diagrams from second-order Barker-Henderson perturbation theory are compared with those from the random-phase approximation for the fluid phase and the van der Waals theory for the solid phase; this comparison shows significantly different phase diagrams. Moreover, with a potential of mean force with primary and secondary minima, calculations using second-order perturbation theory identify conditions where colloidal and protein solutions can present two fluid-fluid regions, each with a critical point; however, the higher-density fluid-fluid region is likely to be metastable. The analytic calculations described here may be useful for interpretation of experimental phase diagrams and for guiding design of separation processes. © Springer-Verlag 2003.