Phase diagram of charge-stabilized colloidal suspensions: van der Waals instability without attractive forces

Abstract

A careful analysis of the classic Derjaguin-Landau-Verwey-Overbeek theory of the interaction energy in a suspension of charge-stabilized, spherical colloidal particles (polyions) in the presence of salt shows that in addition to the usual screened-Coulomb effective pair interaction between polyions, there exists a structure-independent but state-dependent contribution (the “volume” term), which has almost invariably been overlooked. A variational procedure based on the Gibbs-Bogoliubov inequality is used to calculate the contribution of the polyion pair interactions to the free energy of the suspension. The latter is then combined with the “volume” term to derive the phase diagram of the colloidal suspension. Although the effective pair interaction between polyions is purely repulsive, it is shown that the volume term may drive a van der Waals–like instability in highly deionized suspensions (salt concentrations less than 20 μM) for experimentally relevant choices of the polyion radius and charge. If the latter are sufficiently large, the fluid-fluid phase separation is preempted by the fluid-solid freezing transition which broadens considerably. Reentrant behavior is predicted on the solid side of the phase diagram. The predicted phase diagrams may provide an explanation of some surprising recent experimental results. They also show that the observation of a fluid-fluid phase separation in a charge-stabilized colloidal dispersion does not necessarily imply the existence of an attractive component in the effective pair interaction between highly charged polyions.