Electroneutrality and Phase Behavior of Colloidal Suspensions

TL;DR

This paper investigates the phase behavior of highly charged colloidal suspensions, showing that the choice of reference system and electroneutrality constraints critically influence predicted phase stability and separation.

Contribution

It unifies Poisson-Boltzmann and response theories within a perturbative framework emphasizing electroneutrality, clarifying when bulk phase separation predictions are physically consistent.

Findings

Bulk suspensions require density-dependent effective interactions for accurate phase predictions.

Linearized theories predict phase separation only with a closed, electroneutral reference system.

Lower-dimensional systems may behave differently, with density-independent interactions linked to open reference systems.

Abstract

Several statistical mechanical theories predict that colloidal suspensions of highly charged macroions and monovalent microions can exhibit unusual thermodynamic phase behavior when strongly deionized. Density-functional, extended Debye-H\"uckel, and response theories, within mean-field and linearization approximations, predict a spinodal phase instability of charged colloids below a critical salt concentration. Poisson-Boltzmann cell model studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate that effective interactions and osmotic pressures predicted by such theories can be sensitive to the choice of reference system, e.g., whether the microion density profiles are expanded about the average potential of the suspension or about the reservoir potential. By unifying Poisson-Boltzmann and response theories within a common perturbative framework, it is shown here that the choice of reference system is dictated by the constraint of global electroneutrality. On this basis, bulk suspensions are best modeled by density-dependent effective interactions derived from a closed reference system in which the counterions are confined to the same volume as the macroions. Linearized theories then predict bulk phase separation of deionized suspensions only when expanded about a physically consistent (closed) reference system. Lower-dimensional systems (e.g., monolayers, small clusters), depending on the strength of macroion-counterion correlations, may be governed instead by density-independent effective interactions tied to an open reference system with counterions dispersed throughout the reservoir, possibly explaining observed structural crossover in colloidal monolayers and anomalous metastability of colloidal crystallites.