A long-standing debate in colloid science concerns the origin of anomalous phase behavior in deionized suspensions. Aqueous suspensions of highly charged macroions and monovalent microions at sub-millimolar ionic strengths reportedly can display liquid-vapor coexistence, stable voids, and compressed crystals. Several effective interaction ("volume term") theories predict a strikingly similar spinodal instability at low ionic strengths, even after accounting for renormalization of the effective macroion charge. Such predictions have been challenged, however, by studies based on the Poisson-Boltzmann (PB) cell model, which suggest that phase instability may be an artifact of linearization approximations. By carefully analyzing the foundations of PB theory, it is here shown that the osmotic pressure derived from the cell model typically neglects a contribution associated with the Donnan effect. The constraint of global electroneutrality, which leads to unequal partitioning of microions across a semi-permeable interface separating the suspension from an electrolyte reservoir, generates an electrostatic (Donnan) potential difference at the interface. At low (but nonzero) salt concentrations, the density dependence of the Donnan potential and of the macroion self energy can profoundly affect the phase behavior of highly charged colloids. Consistent incorporation of the Donnan effect is shown to formally unify linearized PB and effective interaction theories. These conclusions should dispel concerns of theoretical artifacts and renew experimental interest in the unusual phase behavior of deionized colloidal suspensions.

Supported by the Petroleum Research Fund (PRF 44365-AC7) and the National Science Foundation (DMR-0204020).