Phase Behavior of Charged Colloids: Closed versus Donnan Equilibrium$^1$

The influence of chemical boundary conditions on thermodynamic properties of deionized charge-stabilized colloidal supensions is analyzed. Effective electrostatic interactions and phase behavior are shown to depend fundamentally on whether a suspension is confined to a closed (electroneutral) cell or is in Donnan equilibrium with a microion reservoir, {\it e.g.}, electrolyte solution. Linear-response theory$^2$ predicts that at low ionic strength closed suspensions of highly charged macroions and monovalent microions can phase separate, while microion exchange with a reservoir stabilizes the fluid phase. \\[1ex] $^1$~Supported by National Science Foundation grant DMR-0204020. \\[0.5ex] $^2$~A.R.~Denton, {\it Phys. Rev.} E {\bf 73}, 41407 (2006).