On the fluid-fluid phase separation in charged-stabilized colloidal suspensions

We develop a thermodynamic description of particles held at a fixed surface potential. This system is of particular interest in view of the continuing controversy over the possibility of a fluid-fluid phase separation in aqueous colloidal suspensions with monovalent counterions. The condition of fixed surface potential allows one in a natural way to account for the colloidal charge renormalization. In a first approach, we assess the importance of the so called,volume terms' and find that in the absence of salt, charge renormalization is sufficient to stabilize the suspension against a fluid-fluid phase separation. The presence of salt, on the other hand, is found to lead to an instability. A very strong dependence on the approximations used, however, puts the reality of this phase transition in serious doubt. To further understand the nature of the instability we next study a jellium-like approximation, which does not lead to a phase separation and produces a relatively accurate analytical equation of state for deionized suspensions of highly charged colloidal spheres. A critical analysis of various theories of strongly asymmetric electrolytes is presented, to assess their reliability as compared to the Monte Carlo simulations.