Field-Theoretic Approach to Ionic Systems: Criticality and Tricriticality

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A Landau-Ginzburg functional of two order parameters (charge-density ϕ and mass-density deviation η) is developed in order to yield a field theory relevant to ionic lattice gases as well as a family of off-lattice models of ionic fluids that go beyond the restricted primitive model (RPM). In a mean-field (MF) approximation an instability of a uniform phase with respect to charge fluctuations with a wave-number k≠ 0 is found. This second-order transition to a charge-ordered phase terminates at a tricritical point (tcp). Beyond MF, a singularity of a mass correlation function for k→ 0 occurs at ion concentration lower than that of the MF tcp. An effective functional depending only on η is constructed. For low ion concentration the usual Landau form of the simple-fluid (Ising) functional is obtained; hence in this theory the critical point is in the Ising universality class.