Field-theoretic approach to ionic systems: Criticality and tricriticality

A Landau-Ginzburg functional of two order parameters (charge-density phi and mass-density deviation eta) 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 not equal 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 eta 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. (C) 2000 Elsevier Science B.V. All rights reserved.