Field theory for size- and charge asymmetric primitive model of electrolytes. Mean-field stability analysis and pretransitional effects

The primitive model of ionic systems is investigated within a field-theoretic description for the whole range of size-, \lambda, and charge, Z, ratios of the two ionic species. Two order parameters (OP) are identified, and their relations to physically relevant quantities are described for various values of \lambda and Z. Instabilities of the disordered phase associated with the two OP's are determined in the mean-field approximation. A gas-liquid separation occurs for any Z and \lambda different from 1. In addition, an instability with respect to various types of periodic ordering of the two kinds of ions is found.