Ionic fluid criticality pairing theories

The proper treatment of ionic fluids at low T by appropriate pairing theories is a long-standing concern in standard ionic solution theory which, in the light of theories for ionic criticality, has received considerable new impetus. Pairing theories combine statistical-mechanical theory with a chemical model of ion pair association. The statistical-mechanical treatment is restricted to terms of the Mayer/-functions which are linear in / , while the higher terms are taken care by the mass action law... 

At the present stage, only MSA- and DH-based theories have been widely applied. From these theories, and their comparison with MC data, there is now a quite consistent picture of the major mechanisms driving these ionic phase transitions, although there are still difficulties to predict the critical point on a quantitative level. The results point toward the crucial importance of ion pairing and of interactions between ion pairs and the remainder of the ionic fluid. Deficits in predicting the critical point quantitatively seem to be closely connected to a substantial overestimate of ionicity in almost all pairing theories. 

In the absence of a reliable theory, computer simulations have become the most important means of tackling the question of ionic criticality. In the last ten years there have been numerous attempts to identify the universality class of what is perhaps the most basic model of ionic fluids, the restricted primitive model (RPM). The RPM consists of an equimolar mixture of positively and negatively charged hard spheres with diameter a, immersed in a dielectric continuum with dielectric constant D. The pair potential is,... 

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