Ionic fluid criticality coexistence curves

Generally, a tricritical point is defined as the point, where three phases become identical. Alternatively a tricritical point can be defined as the point where a second order A-line of an order transition becomes first order or else as the point, where a A-line cuts a coexistence curve at its critical point [4, 5], The last definition is relevant for the ionic fluids. 

It has been proposed to define a reduced temperature Tr for a solution of a single electrolyte as the ratio of kgT to the work required to separate a contact +- ion pair, and the reduced density pr as the fraction of the space occupied by the ions. (M+ ) The principal feature on the Tr,pr corresponding states diagram is a coexistence curve for two phases, with an upper critical point as for the liquid-vapor equilibrium of a simple fluid, but with a markedly lower reduced temperature at the critical point than for a simple fluid (with the corresponding definition of the reduced temperature, i.e. the ratio of kjjT to the work required to separate a van der Waals pair.) In the case of a plasma, an ionic fluid without a solvent, the coexistence curve is for the liquid-vapor equilibrium, while for solutions it corresponds to two solution phases of different concentrations in equilibrium. Some non-aqueous solutions are known which do unmix to form two liquid phases of slightly different concentrations. While no examples in aqueous solution are known, the corresponding... 

Singh, R.R., and Pitzer, K.S. Near-critical coexistence curve and critical exponent of an ionic fluid. J Chem. Phys., 1990, 92, p. 6775-8. 

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