Pair potential ionic fluids

Expressions obtained for the free energy, pressure and chemical potentials can be used to study the thermodynamic properties of the electrolyte solutions, in particular, to describe the phase diagram of ionic fluids. Such a possibility is illustrated in Fig. 7, which shows the effect of ion pairing on liquid-liquid coexistence curve in the ion-dipole model as a function of the ion concentration a = ()%/(pi + ps) and reduced temperature T = (Ms)-1/2, bs = Rpl/Rl The solid line corresponds to the ion-dipole model with the parameter of ion association, B = 10. The dashed line corresponds to the ion-dipole... 

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,... 

Here cr is the collision diameter and e is the depth of the potential well at the minimum of zz(r). For molecules we often use combinations of atomic pair potentials, adding several body potentials that describe bending or torsion when needed. For dipolar fluids we have to add dipole-dipole interactions (or, in a more sophisticated description. Coulomb interactions between partial charges on the atoms) and for ionic solutions also Coulomb interactions between the ionic charges. 

In the case of ionic fluids it is simplest to begin with the restricted primitive model, a system of charged hard spheres, all of equal diameter, half of which carry a charge of qi and half of which carry a charge q2 = -q. Thus the pair potential between two particles of charge and qj is of the form... 

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