The restricted primitive model for ionic fluids

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Abstract
The vapour of an ionic fluid at low temperature is known to be dominated by ion pairs and larger neutral clusters with smaller populations of individual ions and charged clusters. Previous cluster treatments of the restricted primitive model are improved to include clusters through the hexamer and to consider interactions between clusters according to the mean spherical approximation. With these refinements it is possible to obtain good results for reciprocal reduced temperatures β* from 6·8 to 20 which include the critical temperature. The distribution among clusters and various thermodynamic properties are calculated. The concentration of charged clusters is reported for comparison with measurements of electrical conductance. Since the approximations of this approach become serious at reduced densities (c* = Na3/V) above 0·02, interpolations are made with an empirical equation of Larsen based in turn on various Monte Carlo simulations. These interpolations yield estimates of the saturated liquid and vapour densities and a critical β* of 14·7. The speculative nature of the estimates of critical properties is emphasized.