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Gas-liquid phase equilibrium in ionic fluids: Coulomb versus non-Coulomb interactions

Using the collective variables theory, we study the effect of competition between Coulomb and dispersion forces on the gas-liquid phase behaviour of a model ionic fluid, i.e. a charge-asymmetric primitive model with additional short-range attractive interactions. Both the critical parameters and the coexistence envelope are calculated in a one-loop approximation as a function of the parameter $α$ measuring the relative strength of the Coulomb to short-range interactions. We found the very narrow region of $α$ bounded from the both sides by tricritical points which separates the models with "nonionic" and "Coulombic" phase behaviour. This is at variance with the result of available computer simulations where no tricritical point is found for the finely-discretized lattice version of the model.