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Gas-liquid phase coexistence and crossover behavior of binary ionic fluids with screened Coulomb interactions: The effect of an interaction range

We study the gas-liquid phase diagram and the crossover behavior of a simple model of ionic fluid: an equimolar binary mixture of equisized hard spheres interacting through screened Coulomb potentials which are repulsive between particles of the same species and attractive between particles of different species. Using our previous results, we obtain explicit expressions for the relevant coefficients of the effective $φ^{4}$ Ginzburg-Landau Hamiltonian in a one-loop approximation. Within the framework of this approximation, we calculate the critical parameters and gas-liquid coexistence curves for different values of the dimensionless inverse screening length $z$. The critical temperature scaled by the Yukawa potential contact value as well as the critical packing fraction rapidly decrease with an increase of the interaction range and then for $z<0.05$ slowly approach the values found for a purely ionic model, i.e., a restricted primitive model (RPM). The both trends are qualitatively consistent with the results of Monte Carlo simulations. We find that gas-liquid coexistence region reduces with an increase of $z$ and completely vanishes at $z\simeq 2.781$. This qualitatively agrees with the results of Monte Carlo simulations indicating a stable gas-liquid coexistence for $z\leq 4$. It is also shown that an increase in the interaction range from the one typical of simple fluids to the one typical of ionic fluids leads to a decrease of the crossover temperature. For $z\simeq 0.01$, the crossover temperature is the same as for the RPM. For $z\simeq 2.781$, our results indicate a tricritical point.