  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 $\varphi^{4}$ 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.
