  We study a fluid-fluid phase transition of the explicit solvent model
represented as a mixture of the restricted primitive model (RPM) of ionic fluid
and neutral hard spherocylinders (HSC). To this end, we combine two theoretical
approaches, i.e., the scale particle theory (SPT) and the associative mean
spherical approximation (AMSA). Whereas the SPT is sufficient to provide a
rather good description of a reference system taking into account hard-core
interactions, the AMSA is known to be efficient in treating the Coulomb
interactions between the ions. Alternatively, we also use the mean spherical
approximation (MSA) for comparison. In general, both approximations lead to
similar qualitative results for the phase diagrams: the region of coexisting
envelope becomes broader and shifts towards larger densities and higher
temperatures when the pressure increases. However, the AMSA and the MSA produce
different concentration dependences, i.e., contrary to the MSA, the AMSA phase
diagrams show that the high-density phase mostly consists of the ions for all
pressures considered. To demonstrate the effect of asphericity of solvent
molecules on the fluid-fluid phase transition, we consider an "equivalent"
mixture in which the HSC particles are replaced by hard spheres (HS) of the
same volume. It is observed that in the case of HSC solvent (RPM-HSC model),
the region of phase coexistence is wider than for the case of the solvent
molecules being of spherical shape (RPM-HS model). It is also found that the
critical temperature is higher in the RPM-HSC model than in the RPM-HS model,
though it becomes the same at higher pressures in the MSA, while in the AMSA
this difference remains essential.
