Fluid-fluid phase behaviour in the explicit solvent ionic model: Hard spherocylinder solvent molecules

Abstract

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. We restrict our attention to the HSC with aspect ratios L2/σ2 = 5 and 10, when the diameter of RPM particles is set to σ1 = σ2. We find the conditions at which the fluid-fluid phase transition is accompanied by the isotropic-nematic phase transition occurring in the solvent component. Furthermore, the effect of asphericity of solvent molecules on the fluid-fluid phase transition is studied by considering an “equivalent” mixture in which the HSC particles are replaced by hard spheres (HS) of the same volume. We discuss an importance of accounting for the association effects which appear between oppositely charged ions.