Hvozd M. Phase behavior of ionic solutions in the bulk and in a porous media: Primitive model with the explicit consideration of solvent

The thesis is devoted to the study of the phase behavior of ionic solutions with the explicit consideration of anisotropic solvent molecules in the bulk and in a disordered porous media. The system is represented by a model that includes two subsystems: (i) ionic system of charged hard spheres (HS); (ii) model of hard spherocylinders (HS). A disordered porous media is represented by a quenched configuration of randomly distributed HS that form so-called matrix. The generalization of the scaled particle theory (SPT) is used to describe the phase behavior of a binary HS/HSC mixture in the bulk. For such a model there is an isotropic-nematic phase transition. Due to the orientational ordering of HS particles the formation of a nematic phase is expected at certain concentrations of mixture components. In this case anisotropic molecules are oriented along a certain direction. Phase diagrams of the coexistence of isotropic and nematic phases are constructed from the bifurcation analysis of the nonlinear integral equation for the singlet orientation distribution function. It is shown that the presence of HS shifts the phase transition to the lower densities of HSC. With increasing of the sizes of HS the coexistence region is expanded and with increasing of the packing fraction of HS the coexistence region becomes narrower. To study the phase behavior of HS/HSC mixture confined in a disordered porous media, we propose an extention of the SPT. Two approaches are used to describe such a system: (i) bifurcation analysis of a nonlinear integral equation for a singlet orientation distribution function; (ii) a thermodynamic approach based on phase equilibrium conditions. The bifurcation analysis shows that the іncreasing of the packing fraction of matrix shifts the coexistence region of the isotropic and nematic phases towards lower packing fraction of a mixture, and the coexistence region gets narrower. In the framework of thermodynamic approach it was shown that at certain concentrations of HS particles the demixing processes occur in the coexisting phases, leading to the nematic phase rich in HSC and the isotropic phase rich in HS. We have studied the phase behavior of the explicit solvent model represented as a mixture of the restricted primitive model (RPM) of ionic fluid and neutral solvent particles (HSC) in the bulk. RPM is represented by equal number of equisized positively and negatively charged HS. To this end, we combine two theoretical approaches, i.e., SPT and the associative mean spherical approximation (AMSA). The effect of asphericity of solvent molecules on the fluid-fluid phase transition is studied by considering an “equivalent” mixture in which the HSC are replaced by HS of the same volume. To study an ion association phenomena on the phase behavior of RPM-HSC and RPM-HS models, we also use the mean spherical approximation (MSA) for comparison. It is shown that due to the mass action law term (MAL) in the AMSA, in contrast to MSA, the asphericity of solvent molecules can significantly changes the phase diagram of the ionic solution. Moreover, the MAL contribution depends on the contact value between ionic particles. To study the phase behaviour of ionic solutions confined in a disordered porous media with the explicit neutral anisotropic solvent, we combine SPT and the AMSA approximation. It was shown that porous matrix favours orientation ordering of HSC in solvent-rich phase. Due to a disordered porous media a coexistence region shifts towards lower number densities and towards higher temperatures. Asphericity of HSC makes a region of phase coexistence wider and shifts towards higher temperatures. The increase of pressure leads shifts coexistence region towards higher densities and higher temperatures. It was found that the degree of ion dissociation along the coexistence curves in the solvent-rich phase is smaller than in the ion-rich phase. It means that the pairing of oppositely charged ions is more preferable for solvent-rich phase. The presence of disordered porous media lowers the degree of ion dissociation. This work is the first theoretical study of ionic solutions, which describes the effect of anisotropy of solvent particles, and, in addition, the effect of a disordered porous medium on the phase behavior of the system.