Studies on the structure and thermodynamics of electrolyte solutions

The theoretical and simulation methods of statistical thermodynamics aim to develope a relationship between the intermolecular potential and various structural and thermodynamic properties for the fluids. Most of the previous work in the literature of electrolyte solutions either depend on primitive model in which only the ionic species are described on a molecular basis or employ sophisticated solvent models to describe the real electrolyte solutions;It has been found that the primitive model does not provide a realistic description for the structure and thermodynamics of electrolyte solutions in a wide range of concentrations. The computer simulations with sophisticated models give reliable results when compared with experiments, but these models can not be incorporated into the theoretical methods and most of the reported results are either at infinite dilution or at very low ionic concentrations;The objectives of the present work is to perform Monte Carlo simulations with simple and theoretically tractable models including point dipolar, dipolar and linear quadrupolar hard spheres and dipolar diatomics; to provide a mean of comparison and examine the quality of the existing theories and to study various thermodynamic and structural properties of the electrolytes;In the simulation, the interaction energy is approximated with periodic boundary conditions, and Ewald summation technique is used to calculate ion-ion, ion-dipole, ion-quadrupole and dipole-dipole interaction energies. It has been found that; at low ionic charges and concentrations solvated ions or pairs of ions are dominant, triple ions form mostly linear or close to linear clusters. At high ionic charges and concentrations bent triplets and bigger clusters are observed. The solvation characteristics of anions and cations which are exactly the same in a dipolar solvent are found to be dissimilar in a dipolar plus quadrupolar solvent. The total internal configurational energy, compressibility factor and static component of the dielectric constant are found to decrease and the specific heat is found to increase with increasing ionic charge and concentration;Numerical solution of the Ornstein-Zernike equation with the mean spherical approximation is obtained for a mixture of ions and dipoles with the same diameter. The dipole-dipole and ion-ion energies are found to be in good agreement with the Monte Carlo simulation results. Solution of a previously developed perturbation theory for a mixture of ions and dipoles with a new Pade approximation is obtained. The overall internal energy is found to be close to simulation results. An alternative to the existing perturbation theory is discussed