Monte Carlo Simulation And Cluster Expansion Theory Study On Electrolyte Model Fluid

Study of electrolyte solution is of great significance for understanding of many natural phenomena and design of industrial processes. Study of electrolyte solution has long been a challenging theme due to the intrinsic complexities involved such as the long-range interionic interaction, the ionic solvation, mixed solvents and mixed electrolytes etc. In recent years, as the development of statistical mechanics, molecular thermodynamics and computer simulation, there is a better understanding on the microscopic structure of electrolyte solution. The main study trend is gradually turned from the traditional solution theory and semi-empirical model to statistical mechanics theory, from primitive to non-primitive electrolyte model, in order to establish some high lever molecular thermodynamic models, which can predict the macroscopic thermodynamic properties of electrolyte solution from microscopic molecular and ionic parameters.The ion-dipole model is the simplest non-primitive model fluid of electrolyte solution, which is based on the explicit account of all the particle species in the solution. Deep insight on the attributes of this model fluid of electrolyte solution helps to describe the intrinsic characteristics of ionic solution, the effective pair-wise interactions among particles, and the variation trend and interrelationship of thermodynamic properties of electrolyte solution with temperature, concentration and dipolarity of solvents. In this thesis, the structure and thermodynamic properties of ion-dipole electrolyte model fluid was studied by using Monte Carlo computer simulation technique and cluster expansion theory.Monte Carlo simulations were performed for symmetrical electrolyte model fluid of 1-1 and 2-2 type with equal diameter, and 1-1 type but with unequal diameter systems. For the latter case, the sodium chloride aqueous solution is taken as a prototype to calculate the simulation conditions. As such the structure and thermodynamic properties of this fluid were obtained and their variations with temperature, density and concentration were analyzed and discussed in detail. The electrolyte solution was represented with a mixture of charged and dipolar hard sphere, and the long-range forces between ion-ion, ion-dipole and dipole-dipole were calculated by an Ewald sum technique. Ion dipole mixture with unequal diameter is more realistic to the actual electrolyte solution, where the Pauling ionic diameters are used and the ionic number densities are calculated from experimental density of NaCl solution at specified temperatures and concentrations. The present study helps to understand the origin of some main characteristics and macroscopic phenomena of electrolyte solutions and lay a solid foundation for building up a molecular thermodynamic model for electrolyte solution via perturbation theory approach.Although the cluster expansion theory has been successfully used to the non-electrolyte system leading to the well-known viral type equation of state, its application to electrolyte solution has been not available to our best knowledge. Considering that the ionic density in electrolyte solution is generally as low as that of low density gas, we explored the applicability of cluster expansion theory to electrolyte solution in this paper. Firstly, based on the charged hard sphere primitive model, an explicit model for electrolyte solution has been developed in terms of the cluster expansion theory, with which the osmotic coefficient and activity coefficient in dilute concentration range can be predicted qualitatively with Pauling ionic diameter. In this treatment, the effects of solvent and mean field of surrounding ions have been considered by virtue of an effective interionic pair potential and the three-body interaction is neglected. The applicability of the model as well as the effective pair potential adopted have been tested with activity and osmotic coefficients of a series of sodium halides aqueous solutions. In addition, we studied the non-primitive model fluid of electro...