On advancing molecular theories for electrolyte-solutions and polar fluids

Abstract

This thesis explores the thermodynamics of electrolyte solutions and polar fluids, with applications in several key industries like petrochemicals, geothermal energy, and carbon capture/storage. It utilises statistical mechanics to investigate challenges in modelling these systems, exploring theories such as Debye-Hückel (DH), Mean-Spherical Approximation (MSA), and the SAFT (Statistical Associating Fluid Theory) framework. The SAFT-𝛾 Mie equation of state is employed to model CO2 solubility in aqueous electrolyte solutions, with both spherical and non-spherical water models evaluated. The non-spherical model is shown to achieve superior accuracy when compared to experiments. A detailed study of NaCl in water evaluates the accuracy of using the MSA, DH, and Born as perturbation terms to describe the chemical potential of the ions in electrolyte solutions. The MSA model aligns more closely with simulation data, while the Born contribution plays a key role in describing ion-solvent interactions. Molecular structure analysis through radial and orientational distribution functions is also presented. The thesis introduces a new molecular theory for the dielectric constant of dipolar fluids, based on the augmented modified mean-field (AMMF) approximation. The theory accurately describes the dielectric constant in dipolar hard-sphere fluids and extends to the Stockmayer fluid, achieving good agreement with simulation data. It also examines the orientational structures of the Stockmayer fluid, correlating theory with dipole angle distributions. In summary, the thesis covers CO2 solubility modelling, ion interactions, and a new molecular theory for the dielectric constant of dipolar fluids, contributing to the understanding of electrolyte solution thermodynamics.