Ion-Ion Association in Electrolyte Solutions: A Theoretical Investigation

This doctoral dissertation explores the concept of ion-ion association within electrolyte solutions, with the aim of enhancing existing methods for predicting the properties of such solutions. This study follows two distinct paths that  eventually come together to form a unified framework. Initially, an emphasis is placed on investigating the electrical conductivity, a key transport property significantly affected by ion pair formation in electrolyte solutions. Within this research, theoretical works dedicated to predicting electrical conductivity are identified from the existing literature. Through a comparative study, their strengths and weaknesses are elucidated. Subsequently, two novel models, one for single-salt and another for multi-salt electrolytes, are developed to predict the electrical conductivity of electrolyte solutions under the assumption of complete dissociation. These models are constructed based on the Ebeling hierarchy of Smoluchowski dynamics and Debye-Hückel-Onsager theory. Rigorous evaluations are conducted by comparing the predictions of the models with experimental data. The findings affirm that the developed models exhibit high accuracy and reliability under conditions where the assumption of complete dissociation holds.

In the second line of research, the issue of ion pairing in electrolyte solutions is approached from a thermodynamic perspective. This research, similar to the first, begins with a thorough examination of the equations of state for charged hard sphere fluids. This investigation involves comparing the predictions of four distinct equations of state, which consider ion pairing and serve as the  foundation for other models, with numerical solutions to the Poisson- Boltzmann equations, Monte Carlo simulations, and experimental data.

Subsequently, a novel equation of state named Binding Debye-Hückel for charged hard sphere fluids is developed. This model draws on the Debye-Hückel theory, Kirkwood theory, Wertheim theory, and the reference cavity approximation. To validate the BiDH model, its predictions are compared with Monte Carlo simulations  documented in the existing literature. The validation specifically focuses on evaluating the mean ionic activity coefficient, the individual activity coefficient, and the osmotic coefficient. Through meticulous evaluations, the study demonstrates the accuracy and reliability of the BiDH model. In the final research phase, the models previously established and verified for electrical conductivity, which did not consider the impact of ion pairing, are combined with the Binding Debye-Hückel model designed to account for ion pairing effects. Initially, an effort is made to predict the properties, particularly electrical conductivity, of diverse electrolyte solutions where ion pairing may play a significant role. This includes aqueous electrolyte solutions, mixed-solvent electrolyte solutions, and ionic liquid-co-solvent systems, all under the assumption of an implicit solvent model.

Subsequently, a novel electrolyte equation of state termed Binding eSAFT-VR-Mie is formulated. Following this, a new unified framework for the development and validation of models for electrolyte solutions is introduced. This unified approach is applied to predict the properties of aqueous electrolyte solutions across a spectrum of affinity for forming ion pairs, ranging from slightly or non-associative to highly associative electrolyte solutions. The study provides evidence of the effectiveness and reliability of this unified framework.