A quantum chemical investigation of Hofmeister effects in non-aqueous solvents

Specific ion effects (SIEs) encompass any phenomenon induced by ions that is dependent on the identity of the ions, and not just their charge or concentration. These occur in salts, electrolyte solutions, ionic liquids, acids and bases and have been known for over 130 years, from which the Hofmeister series originated. They are important in biology, nutrition, electrochemistry and various interfacial or geophysico-phenomena. It is perhaps harder to find a “real-world” system in which specific ion effects don’t occur, than systems where they do. Nonetheless, despite such ubiquity and effect on our daily lives, our understanding of these salty solutions is limited. This thesis addresses the knowledge gap surrounding the lack of parameters (for both ion and solvent) for quantifying SIEs in aqueous and nonaqueous environments. This thesis begins with a deeper introduction to the topic of SIEs and highlights the current state-of-play. The theories underlying quantum mechanics and computational chemistry are discussed to highlight how they may be applied to elucidate some of the fundamental origins of SIEs. These methods were subsequently used to investigate possible energetic origins of counterion and solvent induced reversals to the Hofmeister series, and highlights that the Lewis acidity and basicity (collectively Lewis strength) indices of the cations and anions respectively, can quantify SIEs. Following this revelation, these empirical parameters were recast in terms of intermolecular forces. Electrostatics appeared to govern the Lewis strength indices, so these were replaced with an electrostatic parameter, ϸ (“sho”), that originates from Coulomb’s Law. For anions, ϸ is shown to quantify SIE trends observed in enthalpies of hydration, polymer lower critical solution temperatures, enzyme and viral activities, SN2 reaction rates and Gibbs free energies of transfer from water to nonaqueous solvents highlighting the versatility of ϸ as a new SIE parameter. Cation interactions are more prone to deviations from ϸ correlations. In the absence of any cosolute (i.e., pure ion-solvent interactions) however, cation solvent interactions follow a strong trend with Coulomb’s Law for ~15 different solvents. This supports a conclusion that competing electrostatic interactions between the solvent and a cosolute for the cation may mask each other allowing non-electrostatic contributions to play a dominant role. Furthermore, with similarity to the ion parameterisation, the ϸ values at the negative and positive solvent dipolar atoms correlate with the solvent’s Lewis basicity and acidity respectively. Additionally, these analyses can be related to macroscopic solvent parameters such as the relative permittivity. The data deficiency issue facing the SIE field was more generally addressed in this thesis by the generation of IonSolvR, a repository containing over 3000 distinct QM/MD trajectories of up to 52 ions in 28 bulk solvents on nanosecond-scales. Finally, the key findings of this thesis are summarised and an outlook on the field of SIEs and the broader implications arising from this thesis is presented.