Specific ion effects in non-aqueous solutions

Abstract

Electrolyte solutions play a central role in life and technological processes because of their complexity. This complexity is yet to be described by a predictive theory of the specific effects that different ions induce in solution. The vast majority of investigations of specific-ion effects have been conducted in aqueous solutions. These studies have revealed that amongst the complexity, the effectiveness of the ions often follow trends that are apparent across a number of very different experiments, revealing an underlying order (e.g. the Hofmeister series). It is often assumed that water itself is intricately involved in these trends.

Here I investigate specific-ion effects in non-aqueous solvents rather than water. By extending the investigation to a number of non-aqueous solvents, the role of the solvent in specific-ion effect trends can be elucidated and a better understanding of the general phenomenon gained.

Firstly, a more definite terminology is developed for describing the specific-ion effects trends in order to address the current confusion in the literature and provide a basis for the following investigations. An extensive investigation of the scarce literature demonstrates that water is by no means a special solvent with regards to ion-specificity, and that within the complexity there is universality. An investigation of electrostriction under the conditions of infinite dilution shows that the same fundamental specific ion trends are observed across all solvents, demonstrating that ion-specificity arises from the ions themselves. In this regard the influence of solvents, surfaces and real concentrations of electrolytes can be seen as perturbations to this fundamental series. Further work shows that for systems that are perturbed, the trends in non-aqueous protic solvents can be expected to follow the same trend in water; and in aprotic solvents the cations are more likely to adhere to the trend in water than the anions.

My experimental work focuses on specific-anion effects of seven Hofmeister sodium salts in the solvents: water, methanol, formamide, dimethyl sulfoxide and propylene carbonate. Two very different experiments were performed; the elution of electrolytes from a size-exclusion chromatography column and an investigation of the electrolyte moderated swelling of a cationic brush (PMETAC) using a Quartz Crystal Microbalance (QCM). The trends observed are consistent across these experiments. A forward or reverse Hofmeister series is observed in practically all salt-solvent combinations, and the reversal is attributed to the polarisability of the solvent.

Finally, a qualitative model of ion specific trends is formulated, where the specific-ion effects are fundamentally a property of the ion, and the associated trends correspond to the Hofmeister series for anions and the lyotropic series for cations. When the concentration is increased, or surfaces introduced, the effects of ion-ion interactions and ion-surface interactions can perturb the fundamental series. The perturbation of the series is related to the proticity of the solvent for ion-ion interactions, whereas the polarisability of the solvent and ion are important when a surface is present. This work for the first time individuates the principal properties of the solvent that affect their ordering: proticity and polarisability.