Exploring new protic ionic liquids: From synthesis to fundamental properties

Doctoral thesis, 2023

The ionic liquid community frequently leverages the selling point that more than a million new ionic liquids could conceivably be created. Nonetheless, the number of commercially available compounds is orders of magnitude lower. This highlights the fact that only a small number of all possible ionic liquids are actively being researched, a reality particularly noticeable in the niche field of protic ionic liquids. In such a scenario, research focusing on the development of even a small number of viable alternatives to the popular alkylammonium- and imidazolium-based cations could potentially have a big impact, by paving the way for the synthesis of new families of ionic liquids. However, for these new alternatives to be widely used by the community, they must be easy to synthesize and have desirable properties.

In this thesis, I discuss the challenges that I have encountered and the lessons that I have learned while trying to explore the chemical space of protic ionic liquids. This exploration started with the development of a procedure for the synthesis of pure and dry protic ionic liquids, which was used to make new triazolium-based protic ionic liquids. Additionally, this first work highlights the importance of using air-free techniques to analyze these hygroscopic compounds. Later, these insights were used to develop a new setup for the determination of ionic conductivity in ionic liquids. The latter was used in conjunction with pulsed-field gradient nuclear magnetic resonance diffusion experiments and density functional theory experiments to understand the differences in transport properties between triazolium- and imidazolium-based protic ionic liquids. Finally, we once again turned our attention to the imidazolium cation and explored how simple modifications to its electronic structure, by means of functionalization with electron-withdrawing groups, can enhance its acidity, and how that affects the properties of these nitro- and cyano-functionalized protic ionic liquids.

This thesis aims to highlight the importance of developing new methods for the synthesis and analysis of protic ionic liquids, as well as to explore how computational modeling can be used to rationalize the observed differences in the physicochemical properties of these compounds.