Binary mixtures and cationic modification of protic ionic liquids. Local structure and transport properties

Doctoral thesis, 2021

Protic ionic liquids are a subclass of ionic liquids, which in many cases are obtained by mixing an equimolar amount of Brønsted acid and base. Protic ionic liquids possess an exchangeable proton that gives them distinct features, e.g. the potential to conduct protons and the ability to form extended hydrogen bonds, which is different from the case of their aprotic counterparts.

The protic ionic liquids considered in this thesis are limited to those based on the imidazolium and triazolium cation, which (compared to aprotic ionic liquids) are less developed and have been investigated to a lesser extent. The work included in this thesis has been devoted to investigate strategies that could possibly promote a fast proton motion. These strategies have primarily consisted in mixing a protic ionic liquid with a molecular solvent or another parented ionic liquid and in modifying the cationic structure by either attaching alkyl chains of different length to the imidazolium cation or synthesizing new protic ionic liquids derived from another heterocyclic ring such as triazole. In both cases, a detailed characterization by means of vibrational spectroscopy could provide valuable information about intermolecular interactions. An important finding from the studies conducted in this thesis is that altering the cation's structure or adding a co-solvent leads to a significant change in chemical and transport properties.

This thesis also includes an extensive work devoted to develop a new solvent-free method to synthesize dry and pure protic ionic liquids, in the laboratory scale. An analytical approach to accurately quantify their purity using nuclear magnetic spectroscopy has also been developed.
In addition, the ionic conductivity of the investigated protic ionic liquid systems has been evaluated by use of impedance spectroscopy, from room temperature up to 140 °C.
The results included in my work indicate that the two investigated strategies can potentially be used, each with its limitations, to develop thermochemically stable proton conducting materials based on protic ionic liquids.