Ionic liquids have gained a reputation as ‘designer solvents’ due to the wide
range of properties that are accessible and the fine degree of ‘tuning’ allowed by
varying the constituting ions.
To further widen the parameter space, yet allow the prediction of properties,
mixtures of ionic liquids are analysed in this project. An extensive literature
review is performed, covering the full range of available publications on this
subject at the time of writing. The available literature is extended by original
research on mixtures covering a range of structurally different anions and cations
that have not been reported in mixtures before. Chemical and physical properties
such as conductivity, viscosity, density and solvent properties are evaluated. The
physical properties are found to behave largely ideally with few exceptions, while
the solvent properties of mixtures are found to behave in a systematic manner
according to simple equilibrium laws.
A potential explanation for the dependence of the H-bond acidity of ionic liquid
cations on the basicity of the corresponding anion based on multiple available
acidic protons of the cation is discussed and evaluated.
Computational methods to predict the acidity and basicity parameters α and β of pure ionic liquids are developed and evaluated. For the H-bond acidity α the
electrostatic potential, complex formation with NCH and electronic excitations
of the complex with Reichardt’s Dye are found successful. For β, properties of
the complex with HF are found to correlate well with the H-bond basicity.
The properties of a siloxane substituted cation are analysed by DFT calculations
to explain the low viscosity of the corresponding ionic liquids. The cation is found
to form intramolecular interactions and have lowered barriers for the movement
of the substituent, thus minimising cation–cation interactions, increasing entropy
and facilitating the movement of anions.