Structure Effects on the Ionicity of Protic Ionic Liquids

We report on the characterisation of 16 protic ionic liquids (PILs) prepared by neutralisation of primary or tertiary amines with a range of simple carboxylic acids, or salicylic acid. The extent of proton transfer was greater for simple primary amine ILs compared to tertiary amines. For the latter case, proton transfer was increased by providing a better solvation environment for the ions through the addition of a hydroxyl group, either on the tertiary amine, or by formation of PIL/molecular solvent mixtures. The library of PILs was characterised by differential scanning calorimetry and a range of transport properties (i. e. viscosity, conductivity and diffusivity) were measured. Using the (fractional) Walden rule, the conductivity and viscosity results were analysed with respect to their deviation from ideal behaviour. The validity of the Walden plot for PILs containing ions of varying sizes was also verified for a number of samples by directly measuring self-diffusion coefficients using pulsed-field gradient spin-echo (PGSE) NMR. Ionicity was found to decrease as the alkyl chain length and degree of branching of both the cations and anions was increased. These results aim to develop a better understanding of the relationship between PIL properties and structure, to help design ILs with optimal properties for applications.