Structure Effects on the Ionicity of Protic Ionic Liquids

Abstract

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.

The unseen evidence of Reduced Ionicity: The elephant in (the) room temperature ionic liquids

“…Spectroscopic approaches have been proposed to quantify the proton transfer [43], but the method proposed relies on the assumption that the solvation shells and dielectric constant of the systems stay unperturbed upon changing the composition of the system, which is questionable. MacFarlane's group extensively addressed the proton transfer topic [39,44,45], but always accounting for ΔpKa values, which are strictly meaningful only in aqueous systems. They also pointed to the proton transfer's qualitative spectroscopic determination [44], but again overlooking the different chemical environments experienced by the species in the Brønsted precursors and the ILs.…”

“…MacFarlane's group extensively addressed the proton transfer topic [39,44,45], but always accounting for ΔpKa values, which are strictly meaningful only in aqueous systems. They also pointed to the proton transfer's qualitative spectroscopic determination [44], but again overlooking the different chemical environments experienced by the species in the Brønsted precursors and the ILs. Ionicity and proton transfer cannot be used as synonyms, albeit directly correlated [46].…”

Synthesis and characterization of ionic liquid microneedle patches with different carbon chain lengths for antibacterial application

“…An IL is a kind of pollution-free, recyclable, stable, non-volatile green solvent. 3 Imidazole salts, piperidine salts, pyridine salts, pyrrole salts and other ILs have the effect of inhibiting the growth of pathogenic and non-pathogenic bacteria and fungi. [4][5][6] ILs can interact with the cell wall of electronegative bacteria through the Coulomb interaction.…”

Carbon dioxide capture by new DBU‐based DES: The relationship between ionicity and absorptive capacity

“…Unlike other ionic liquids, PILs have a large number of rapidly transferring protons, leading to the presence of a hydrogen bond network 17‐20 . It is worth noting that PILs are not only composed of ions, but also have molecular components, which cannot be ignored 21‐26 …”