Phase behaviour and heat capacities of 1-butyl-1-methylpyrrolidinium-based ionic liquids with fluoro-sulfonate anions

Pyrrolidinium-based ionic liquids belong to the relatively common classes of these compounds; however, there are serious gaps in the knowledge and interpretation flaws of their physico-chemical properties, except for those containing the archetypal bistriflimide [NTf₂] anion. Compounds combining the 1-butyl-1-methylpyrrolidinium cation [C₄C₁Pyrr] with various fluorinated sulfonate anions exhibit vast polymorphism, rendering the thermodynamic description even more complicated. Four 1-butyl-1-methylpyrrolidinium ionic liquids, available commercially in high purities, were subjugated to a combined calorimetric and crystallographic study with a special emphasis on establishing a complete interpretation of their phase behaviour.

The samples were dried by vacuum heating and their purity was verified using the Karl Fischer titration and van't Hoff analysis of the melting peaks. Polymorphism was readily detected for all four compounds, but a complete description of their phase behaviour required an intensive and careful labour. Despite our experimental efforts, some of the polymorphs were found to be too elusive for a complete characterization by any of the used techniques. The obtained phase-transition temperatures and enthalpies were critically compared to the available literature with a variable agreement. While the existing thermodynamic data for [C₄C₁Pyrr][NTf₂] were found complete and sensible, literature data for the other compounds were incomplete or ambiguous. Heat capacities of the four compounds were determined using a Tian-Calvet calorimeter, the results of which were in a good agreement with the existing adiabatic-calorimetric data for [C₄C₁Pyrr][NTf₂]. Several crystal structures were newly resolved within the phase behaviour study and they are reported in the paper.