Ultraslow Phase Transitions in an Anion-Anion Hydrogen-Bonded Ionic Liquid

A Raman spectroscopy study of 1-ethyl-3-methylimidazolium hydrogen sulfate, [C₂C₁im][HSO₄], as a function of temperature, has been performed to reveal the role played by anion-anion hydrogen bond on the phase transitions of this ionic liquid. Anion-anion hydrogen bonding implies high viscosity, good glass-forming ability, and also moderate fragility of [C₂C₁im][HSO₄] in comparison with other ionic liquids. Heating [C₂C₁im][HSO₄] from the glassy phase results in cold crystallization at ∼245 K. A solid-solid transition (crystal I → crystal II) is barely discernible in calorimetric measurements at typical heating rates, but it is clearly revealed by Raman spectroscopy and X-ray diffraction. Raman spectroscopy indicates that crystal I has extended ([HSO₄]^-)_n chains of hydrogen-bonded anions but crystal II has not. Raman spectra recorded at isothermal condition show the ultraslow dynamics of cold crystallization, solid-solid transition, and continuous melting of [C₂C₁im][HSO₄]. A brief comparison is also provided between [C₂C₁im][HSO₄] and [C₄C₁im][HSO₄], as Raman spectroscopy shows that the latter does not form the crystalline phase with extended anion-anion chains.