A Raman spectroscopy and rheology study of the phase transitions of the ionic liquid choline acetate

Abstract Melting of the ionic liquid choline acetate, [Chol][Ac], at ~335 K is manifest in the differential scanning calorimetry (DSC) curve as a broad band covering ~30 K of temperature range. The events taking place in the pre-melting of [Chol][Ac] are revealed by Raman spectroscopy and rheological measurements. The low-frequency range of the Raman spectrum exhibits changes in the intermolecular vibrational modes and the higher frequency range indicates conformational changes of the choline cation. Oscillatory rheological measurements show that the sample experiences a stiffening of its rheological characteristics within the temperature interval encompassing the melting process. Taking together the spectroscopic and rheological data, it is concluded that the sample experiences partial recrystallization during the pre-melting of [Chol][Ac].