Ionic liquids (IL) have important application in separations, energy storage, and tribology. The temperature dependent morphologies of ILs, i.e. freezing, glass transition, and melting, represent important physical processes for these applications. Since most of the molecular ions which make up ILs are bulky and asymmetric, the crystallization process is known to be very complex. Building an understanding of their phase transition behaviors would be very valuable, particularly as they deviate from bulk to interfacial environments. Here, we use differential scanning calorimetry (DSC) and temperature controlled spectroscopic measurements to study ILs of the tetra-alkylammonium cation class. Our data shows significant changes in crystallization and melting events as a function of cooling and heating rates. Specifically, when butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [N1114][TFSI] is cooled from 30 °C at 15 °C min-1 to -75 °C and heated at 5 °C min-1 to 30 °C, only one crystallization event is observed. However, when [N1114][TFSI] is cooled at 15 °C min-1 and heated at 15 C min-1, two crystallizations are observed, which is accompanied by increasing complexity in the subsequent pre-melting and melting features. These different crystallization events are probed via temperature controlled spectroscopy, which provides data revealing clear structural differences between the two crystallization states, particularly changes in the CF3 and SO2 regions of the vibrational spectrum.

Spectroscopic and calorimetric analysis of multiple ionic liquid phase transitions in bulk and thin film systems

255th National Meeting (2018)

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