Phase Transitions of the Ionic Liquid [C2C1im][NTf2] under High Pressure: A Synchrotron X-ray Diffraction and Raman Microscopy Study

Abstract

The interplay between crystallization and glass transition in the archetypal ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2C1im][NTf2], has been studied as a function of pressure up to ca. 12 GPa. Besides heterogeneous crystal nucleation, homogeneous nucleation in the sample inside the diamond anvil cell was also observed depending on compression/decompression rate. Amorphization of the crystal and glass formation under pressure has been followed by synchrotron X-ray diffraction. The characteristic Raman bands of the [NTf2]− anion provide a microscopic probe of the different phases. The crystalline phase is composed of the [NTf2]− cisoid conformer, but moisture implies formation of crystal with the transoid conformer. Raman spectra show that crystalline phases might become microscopically heterogeneous because of [NTf2]− conformational disorder. Raman mapping reveals the order–disorder evolution from crystal to glass. Crystals of [C2C1im][NTf2] formed under high pressure and room temperature are similar to previously reported low temperature and atmospheric pressure crystals. Thus, it is concluded that density is the main factor controlling crystallization and glass formation under high pressure of [NTf2]− based ionic liquids due to hindrance of efficient ion packing. The results highlight that ionic liquids are good models to understand fundamental questions related to the mechanism of crystallization and glass transition.