In Situ Observation of Multiple Phase Transitions in Low-Melting Ionic Liquid [BMIM][BF₄] under High Pressure up to 30 GPa

Abstract

In situ characterization of phase transitions and direct microscopic observations of a low-melting ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF(4)]), has been performed in detail by Raman spectroscopy. Compression of [BMIM][BF(4)] was measured under hydrostatic pressure up to ~30.0 GPa at room temperature by using a high-pressure diamond anvil cell. With pressure increasing, the characteristic bands of [BMIM][BF(4)] displayed nonmonotonic pressure-induced frequency shifts, and it is found to undergo four successive phase transitions at around 2.25, 6.10, 14.00, and 21.26 GPa. Especially, above a pressure of 21.26 GPa, luminescence of the sample occurs, which is connected with the most significant phase transition at around this pressure. It was indicated that the structure change under high pressure might be associated with a conformational change in the butyl chain. Upon releasing pressure, the spectrum was not recovered under a pressure up to 1.16 GPa, thereby indicating that this high-pressure phase remains stable over a large pressure range between 30 and 1.16 GPa in low-melting ionic liquid [BMIM][BF(4)]. Although the sample was kept under the normal pressure for 24 h, the spectrum was recovered, and it showed that the phase transition of [BMIM][BF(4)] was reversible. In other words, such a low-melting ionic liquid [BMIM][BF(4)] remains stable even after being treated under so a high pressure of up to 30 GPa.

Ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion for high‐pressure Raman spectroscopy measurements

“…Previous high‐pressure Raman spectroscopy studies have shown crystal phase transitions of [C 4 C 1 Im]Cl and [C 4 C 1 Im]Br, and liquid–crystal transition of [C 4 C 1 Im]PF 6 below 1.0 GPa . In contrast, no crystallization of [C 4 C 1 Im]BF 4 has been observed even at 30.0 GPa . Interestingly, in case of ionic liquids based on the N,N‐diethyl‐N‐methyl‐N‐(2‐methoxyehtyl)ammonium cation, crystallization was observed at 1.2 GPa for the case of BF 4 ‐ anion, whereas crystallization was not observed for the case of [Tf 2 N] ‐ anion even at 5.5 GPa .…”

Pressure and temperature effects on intermolecular vibrational dynamics of ionic liquids

“…19,20 Ionic liquids under high pressure have been studied by Raman spectroscopy in the high frequency range of intramolecular normal modes. These studies showed conformational changes, 21,22 phase transitions, [23][24][25] and enhancement of anion-cation hydrogen bonding interactions, 26 for several ionic liquids under high pressure. However, in the authors' knowledge, there is no previous study concerning the effect of pressure on the low frequency range of Raman or OKE spectra of ionic liquids.…”

Structural and conformational properties of 1-decyl-3-methylimidazolium tetrafluoroborate under high pressure

“…In the case of imidazolium-based ILs, which is one of the most studied ILs, high-pressure phase behavior of some 1-alkyl-3-methylimidazolium tetrafluoroborate ([C n MIM][BF 4 ]) have been investigated in previous studies [16][17][18][19].…”