Pressure-induced Vitrification and Decompression-induced Crystallizations of Ionic Liquids

Abstract

In this article, we have reviewed the results on pressure-induced Raman spectral changes of two aliphatic quaternary ammonium-based room temperature ionic liquids, N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF₄]) and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) up to ～6 GPa. We focus on the behavior whether they crystallize or hold a liquid state at high pressures. Here we report the very unique behavior of ionic liquids under high pressure that [DEME][BF₄] form a superpressed (glassy) state upon compression, and the crystallizations occur by releasing the pressure on the superpressurized liquid. On the other hand, [DEME][TFSI] also enters the glassy state as well, but the results together with the visual observations show that no crystallization occurs upon decompression. We believe that the results provide new insights into the phase transition behavior of ionic liquids, except for the liquid to solid or solid to solid transitions upon compression.