Phase transition of [Cn-mim][PF6] under high pressure up to 1.0 GPa

Abstract

Behavior of the phase transition of an ionic liquid, [Cn-mim][PF(6)], has been investigated under pressures up to 1.0 GPa by using a high-pressure differential thermal analysis (DTA) apparatus. The T versus P phase diagrams of [BMIM][PF(6)] and [EMIM][PF(6)] are constructed. The DTA curve of [BMIM][PF(6)] shows one endothermal valley in heating course at each given pressure, which indicates that a simple phase transition from solid to liquid has taken place under high pressure and that the melting point is an increase function of pressure. However, the DTA curve of [EMIM] x [PF(6)] shows two endothermal valleys in the heating course within the tested pressure range, implying that there may exist another phase. After treatment of [EMIM][PF(6)] at different temperatures under high pressure, the structures of the recovered samples are also investigated by wide-angle x-ray scattering. By considering the results above, it indicates that another crystalline phase exists between the solid and liquid of [EMIM][PF(6)].

Structural organization and phase behaviour of 1-butyl-3-methylimidazolium hexafluorophosphate: an high pressure Raman spectroscopy study

“…2 with results from the recent reports from Su et al obtained using DTA and Raman spectroscopy, respectively. 19,20 Only their data for p o 200 MPa agree with our present results as well as the ones from the other mentioned groups. In particular the results from Su's DTA measurements agree with the curve obtained for the S2-L equilibrium.…”

“…The range up to 100 1C was explored and at that temperature the L-S2 equilibrium pressure was found to be 465 MPa. A comparison between our present equilibrium data and the data reported by de Azevedo et al, 18 and Li et al 19,20 shows that S2 corresponds to the same crystalline phase whose melting point had been previously investigated. Such a comparison is presented in Fig.…”

“…The study based on DTA measurements 19 detected the existence of just one crystalline phase, whose transition to the liquid state follows the behaviour found by de Azevedo and co-workers, 18 Sawamura and co-workers [unpublished data] and us (this work) up to 65 1C/250 MPa for the S2-L equilibrium. Above this temperature/pressure the melting line of this crystalline phase (our S2) deviates from the trend that our study indicates (we notice that the other studies do not access this high p/T range).…”

“…18 The latter report also provided information on the high pressure-high temperature phase diagram of [ 6 ]), at pressures up to 1 GPa, providing a description for the phase diagram of this salt and highlighting the existence of one crystalline phase over this pressure range. 19 More recently the same group presented new results based on an extensive high pressure Raman spectroscopy investigation on [bmim] [PF 6 ] up to 2 GPa, focusing on the in situ crystallization of this ionic liquid. 20 This study confirmed their previous results on the existence of one crystalline phase for [bmim] [PF 6 ] under high pressure conditions.…”

“…34,35 These experimental results are in contrast with recent results from high pressure DTA and Raman experiments, where only one crystalline phase was discernible up to 2 GPa. 19,20 The existence of different crystalline polymorphs at comparable thermodynamic conditions points to the occurrence of several local conformational minima in the potential energy surface. 27 This might be one of the reasons for the low melting point and easy supercooling of the liquid state in ILs.…”