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

Behavior of the phase transition of an ionic liquid, [Cn-mim][PF6], 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][PF6] and [EMIM][PF6] are constructed. The DTA curve of [BMIM][PF6] 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][PF6] shows two endothermal valleys in the heating course within the tested pressure range, implying that there may exist another phase. After treatment of [EMIM][PF6] 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][PF6].