Very Slow Phase Transition from the Liquid to Mesophase and Phase-Coexistence in the Ionic Liquid [C8mim]BF4

Abstract

Abstract The room temperature ionic liquid, 1-methyl-3-octylimidazolium tetrafluoroborate, abbreviated as [C8mim]BF4, has been known as a good glass-former, which can be cooled or heated at normal scanning rates without any phase transition. However, after cooling to 183 K, just below the glass transition temperature, 190 K, followed by heating to 223 K, Tg + 33 K, a novel phase transition from the supercooled liquid to a partially ordered phase was observed by X-ray diffractometry during a slow isothermal annealing at that temperature. The XRD profiles showed that the ordered phase is not crystal, but a smectic-A phase of a bilayer structure as seen in typical lamellar phases of surfactant solutions or lipid polymers. It is remarkable that the liquid phase still remains even after the existence of the ordered SmA phase for more than 90 hours, which is supposed to be the coexistence, or partial ordering, of the liquid and liquid crystalline. This cannot be seen in conventional one-component systems, such as ordinary molecular liquids. The ionic liquid structure is expected to be very stable due to the original mesoscopic order, which is predicted by MD simulation, Raman spectroscopy and neutron scattering.