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  • Title: Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature.
    Author: Yoshimura Y, Shigemi M, Takaku M, Yamamura M, Takekiyo T, Abe H, Hamaya N, Wakabayashi D, Nishida K, Funamori N, Sato T, Kikegawa T.
    Journal: J Phys Chem B; 2015 Jun 25; 119(25):8146-53. PubMed ID: 25988295.
    Abstract:
    To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4]) homologues with different alkyl chain lengths for 2 ≤ n ≤ 8 up to ∼7 GPa at room temperature. The ionic liquids exhibited complicated phase behavior, which was likely due to the conformational flexibility in the alkyl chain. The present results reveal that [Cnmim][BF4] falls into superpressed state around 2-3 GPa range upon compression with an implication of multiple phase or structural transitions to ∼7 GPa. Remarkably, a characteristic nanostructural organization in ionic liquids largely diminishes at the superpressed state. The behaviors of imidazolium-based ionic liquids can be classified into, at least, three patterns: (1) pressure-induced crystallization, (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. Interestingly, the high-pressure phase behavior was relevant to the glass transition behavior at low temperatures and ambient pressure. As n increases, the glass transition pressure (pg) decreases (from 2.8 GPa to ∼2 GPa), and the glass transition temperature increases. The results indicate that the p-T range of the liquid phase is regulated by the alkyl chain length of [Cnmim][BF4] homologues.
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