title: Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature.
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.
authors.affiliation: †Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan.
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Yoshimura
authors.affiliation: †Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan.
authors.forename: Machiko
authors.initials: M
authors.lastname: Shigemi
authors.affiliation: ‡Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo 112-8610, Japan.
authors.forename: Mayumi
authors.initials: M
authors.lastname: Takaku
authors.affiliation: ‡Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo 112-8610, Japan.
authors.forename: Misaho
authors.initials: M
authors.lastname: Yamamura
authors.affiliation: †Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Takekiyo
authors.affiliation: §Department of Materials Science and Engineering, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Abe
authors.affiliation: ‡Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo 112-8610, Japan.
authors.forename: Nozomu
authors.initials: N
authors.lastname: Hamaya
authors.affiliation: ∥Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Wakabayashi
authors.affiliation: ∥Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Nishida
authors.affiliation: ∥Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan.
authors.forename: Nobumasa
authors.initials: N
authors.lastname: Funamori
authors.affiliation: ⊥Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Sato
authors.affiliation: #Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan.
authors.forename: Takumi
authors.initials: T
authors.lastname: Kikegawa