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.

Investigation of Dynamics in BMIM TFSA Ionic Liquid through Variable Temperature and Pressure NMR Relaxometry and Diffusometry

“…42 While temperature dependence is a commonly used tool to understand the energetic profiles of physical processes in ILs, pressure dependence can provide unique information on mechanistic and mechanical factors underlying IL physical properties, such as conformational changes in cations and anions, and hydrogen bonding interactions. 27,30–32,43–48 This is the second in a series of studies aimed at elucidating the characteristic dynamics of C n MIM TFSA ILs using NMR techniques.…”

“…33, 42, 47, 49 However, previous high pressure studies of BMIM-TFSA have only gone as high as 40 MPa at room temperature. Here we observe these ILs at pressures up to 250 MPa.…”

Long Periodic Structure of a Room‐Temperature Ionic Liquid by High‐Pressure Small‐Angle X‐Ray Scattering and Wide‐Angle X‐Ray Scattering: 1‐Decyl‐3‐Methylimidazolium Chloride

“…The pressure‐induced phase changes have also been investigated from the viewpoint of the alkyl side‐chain effect of RTILs . The glass transition pressure ( P g ) of [C n mim][BF 4 ] ( n =2–8) was plotted on an n scale.…”

Communication: Anion-specific response of mesoscopic organization in ionic liquids upon pressurization

“…The interest toward this feature is witnessed by the wealth of studies addressing its nature and dependence upon chemical nature of the ions, [25][26][27][28][29][30] temperature, 13,15,31 mixture composition, [32][33][34][35][36][37][38][39][40][41] and, only recently, pressure. [42][43][44][45] High pressure studies are an important tool to explore the details of microscopic and mesoscopic correlations in complex materials. Pressure changes allow investigating intermolecular interactions, by inducing density changes without introducing the chaotic perturbation that temperature changes induce.…”

“…Pressure changes allow investigating intermolecular interactions, by inducing density changes without introducing the chaotic perturbation that temperature changes induce. As a matter of fact, while temperature dependence studies of morphology in RTILs have been reported in the past, 13,15,31 the pressure dependence of the mesoscopic organization in aprotic RTILs has been explored by means of experimental X-ray studies [42][43][44] and simulations [45][46][47][48][49][50] 42 Therein the progressive position shift and amplitude vanishing of the low Q peak centred at ca. 3 nm −1 had been observed.…”