High-pressure glass formation of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide homologues

Abstract

We investigated the stability of the liquid phase of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cmim][TFSI]) homologues with different alkyl chain lengths for 3 ≤ n ≤ 10 at room temperature. We found that all [Cmim][TFSI] samples (n = 3-10) formed a glassy state when pressure was applied. Intriguingly, the glass transition pressure (p) slightly increases up to n = 5, reaches a plateau at n ≧ 8, and increases again at n = 10. This is completely different from the high-pressure glass formation of [Cmim][BF], where the p decreases as n increases. We discussed the local structural changes occurring in [Cmim][TFSI] in view of the conformational changes of the cation and anion, and small-angle X-ray scattering data. It seems that [Cmim][TFSI] is resistant to external pressure and retains its local liquid structure by conformational adjustments of the cation and anion.

Pressure and Temperature Dependence of Local Structure and Dynamics in an Ionic Liquid

“… 7 , 29 , 33 While the length scale of the apolar domains, defined by the position of a low Q peak in the static structure factor, follows the trend of the overall density change, the intensity of the peak decreases with increasing temperature and increasing pressure. 23 , 31 − 33 With increased temperature, the driving force for segregation into apolar domains is decreased, whereas conformational changes of the alkyl side chains have been suggested to be responsible for the collapse of the domains at a high pressure, >2 GPa. 32 The position of the charge ordering peak has also been shown to follow the density as a function of temperature but with a weaker dependence.…”

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

“…57 Figure 1 shows then the pressure dependence (from ambient pressure to 2.5 GPa) of a portion (Q < 20 nm −1 ) of the computed X-ray diffraction pattern; in the inset, the published data by Yoshimura et al for the same RTIL are shown [note that these data sets cover a more limited Q range (Q < 5 nm −1 )]. 44 The computed patterns show three diffraction features centred at approximately Q 1 = 3.5, Q 2 = 8, and Q 3 = 13 nm −1 : they have been related to polarity, charge, and adjacency alternations, respectively. [17][18][19] It is noteworthy that, similarly to the experimental data, the peak centred at Q 1 slightly decreases its amplitude when passing from ambient to 0.68 GPa but then maintains essentially unaltered upon further pressurization (we also stress the fact that experimental patterns do not show indication of the appearing of Bragg peaks that might indicate an amorphous-crystalline phase transition over the probed pressure range).…”

“…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.…”

“…58 with experimental results. 44 A decomposition of the computed diffraction patterns at ambient pressure and at 2.5 GPa in terms of the polar-polar, apolar-apolar, and polar-apolar contributions (see Fig. 2), as proposed originally by Margulis et al, [17][18][19] indicates that even up to pressures of the order of 2.5 GPa, the characteristic polar-apolar alternation that characterises the RTILs' morphology at ambient conditions is maintained essentially unaffected: the observed low Q peak centred at Q 1 fingerprints the existence of alternating polar and polar domains and this structural organization at mesoscopic scales is hardly affected by pressures as high as 2.5 GPa.…”

“…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.…”