Comparing two tetraalkylammonium ionic liquids. II. Phase transitions

Abstract

Phase transitions of the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N1114][NTf2], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N1444][NTf2], were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) measurements, and Raman spectroscopy. XRD and Raman spectra were obtained as a function of temperature at atmospheric pressure, and also under high pressure at room temperature using a diamond anvil cell (DAC). [N1444][NTf2] experiences glass transition at low temperature, whereas [N1114][NTf2] crystallizes or not depending on the cooling rate. Both the ionic liquids exhibit glass transition under high pressure. XRD and low-frequency Raman spectra provide a consistent physical picture of structural ordering-disordering accompanying the thermal events of crystallization, glass transition, cold crystallization, pre-melting, and melting. Raman spectra in the high-frequency range of some specific cation and anion normal modes reveal conformational changes of the molecular structures along phase transitions.

Nanostructure domains, voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size

“…1,5,10,11 Alkylammonium-based ILs have been studied well. 6,[13][14][15][16] Trialkylammonium ILs were extensively studied by Pott and Méléard 17 using X-ray scattering experiments, and by Shimizu et al 18 using molecular dynamics (MD) simulations. These studies reveal that the change in alkyl chain length strongly affects the structure, but it does not affect the intermediate and high-k peaks 17,18 obtained from X-ray experiments.…”

“…It is well established that the change in alkyl chain length has a signicant impact on segregation, diffusion, and domain formation. 15,16,[35][36][37] The simulations were further analyzed to get insight into the domain formation. The DMA molecules show different domain counts in different mixtures.…”

Pressurizing the mixtures of two ionic liquids: Crystallization versus vetrification

“…For instance, N,N‐diethyl‐N‐methyl‐N‐(2‐ethoxyethyl)ammonium bis(trifluoromethylsulfonyl) imide does not crystallize neither upon cooling nor upon compression, although Raman spectra characteristic of different p – T regions remarkably differs in both the CH stretching region and below 450 cm −1 . Also ILs with a triflate anion, n‐butyl‐trimethylammoniumbis(trifluoromethanesulfonyl)imide and methyl‐tributylammonium bis(trifluoromethanesulfonyl)imide display different low‐temperature and high‐pressure phases . A combined study conducted by means of X‐ray diffraction and Raman spectroscopy on [bmim][PF 6 ] as a function of pressure and temperature evidenced a complex phase diagram with at least three crystalline phases, with different conformers distribution, structural arrangements, and disorder.…”

“…Phase transitions have been identified by various methods: a slope change of the pressure dependence of some vibrational wavenumbers; the broadening of the fluorescence lines emitted from the ruby spheres used for measuring pressure; the appearance of a frozen dynamics below 100 cm −1 , in the case of glass transition; and the change in the population of different conformers of ions . As to the latter method, most studies were performed on ILs containing imidazolium‐based cations, which possess isomers, and anions, which do not possess conformers, such as BF 4 , PF 6 , NO 3 , and TfO .…”

A Computational and Experimental Study of the Conformers of Pyrrolidinium Ionic Liquid Cations Containing an Ethoxy Group in the Alkyl Side Chain

“…Previous systematic studies of the conformers of the ions composing ILs have been reported for TFSI and PYR 14 [3,4,[12][13][14], bis(fluorosulfonyl)imide (FSI) [15][16][17], N,N-diethyl-N-methyl-N-propylammonium [18], imidazolium [11,[19][20][21][22], and ammonium ions [23,24]. It must be noted that the presence of different conformers can alter the physicochemical properties of ILs.…”