Thermal properties of imidazolium ionic liquids (original) (raw)
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Journal of Molecular Liquids, 2016
The ionic liquids (ILs) are green solvents with numerous applications in technology and science. Among the physical properties of the ILs which have a great effect on their application, thermal stability is an important issue which has been discussed in this paper. Kinetics aspect of thermal decomposition of the ILs is considered as a scale of thermal stability. The ILs which have been studied here are composed of 1-hexyl-3-methylimidazolium cation with halide anion [Hmim][X], (X = F, Cl, Br). Three probable paths for thermal decomposition of ILs have been proposed. The first and second paths are the SN2 mechanism in which there are two possible sites on the imidazolium ring position, methyl and hexyl chain. Third path is E2 mechanism which is initiated from hexyl chain. In the case of [Hmim][X] (X = Cl, Br), since the first path has a lower activation energy, it occurs faster than the second path, while in the case of [Hmim][F], path B has a lower barrier than path A and occurs faster. Energy analysis for three studied ILs, showed that [Hmim][Br] with E a = 137.5 kJ mol −1 is decomposed faster than other ILs.
Thermal stability of imidazolium-based ionic liquids
French-Ukrainian Journal of Chemistry, 2016
This work highlights the factors tuning the thermal stability of imidazolium-based ionic liquids (IL) associated to bis(trifluoromethanesulfonyl)imide anion [NTf2]. The decomposition temperatures (Td) were evaluated by thermogravimetric analyses (TGA) with optimized parameters to obtain reproducible Td. The impact of the alkyl chain length and of the presence of functional groups and unsaturations on Td were evaluated. The thermal behaviour was governed by Van der Waals interactions between alkyl chains, and by inter and intra coulombic interactions such as hydrogen bonds.
Limited thermal stability of imidazolium and pyrrolidinium ionic liquids
Thermochimica Acta, 2009
Ionic liquids, with their vast applications, have been touted as being thermally stable to very high temperatures. However, decomposition not detected by standard TGA and NMR techniques are observed with spectroscopic techniques sensitive enough to see small amounts of impurities. Decomposition temperatures of common ionic liquids appear to occur at hundreds of degrees below those temperatures previously reported.
Journal of Chemical & Engineering Data
In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed anion, bis[(trifluoromethane)sulfonyl]amide, were investigated. The imidazolium cations included 1-heptyl-3-methylimidazolium, 1-(cyclohexylmethyl)-3-methylimidazolium, 1-benzyl-3-methylimidazolium, 1,3-dibenzylimidazolium, and 1-(2-naphthylmethyl)-3-methylimidazolium. Structureproperty relationships were investigated regarding the substituent effects on the imidazolium cation, including n-alkyl versus cycloalkyl and aromatic versus aliphatic, as well as the effects of cation symmetry and larger aromatic polycyclic functionalities. Thermophysical properties investigated include density, thermal transition temperatures, and decomposition temperatures. The densities of the ionic liquids are governed by the substituents on the cation: n-alkyl < cycloalkyl < aromatic. The group contribution method is applicable for the density estimation of ionic liq...
The Journal of Chemical Thermodynamics, 2012
Vaporization enthalpies for a series of ten ionic liquids (ILs) 1-alkyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide [C n mim][NTf 2 ], with the alkyl chain length n = 4, 6, 8, 10, 12, 14, 16, and 18 were determined using the thermogravimetric method. An internally consistent set of experimental data and vaporization enthalpies at 540 K was obtained. Vaporization enthalpies at 540 K have shown a linear dependence on the chain length of the alkyl-imidazolium cation in agreement with the experimental results measured previously with a quartz crystal microbalance. Ambiguity of D g l C o p m -values required for the extrapolation of experimental vaporization enthalpies to the reference temperature 298 K has been discussed.
Materials Today: Proceedings, 2021
Abstract This study focused on the effects of anions and the alkyl chain length of cations on the thermophysical properties of imidazolium-based ionic liquid (ILs). The data were obtained from references and the NIST database. The studied parameters include the melting temperature (Tm), glass transition temperature (Tg), decomposition temperature (Td), enthalpy of fusion (ΔHm), and thermal conductivity ( λ ). The variations of Tm with the anions in [C16MIM][A] where A = Br, Cl, BF4, PF6, and TFO, generally decreased with increasing anion radius, except for A = PF6, due to strong hydrogen bonds for the sake of an F atom. The values of the critical temperatures (Tm, Tg, and Td) generally show a strong variation with the number of carbon atom or alkyl chain length (the number of n in [CnMIM][A] for A = BF4, NTf2, and PF6). The variation of Tm corresponding to n shows non-monotonous variations. This characteristic is the result of the combination of interaction potentials in the crystalline and liquid phases. The variation of Tg corresponding to n shows even–odd alternation, presumably due to the competition between the electrostatic and van der Waals forces. Many types of ILs have a relatively high Td value, enabling them to remain in the liquid state above 400 °C, they have excellent catalytic activity and dynamic properties. The variation of Td with n appears different to that of Tm. Furthermore, the variation of ΔHm with n seems to follow that of Tm. This behaviour is in accordance with the thermodynamic relation between ΔHm, Tm, and the entropy of the system. The values of λ varied weakly with the alkyl chain length, and strongly depend on the type of anion.
Solution and thermal behaviour of novel dicationic imidazolium ionic liquids
Organic & Biomolecular Chemistry, 2013
A new class of functionalised dicationic ionic liquids, containing a central cationic unit capped by a basic functionality (imidazole), has been synthesised. These salts have been characterised in isotropic solution using proton and 2D-NMR spectroscopy, and their thermal stability has been studied by DSC and TGA. All these novel salts contain the 1-(1-imidazolylmethyl)-3,5-di{1-(3'-octylimidazolylmethyl)}-benzene cation as a defining structural motif. Salts of both singly and doubly charged anions were prepared and, in particular, the selected monoanions (Br − , [BF 4 ] − , or [NTf 2 ] − ) differ in size, shape and hydrogenbonding ability, whereas the dianions differ in the nature of the spacer, such as 1,4-benzenedicarboxylate, 2,6-naphthalenedicarboxylate, 1,5-and 2,6-naphthalenedisulfonate, 1,4-butanedicarboxylate, and 1,6-hexanedicarboxylate. These ionic liquids exhibit the presence of different conformers in solution, whose distribution is affected by the nature of the anion. The nature of the anion also affects their thermal stability. † Electronic supplementary information (ESI) available: 1 H NMR and 2D NMR spectra, DSC and TGA traces and plot of correlation between Δδ ( ppm) and T (K). See
Thermal stability of low temperature ionic liquids revisited
Thermochimica Acta, 2004
The range of thermal stability of low temperature ionic liquids published in the literature (often >400 • C) is severely overrated. The decomposition temperature calculated from fast TGA scans in a protective atmosphere does not imply a long-term thermal stability below that temperature. Even at temperatures as low at 200 • C, 1-alkyl-3-methylimidazolium phosphates (alkyl = C4-C10) and 1-decyl-3-methylimidazolium triflate showed a slow, but appreciable mass loss. On the other hand, 1-butyl-3-methylimidazolium triflate was stable at 200 • C. The carbonization occurred in most studies salts irrespectively of the nature of the anion (hexafluorophosphate, triflate), but the salts with a shorter side chain (C4) did not show changes in their color after conditioning for 10 h at 200 • C in air. 1-Butyl-3-methylimidazolium triflate shows extremely good wettability against aluminum oxide and silver at elevated temperatures. Addition of silica (amorphous or quartz) accelerates the thermal decomposition of 1-alkyl-3-methylimidazolium phosphates and triflates at 200 • C, while the effect of other ceramic powders (titania, alumina) is less significant.
Thermal stabilities of di-alkylimidazolium chloride ionic liquids
International Journal of Thermal Sciences, 2008
Thermal stability of 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl) ionic liquids was investigated using thermogravimetric analysis (TGA). The long-term isothermal TGA studies have revealed that both the ionic liquids exhibit appreciable decomposition at temperatures significantly lower than indicated by the peak or onset decomposition temperature (T onset ) determined from fast scan TGA experiments. The long-term thermogravimetric studies of both the ionic liquids showed linear weight loss as a function of time at each temperature of 10 • C interval in the range 150-170 • C over a period of 15 hours. However, the linear weight loss regime decreased with increasing temperature for the range 180-200 • C. The kinetics of isothermal decomposition of ionic liquids was analyzed using pseudo-zeroorder rate expression. Arrhenius activation energy for the decomposition of di-alkylimidazolium chloride ionic liquids in the range 150-200 • C was determined.