Compared with organic solvents, most ILs have relatively high thermal stability. The decomposition temperatures reported in the open literature are generally >200 °C, and they are liquid state in a wide range of temperatures (from 70 to 300-400 °C). The decomposition temperatures of ILs that dissolving cellulose are listed in Table 1.4.

Many literature works have investigated the thermal stability of ILs on imidazolium and anion structures. The onset of thermal decomposition is similar for the different cations but appears to decrease as the anion hydrophilicity increases. Ngo et al. [73] found that the thermal stability of the imidazolium- based ILs increases with increasing linear alkyl substitution. Owing to the facile elimination of the stabilized alkyl cations, the presence of nitrogen substituted secondary alkyl groups decreases the thermal stability of ILs. They also found that the stability dependence on the anion is [PF6]~ > [Tf2N]_ ~ [BF4]~ > halides. Fox et al. [74] found that the alkyl chain length does not have a large effect on the thermal stability of the ILs.

However, the thermal stability of ILs has been revised [4]. The range of thermal stability of ILs published in the open literature is overstated. The decomposition temperature of ILs calculated from fast thermo gravimetric analysis (TGA) scans in a protective atmosphere and does not imply a long-term thermal stability below those temperatures [44]. Fox and his group have done some nice study on the thermal stability of ILs [74—76]. Compared with the data from both isothermal and constant ramp rate programs for the decomposition of 1-butyl-2,

3- dimethylimidazolium tetrafluoroborate ([BMMIM][BF4]) under N2, they found that isothermal TGA experiments may be the more appropriate method for evalu­ating the thermal stabilities of ILs [75]. Based on TGA pyrolysis data of 1, 2,

3- trialkylimidazolium room temperature ILs, They also found that although the calculated onset temperatures were above 350 °C, significant decomposition does occur 100 °C or more below these temperatures.

Singh et al. [77] analyzed the thermal stability of imidazolium based ILs [BMIM][PF6] in a confined geometry. They found that [BMIM][PF6] in confined geometry starts at an earlier temperature than that for the unconfined ILs. The loss of alkyl chain end groups of [BMIM] cation of ILs assign to the early decomposi­tion by using a phenomenological ‘hinged spring model’. The idea of ‘hinged spring’ model is that the imidazolium ring is supposed to be ‘hinged’ to the SiO2 matrix pore walls by surface oxygen interacting with the C-H group of the imidazolium ring.

Researchers have reported a new method to study the changes that occur during thermal aging of ILs. The method is potentiometric titration, which is precise, low-cost and quick analytically. To a small extent, they found that imidazolium salts start to decompose at much lower temperatures than those obtained from thermo gravimetric analysis by using this method. They also concluded that the stability of ILs is also influenced by water, except by their composition, such as anion type and alkyl substituent at the imidazolium ring. For instance, 2 wt% of water in ILs could bring about increased degradation of [BMIM]Cl at 140 °C. Furthermore, [BMIM] [BF4], [EMIM][CH3SO3], [BMIM] [CH3SO3] and [BMIM][Tf2N] are completely stable at 140 °C for 10 days [78]. Finally, from these data, long-term stability of ILs is a complicated problem with obvious and serious implications for their use as solvents media of chemical reactions.