Thermal Stability of ILs

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It is well known that halide anions lower the thermal stability of ILs due to their nucleophilic nature and their ability to decompose by S l or 5 2 nucleophilic decomposition [20]. ILs that are not thoroughly purified and examined using ion chromatography for the presence of trace levels of halide anions will produce significant colunm bleed at relatively low-column temperatures. Excessive decomposition/volatilization of [C4QIm]Cl has been observed starting at 145°C [21]. 

Usually, the thermal stability of ILs is characterized by thermogravimetrical analysis (TGA) at ambient pressure in overflow of an inert gas at a constant heating rate, typically 1-20 Kmin [41-43]. The so-called onset temperature (T gg,), at which a certain detectable mass loss of, for example, 1% is reached, is used as a synonym for the decomposition temperature to define the stability of ILs [3, 44]. The application of may be useful as a comparative value, but not as a quantitative measure of the stability due to the following reasons [25, 27] ... 

Thus, one of the greatest advantages of ILs as solvents is their exceptional thermal stability [14], As such, multiple studies investigating the thermal stability of ILs have been reported in the literature, and we will discuss some of these studies... 

Alternatively, the thermal stability of ILs can be modulated by solute composition [45]. Smiglak and co-workers investigated the flammability of 20 imidazolium and phosphonium ELs and found that while ILs are not flammable themselves, they are not necessarily safe to use near fire and other sources of heat because solute impurities and the products of thermal decomposition are sensitive to composition [46, 47]. This is especially tme when F, Cl, P, S, C, H, N, and O in ammonium, imidazolium, pyridinium, tetrazolium, and aminotetrazole cations are paired with anions such as [N03] , [CH3C02] , [N(CN)2] , [C(CN)3] , [PFg], [NTfi]", and [N(N02)2] [32, 48, 49]. Despite the vapor pressure and flammability issues... 

For many applications, the thermal stability of ILs and SILLPs is a critical parameter.i In this regard we have carried out thermogravimetiic (TGA) and differential scanning... 

ILs with a variety of different functionalities have been developed for electrochemical applications. These ILs can be customized to have a wide range of viscosities, conductivities and electrochemical stability by incorporating different take-specific functional groups and making various ion pairs. In addition, the non-volatility and high chemical/thermal stability of ILs make them ideal and versatile electrolytes for many electrochemical applications. A new trend is to combine the task-specific ILs and nanomaterials for constructing novel sensors. There is still a need for further understanding of structure/functionahty-property-application relationship, especially more data on physicochemical properties of task-specific ILs. 

The solubilities of the various gases in [BMIM][PFg] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and CO2 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub CO2 from stack gases composed of N2 and O2. Since we know of no measurements of H2S, SO, or NO solubility in [BMIM][PFg], we do not loiow if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

Most recently, it was found that the thermal stability of geminal dicationic ILs is considerably higher than their monocationic analogs [22]. Figure 4.1 illustrates the increase in thermal stability of dicationic ILs containing the [TfjN] anion (traces D-G) compared to the monocationic l-butyl-3-methyl-imidazolium ILs containing Cl , PF, and TfjN anions. 

ILs viscosity iuCTeased when the chain lengA of the side chain increases, the side chain is composed of fluoride or an imidazole ring in position 2 methylation, or there is increased cation symmetry. Generally, when the cation is composed of the thermally stable imidazolium ion, the anion determines the thermal stability of the entire IL, increasing in the order TfjN >BF >PF ">halogen ions. Minami et al. [18] studied thermo-oxidative stability and volatility of ILs and found that ILs have greater thermal stability than conventional synthetic lubricants. It was also found that the stability of an ILs depends on the structures of both the anion and the cation. For example, the stability of 1, 3-dialkyl-imidazoUum is more stable than quaternary ammonium. 

Lipases are the most extensively studied enzyme in ILs. Sheldon et al. studied the thermal stability of CaLB in [BMIM][PFg] at 80°C by measuring its activity at different time points. CaLB in both free form (SP 525) and immobilized formulation... 

Pig. 10.4 (a) Crystallographic structure of moneUin (PDB id 4mon) (b) Thermal stability of moneUin in ILs change in emission maxima of moneUin in water (filled square) and in [BMPY] [TfjN] (empty square) (Reproduced from Ref. [48], with kind permission of The Royal Society of Chemistry)... 

MoneUin is a sweet protein which could be used as artificial noncarbohydrate sweetener. But it denatures at high temperature and becomes unsuitable for food processing. It is a smaU protein of 11 kDa containing a single tryptophan residue, a 17 residue a-helix, and an antiparallel (3 sheet formed by five (3 strands (Fig. 10.4a). Thus, it is interesting both from application and structural point of view to study the thermal stability of moneUin in ILs since ILs are known to have unconventional properties toward preservation of enzyme structure as observed above for lipases. 

The solubility of different gases in the various ILs, as discussed above, suggests that ILs should be excellent candidates for a wide variety of industrially important gas separations. There is also the possibility of doing higher temperature gas separations due to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric... 

Their physical properties, such as viscosity, density, thermal stability, or surface tension, are important to consider during the design phase of new processes. The demonstration of the thermal and chemical stability of ILs and the recyclability can only be proven through continuous pilot plant runs. On the other hand, the sensitivity to feed impurities that can accumulate in the ionic phase requires feed pretreatments or guard beds for some scheduled applications. 

It has been shown that the thermal stability of a series of l-alkyl-3-methyl-imidazolium-based ILs increases as the length of the alkyl chain increases [16]. 

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