1.3- dimethylimidazolium hexafluorophosphate

The nature of the alkyl substituents was found to rule the rate of cleavage for the N-C bonds [63, 64]. Methyl substitution in the 2 position (i.e., between the two N atoms) enhances the thermal stability. This may be due to the strong acidic character of the C-2 proton. It was observed that the thermal stability of imidazolium was also affected by the type of isomeric structure of the alkyl side group. This was evidenced by the observation that both l-butyl-2,3-dimethylimidazolium tetrafluoroborate and l-butyl-2,3-dimethylimidazolium hexafluorophosphate salts had higher onset decomposition temperatures than l,2-dimethyl-3-isobutylimidazolium tetrafluoroborate and l,2-dimethyl-3-isobutylimidazolium hexafluorophosphate salts. This reaction presumably proceeds via SnI, as shown in Scheme 2.8. 

In 2001, Hanke, Price and Lynden-BelP were the first to conduct an atomistic simulation of compovmds that can be called ionic liquids under our definition. They used molecular dynamics to model the crystalline state of 1,3-dimethylimidazolium chloride ([Cimim][Cl]), 1,3-dimethylimidazolium hexafluorophosphate ([CimimllPFg]), l-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), and l-ethyl-3-methylimidazolium hexafluorophosphate ([C2 mimJlPFg]). They also modeled the liquid state of [Cimim][Cl] and [Cimim] [PFg], both of which are relatively high melting substances. Because of this (and the need to speed dynamics and thus limit computation times), the liquid simulations were carried out at temperatures between 400 and 500 K. The form of the potential function they used was... 

Fig. 1 Diagrams depicting a a layer of a cubic sodium chloride crystal b a monoclinic 1,3-dimethylimidazolium chloride ionic-liquid crystal c two radial distribution functions (RDFs) in liquid l-dodecyl-3-methylimidazolium hexafluorophosphate. Anions and cations are depicted in red and blue. In the cases of b and c the blue circles represent the centroid of the imidazolium rings of the cations. The alternating sequences of red and blue circles in a and b as well as the two curves in phase opposition in c clearly indicate the existence and nature of the polar networks in ionic condensed phases...

The incorporation of imidazolium chloride salts between clay layers drastically increased their initial decomposition temperature, whereas no significant improvement was observed in the thermal stability of the intercalated tetrafluoroborate and hexafluorophosphate salts [64]. Apparently this effect was connected with the removal of the anion and depended on its nucleophilicity. Gilman and co-workers reported that the replacement of sodium in natural MMT by l-alkyl-2,3-dimethylimidazolium salts yielded organophilic MMT with a 100 °C improvement in thermal stability (in N2) as compared to the alkylammonium-treated MMT [66]. In Langat et al. s work the initial temperature of decomposition for clays modified with l-hexadecyl-3-(10-hydroxydecyl)-2-methylimidazolium and l-hexadecyl-2-methylimidazolium chloride was also very high - around 370 °C [67]. 

McFarlane et alP measured the solubility of methylimidazolium-based bis[(trifluo-romethane)sulfonyl]imides, and found that when the length of the alkyl chain increased from 1-butyl, 1-hexyl, to 1-octyl, the solubility of the ionic liquid decreased from 16 mM to 4.7 mM at 22°C. When the 1-butyl form was combined with a more hydrophilic anion (hexafluorophosphate), the solubility of that ionic liquid was 63 mM. The chemical composition of the anion affected the solubility of foiu alkylimidazolium-based ionic liquids. " When the anion was bis[(trifluoromethyl)sulfonyl]azanide, the ionic liquids were less soluble in water when the anion was hexafluorophosphate. For example, the solubility of 1-butyl-3-methylimidazolium hexafluorophosphate was 2.12 wt% whereas the solubility of the relatively more hydrophobic 1-butyl-2,3-dimethylimidazolium bis[(trifluorom-ethyl)sulfonyl]azanide was measured as 0.61 weight percent. Overall, the solubility of each ionic liquid did not exceed 0.075 mM. 

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