Electrochemistry of Room-Temperature/ Articles about substances designated as ionic liquids have begun to appear with increasing regularity in chemistry journals around the world, страница 6

III.         FUNDAMENTAL PROPERTIES OF ROOMTEMPERATURE IONIC LIQUIDS

As a class of solvents, RTILs display many interesting and useful physical and chemical properties. However, newcomers to this field need to know that there are also limitations and special requirements associated with these ionic solvents that need to be understood before employing them as solvents for electrochemistry.

1.  Thermal Stability

The use of RTILs at elevated temperatures requires information about their thermal stabilities. The thermal stability of a RTIL is usually defined through experiments carried out with differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), or differential thermal analysis (DTA). Although these methods are easy to use and show good reproducibility, the results obtained with these techniques, especially fast TGA scans (10 ~ 20 K  min^-1), do not always provide an accurate assessment of the thermal stabilities of the RTILs.^35,269-274 Table 3 shows thermal decomposition rates at different temperatures for some popular RTILs compared to the decomposition onset temperature as measured by TGA-DTA. Obviously, significant decomposition occurs at temperatures considerably lower than the onset of the decomposition process. Therefore, electrochemical experiments should not be done at temperatures close to the decomposition temperature as determined by TGA/DTA, especially when the experiments are expected to last for a considerable period of time. Furthermore, several research groups have compared the thermal stabilities of selected RTILs in moist air and dry N₂^139,272,275 and found notable differences.^272,275 These differences are directly related to the reactivity of the RTILs with atmospheric moisture. In addition to this factor, sample quantity,²⁷² scan rate,²⁷² and the material used to make the sample crucible^271,272,276 can also affect the results.

Limited investigations have been carried out on the pyrolysis reactions associated with decomposition of the organic cations. The main products of the pyrolysis process seem to be related to the nucleophilicity of the anions and the basic skeleton of the cations. Two general decomposition models have been proposed for tetraalkylammonium cations: the well-known Hofmann elimination with the production of an alkene (Eq. 1) and the reverse Menschutkin reaction, leading to a tertiary amine and alkyl halide (Eq.

2):20,277

                       R₄N⁺X^–  →  R₃NH⁺X^–  +  CH₂=CH₂                          (1)

                                 R₄N⁺X^–  →  R₃N  +  RX                                   (2)

Similar investigations using phosphonium-based RTILs have been attempted, but the decomposition mechanisms are more complicated than for the tetraalkylammonium cations and often depend on the anion. The original literature should be consulted for more information.^278,279 

In the case of those RTILs based on dialkylimidazolium cations, the decomposition pathway most likely proceeds via S_N1 (Eq. 3) or S_N2 (Eq. 4) reactions:^74,275 

CH₃                           ⏐

CH ⎯C⎯CH

The reaction pathway depends on the nucleophilicity of the anions and the structure of the alkyl group on the nitrogen atom of

82                                                                           Tetsuya Tsuda and Charles L. Hussey

the imidazolium ring. Thus, if the anions are halides having strong nucleophilicity, and the cations have linear alkyl chains, the reaction will likely follow the S_N2 pathway.^74,275 

Several groups have listed the thermal stabilities of the imidazolium cation-based RTILs. For example, Ngo et al.²⁷⁶ indicates PF₆^– > Beti^– (bis(perfluoroethylsulfonyl)imide) > Tf₂N^– ≈ BF₄^– > Tf₃C ^– ≈ AsF₆^– > I^–, Br^–, and Cl^–. Awad, et al.²⁷⁵ gives the following order: PF₆^– > Tf₂N^– > BF₄^– > Br^–, and Cl^–, whereas Fredlake et al.²⁸⁰ proposes Tf₂N^– > Tf₃C^– > TfO^– > BF₄^– > N(CN)₂^– > Br^–. There is little doubt that the anion plays a crucial rule in the thermal stability. Overall, imidazolium-based cations are reported to be more stable than ammonium-based cations.²⁷⁶ The reason is unclear, but the aromatic properties of the imidazolium ring may contribute to the increased stability.