The effect of atmospheric oxygen on the decomposition of imidazolium salts was also investigated by Awad, et al.275 They reported that the tendency to undergo oxidative decomposition increased as the length of the alkyl chain bound to the nitrogen of the imidazolium ring was increased. However, this was not the case for the halide salts, leading them to propose that the activation energy for oxidative decomposition was higher than the activation energy for thermal decomposition. There is also evidence to indicate that the thermal stabilities of dialkylimidazolium salts declines considerably after the addition of strong nucleophilies.281 The details of the pyrolysis reactions of the various classes of RTILs are just emerging, but more will be known as further investigations are conducted in this field.
The contamination of RTILs by water is a long standing problem. The discovery that some RTILs possess hydrophobic properties has probably led to inattention during the purification of these materials in some cases because researchers assumed that the water content of these materials would be diminutive. Only recently has it come to be appreciated that hydrophobic RTILs may contain appreciable amounts of water and that this water contamination may affect the physicochemical properties of these liquids. In fact, it is now established that adventitious water can result in measurements that lead to lower densities,76,282-285 lower viscosities,76,282-287 and higher conductivities284,287 than neat RTILs. In addition, wet RTILs may exhibit a smaller electrochemical window because the reduction of water to produce hydrogen gas is likely to become the limiting cathodic process.284,288,289 In the latter case, it is important to note that the electrochemical window is highly dependent on the material used for the working electrode.288,290 For example, glassy carbon may give the illusion that the RTIL under study has a large electrochemical window because it displays a large hydrogen overpotential. Platinum, which exhibits a relatively low hydrogen overpotential compared to glassy carbon, may give an entirely different result.
Many different methods have been used for the detection of water in RTILs, including NMR spectroscopy, infrared spectroscopy, Karl-Fischer (K-F) titration methods, and even voltammetry at platinum electrodes. The general consensus is that the spectroscopic techniques are only adequate for detecting relatively large amounts of water, whereas K-F titration and voltammetric methods are better suited for the detection of trace contamination. Of these two techniques, the voltammetric method is more universally applicable because some RTILs may react with the K-F reagents. The voltammetric method is easy to apply, provided that it has been established beforehand that the current for the reduction of water or H+ varies linearly or in some predictable way with the concentration of these impurities so that a standard curve can be prepared. Then the observed current recorded in a contaminated RTIL can be compared to the standard curve to estimate the water or H+ content. If the current versus concentration response is linear, then it is also possible to employ a simple standard addition method. However, the application of platinum electrodes for the detection of trace water or H+ contamination is not without complication; it may be necessary to activate the platinum surface by using one of the many literature recipes in order to obtain the lowest overpotential and greatest sensitivity and reproducibility.
As discussed above, the electrochemical window of most wet RTILs is expected to be smaller than that of their dry counterparts. However, in some cases, wet RTILs display a potential window comparable to dry ones. This phenomenon appears to be related to the formation of symmetric hydrogen bonds between the water and RTIL anions, e.g., BF4–, PF6–, and NTf2–.291-294 This hydrogen bonding apparently results in highly associated water that is less
Чтобы распечатать файл, скачайте его (в формате Word).
Ссылка на скачивание - внизу страницы.