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 8

84                                                                           Tetsuya Tsuda and Charles L. Hussey

reactive than water in the free state. Water also plays a more insidious role in RTILs. Swatloski, et al.293 report that 1-butyl-3methylimidazolium hexafluorophosphate is readily hydrolyzed by adventitious water to produce HF among other products.

It should be realized that most RTILs readily absorb water if exposed to the atmosphere, even those considered to be hydrophobic.282,284,295 Therefore, it is often necessary to handle RTILs under an inert atmosphere to obtain accurate data, regardless of their supposed inertness. The procedure for removing water from ionic liquids is straightforward, if the material under investigation is water stable, i.e., the anion does not readily hydrolyze. Most water can be removed with a liquid N2-trapped vacuum line (< 1 × 10-3 torr) using conventional procedures, under moderate heating and vigorous stirring. Also, water can be removed by sparging the RTIL with a dry inert gas such as N2 or Ar. When ultrapure RTILs are needed, further treatment should be done by using controlledpotential electrolysis at an applied potential sufficiently cathodic so as to reduce any water or H+ without reductive decomposition of the RTIL. To attain maximum benefit, this procedure should be carried out under an inert atmosphere in a divided cell. For RTILs containing fluoroanions such as MFx-, where M: B (x = 4); P, As, Nb, Ta (x = 6); W (x = 7), care should be taken to avoid any HF that may result from the hydrolysis of these anions. In particular, the contamination of wet RTILs by protons, chloride, and hard acid impurities requires special attention, because, under such conditions, these impurities will catalyze anion hydrolysis.296-301

                           3.    Chloride Ion Contamination

Most RTILs are produced from a halide salt intermediate, usually a chloride salt, because such salts are easy to prepare and purify. However, RTILs produced in this way often contain residual chloride. In a careful study, Seddon et al.282 found that some physicochemical properties are highly dependent on the amount of residual chloride in the RTIL. Generally speaking, chloride ion, because of its strong nucleophilicity, increases the viscosity and decrease the density of the ionic liquid. In addition, because chloride is relatively easy to oxidize, RTILs containing chloride ion impurities exhibit a smaller electrochemical window.302 Chloride contaminated RTILs often show decreased thermal stabilities as mentioned above (see Section III.1). Needless to say, if RTILs containing chloride contaminants are employed as reaction media, the results may be much different from those that would be obtained in the neat RTILs. Therefore it is very important to purify RTILs prepared from halide salts. Recommended procedures for the purification of RTILs are described in a recent monograph.303 After the RTILs have been purified, the Cl- concentration should be examined in some way. A simple approach is to add a few drops of a dilute silver nitrate solution and watch for the formation of a precipitate of silver chloride. However, quantitative analysis of the chloride concentration is best done with ion chromatogra-


                                4.    Oxygen Contamination

Oxygen does not react with most RTILs. However, the electrochemical reduction of dissolved O2 in dry RTILs is similar to that observed in anhydrous aprotic organic solvents such as acetonitrile, dimethylformamide, dimethylsulfoxide, and similar solvents under anhydrous conditions and results in the formation of superoxide ion, O2–:305-313

                                        O+  e  '  O2                                           (5)