The Cations And The Anions

Despite the discovery of room-temperature ionic liquids in 1914, by Paul Walden, no attention was paid to them until 1948,46 when Hurley and Wier used fluids based on the N-ethylpyridinium cation, [NEtpy]+.47,} However, it was not until Osteryoung investigated the N-butylpyridinium cation, [NBupy]+, that the field was really born.48 A tremendous boost to this area was achieved when Hussey and co-workers49 discovered that the 1-ethyl-3-methylimidazolium cation, [emim]+, conferred even better properties on the systems under study, which were then mostly based upon the tetra-chloroaluminate(III) anion. The next step in expanding the area came when Wilkes and Zaworotko discovered that there were many more ionic liquid systems possible with the [emim]+ cation, if the anion were to be changed to, say, tetrafluoroborate or ethanoate.50 Moreover, these new ionic liquids had the added advantages over the traditional tetrachloroaluminate(III) systems that they were air- and water-stable. In our own laboratories, we were investigating the effects of modifying the cation

8 12 Alkyl Chain Length (n)

Figure 5.5 Phase diagram for the ionic liquids [Cn mim][PF6]. Open squares are glass transitions, closed squares are melting points, and circles are clearing transitions; LC is a liquid crystalline phase.

}Two parallel, and equally useful, systems of abbreviations exist: [NEtpy]+ is also referred to as [C2 py]+, [NBupy]+ as [C4 py]+, [emim]+ as [C2 mim]+, and so forth.

(by replacing the N-alkyl substituents with longer chains) to form [Cn mim]+ and [Cn py]+ (n = 2-18),51 the effects of changing the anion,52'53 the effects of hydrogen bonding,54 the nature of the anionic equilibria,55,56 purification techniques,55 and the properties of binary and ternary mixtures of ionic liquids.57

These changes in anion and cation were not merely a case of methyl, ethyl, propyl, butyl, and then futile. The change of anion dramatically affects the chemical behavior and stability of the ionic liquid; the change of cation has a profound effect on the physical properties, such as melting point, viscosity, and density.52,53,58 This readily can be seen by examining the phase diagrams for the hexafluorophosphate52 and tetrafluoroborate53 salts (see Figures 5.5 and 5.6, respectively).

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