Other types of ionic liquid

Tetraalkylammonium bis(trifluoromethanesulfonyl)amide 15 is prepared from a commercially available bromide derivative by simple anion metathesis. A similar procedure is used for sulfonium bis(trifluoromethanesulfonyl)amide 16.  

In parallel, new types of anion have also been described, usually associated with the dialkylimidazolium cation. For example, dicyanamide 18 salts are water-soluble ionic liquids with low viscosity.Other types of anion-containing metals have been reported, such as hexafluoroniobate and tantalate 19 and Co(CO)4 20, but these have found only limited applications. Ionic liquids formed with 

In fact, a great number of different ionic liquids can be synthesized by combining the available cations and anions Seddon suggests that, if binary and ternary mixtures are considered, up to 10 room-temperature ionic liquids can be prepared.  

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The room temperature conductivity data for a wide variety of ionic liquids are listed in Tables 3.6-3, 3.6-4, and 3.6-5. These tables are organized by the general type of ionic liquid. Table 3.6-3 contains data for imidazolium-based non-haloaluminate alkylimidazolium ionic liquids. Table 3.6-4 data for the haloaluminate ionic liquids, and Table 3.6-5 data for other types of ionic liquids. There are multiple listings for several of the ionic liquids in Tables 3.6-3-3.6-5. These represent measurements by different researchers and have been included to help emphasize the significant vari-... 

Two other types of ionic liquids are very promising candidates for conducting polymers. They are ionic liquids based on choline chloride, which have already shown superior properties in electrochemical processes (e.g. metal finishing) [46], and s mg/e-ended or double-ended diallylammonium ionic liquids, which are protic compounds with a high potential for excellent proton conductivity [47]. 

We note that there has been very recent work devoted to the development of force fields for other types of ionic liquid cations including pyridinium [123] and triazolium [124]. Moreover, an electronically polarizable model has been developed and applied to the simulation of [EMIM][N03] [125]. Results from these studies will be discussed in the next section. 

The types of ionic liquids shown in Figure 5.4 have been most extensively studied, especially ones based on chloroaluminate. Whilst these chloroaluminate materials also display useful Lewis acid properties they are highly air and moisture sensitive, which renders them relatively commercially unattractive. Newer ionic liquids containing C104 and NOa anions, for example, which are less air and moisture sensitive, are now being more widely studied, but these are less catalytically active. Other than lack of vapour pressure and catalytic properties there are several other features common to most ionic liquids that make them attractive reaction solvents. These include ... 

This simple example may illustrate that in general the reaction of an organic halide salt [cation]X with an excess of a Lewis-acid MXy can result in new catalytic materials even if other Lewis-acids are applied than AICI3. In contrast, the use of other Lewis-acids to form the ionic liquid of type [cation][MXy+i] + excess MXy (the excess of MXy may be dissolved in the neutral ionic liquid or may form acidic anionic species such as e.g. [M2X2y+i]-) gives access to new combinations of properties (e.g. a liquid, less oxophilic, Lewis-acidic catalyst with defined solubility and acidity properties). In Table 2 other examples of ionic liquids are presented which are formed by the reaction of an organic halide salt with different Lewis-acids. All these systems should be in principle useful acidic catalysts for synthetic organic chemistry even if not all displayed examples have been already discribed in the literature for this application. 

RUCI3 as a simple complex is able to catalyze the aerobic oxidation of alcohols in different types of ionic liquids [44]. A new fluorinated ionic liquid, l-n-butyl-3-methyUmidazolium pentadecafluorooctanoate [bmi.COO(CFj) CF3], was synthesized because of its ability to dissolve higher amounts of oxygen which demonstrates better performance than the other commonly employed ionic liquids. 

The relationship between conductivity and viscosity may be viewed through the use of a Walden plot (log A versus log (1// )) [61]. Plotting the molar conductivity (A) instead of the absolute conductivity (k), to an extent, normalizes the effects of molar concentration and density on the conductivity and, thus, gives a better indication of the number of mobile charge carriers in an ionic liquid. Fig. 3.6-4 shows the Walden Plot for the data in Tables 3.6-5-3.6-8. Data for each of the various types of ionic liquids (haloaluminates, non-haloaluminate imidazoliums, ammoniums, other ionic liquids) were plotted separately on the graph. However, as is clearly shown in Fig. 3.6-4, no difference in the behavior of any of the types of ionic liquids was observed. 

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubility of ionic liquids in other compounds and the solubility of other compounds in the ionic liquids. For instance, an increase in allcyl chain length decreases the mutual solubility with water, but some anions ([BFJ , for example) can increase mutual solubility with water (compared to [PFg] , for instance) [1-3]. While many mixture properties and many types of phase behavior are important, we focus here on the solubility of gases in room temperature IFs. 

So far, we have focused on the melting points and polarities of ionic liquids. Like conventional solvents, other properties such as viscosity and density are also very important when selecting a solvent for synthetic applications. Whilst this type of data is well known for other solvents, relatively little has been reported for ionic liquids. Table 4.6 lists available melting points, thermal stability, density, viscosity and conductivity data for the better studied ionic liquids. 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

Tetrafluoroborate(BF4) and hexafluorophosphateCPFg), are among other types of anions that are attracting the interest of ionic liquids research groups. These ions do not combine with their corresponding Lewis acids and are therefore not potentially acidic. 

The temperature range within which ionic liquids occur in the liquid state is very characteristic it is assumed that it is never greater than 300°C. No other type of commonly used solvent occurs as a liquid over such a great range. Table 19.7 lists the physicochemical properties of frequently used solvents. 

Ionic liquids have also been separated into first and second generation liquids [10] where first generation liquids are those based on eutectics and second generation have discrete anions [17]. Others have sought to further divide the first generation liquids into separate types depending on the nature of the Lewis or Bronsted acid that complexes [18]. While there is some dispute whether eutectics with Bransted acids constitute ionic liquids at all there are others who seek to widen the description of ionic liquids to include materials such as salt hydrates [19]. 

The most widely used synthetic approach used to prepare ionic liquids is summarised in Scheme 2.1. The scheme depicts the preparation of imidazolium-based ionic liquids although the method is widely applicable to other types of cations, notably pyridinium systems. 

We have tried to cover important aspects of the physical chemistry of the ionic liquids currently under study, and to relate them to what is known about other types of low-melting ionic media. In concluding, we must emphasize that much of the success in their application, particularly in the Green Chemistry area where there is hope they will replace volatile solvents of environmentally hostile character, will depend on the important chemical properties of these media. These we have not addressed at all in this chapter. Properties such as donor and acceptor character, acidity and basicity, are in fact aU within the capacity of physics to describe, though they are most commonly invoked in a more empirical manner based on experience, as described in [1—4]. An excellent treatment of acid base character of ionic liquids has recently been given by MacFarlane and Forsyth [45]. 

Clearly this is the least mature field within the solvent alternatives arena however, this also means that, as with tailor-made ionic liquids, it is likely that tailor-made switchable solvent systems will continue to advance and become an increasingly important area of research during the coming decades. As with all areas of clean technology, synergies and overlaps with other areas of sustainable development will increase and lead to new advances. For example, in the area of gas expanded liquids, the focus has so far been on petroleum-sourced VOCs and therefore significant advances could be made by investigating other types of gas expanded media, whether they be renewably sourced VOCs or non-volatile alternatives. 

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