What is an Ionic Liquid

What constitutes an ionic liquid, as distinct from a molten salt It is generally accepted that ionic liquids have relatively low melting points, ideally below ambient temperature [1, 2]. The distinction is arbitrarily based on the salt exhibiting liquidity at or below a given temperature, often conveniently taken to be 100 °C. However, it is clear from observation that the principle distinction between the materials of interest today as ionic liquids (and more as specifically room-temperature ionic liquids) and conventional molten salts is that ionic liquids contain organic cations rather than inorganic ones. This allows a convenient differentiation without concern that some molten salts may have lower melting points than some ionic liquids . 

Ionic liquids with discrete anions have a fixed anion structure but in the eutectic-based liquids at some composition point the Lewis or Bronsted acid will be in considerable excess and the system becomes a solution of salt in the acid. A similar scenario also exists with the incorporation of diluents or impurities and hence we need to define at what composition an ionic liquid is formed. Many ionic liquids with discrete anions are hydrophilic and the absorption of water is found sometimes to have a significant effect upon the viscosity and conductivity of the liquid [20-22], Two recent approaches to overcome this difficulty have been to classify ionic liquids in terms of their charge mobility characteristics [23] and the correlation between the molar conductivity and fluidity of the liquids [24], This latter approach is thought by some to be due to the validity of the Walden rule... 

The conductivity and viscosity of an ionic liquid is often combined into what is termed Walden s rule [Equation (3.6-4)] [54],... 

What reaction can be carried out in an ionic liquid that is not possible in organic solvents or water Many convincing examples have been described in Chapters 5-8. 

In an ionic liquid, the cations are much larger than in typical ionic substances. As a result, the anions and cations cannot be packed together in an orderly way that balances both the sizes and distances of the ions with the charges between them. As a result, they remain in a loosely-packed, liquid form. Because the charges on the cations have the capacity to bind more anions, an ionic liquid has a net positive charge. Notice the large cation of the ionic liquid molecule shown below. This is what makes ionic liquids suitable solvents. 

The nature of this ionic/hydrophilic liquid phase can be quite diverse it can be made by an onium salt (e.g., ammonium or phosphonium), by an ionic liquid (e.g., imidazolium salts), by polyethyleneglycols, and even water. What is required is that the catalyst-philic phase is not miscible with the other phases... 

Supporting ionic liquids in the pores of solid materials offers the advantage of high surface areas between the reactant phase and that containing the supported liquid catalyst. This approach is particularly useful for reactants with less than desired solubility in the bulk liquid phase. Another incentive for using such catalysts is that they can be used in continuous processes with fixed-bed reactors (26S). The use of an ionic liquid in the supported phase in addition to an active catalyst can help to improve product selectivity, with the benefit being similar to what was shown for biphasic systems. However, care has to be taken to avoid leaching the supported liquids, particularly when the reactants are concentrated in a liquid phase. 

The recognised definition of an ionic liquid is an ionic material that is liquid below 100 °C but leaves the significant question as to what constitutes an ionic material. Some authors limit the definition to cations with discrete anions e.g. BF4-, NO3. This definition excludes the original work on chloroaluminate systems and the considerable work on other eutectic systems and is therefore unsatisfactory. Systems with anionic species formed by complex equilibria are difficult to categorise as the relative amounts of ionic species depend strongly on the composition of the different components. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

The parameter x now invites consideration. In the gas phase, x is the mean time between collisions. What is the significance of t in an ionic liquid ... 

It has just been argued that the conductivities of simple ionic liquids, on the one hand, and liquid sihca and water, on the other, are vastly different because a fused salt is an unassociated liquid (it consists of individual particles) whereas both molten silica and water are associated liquids with network structures. What is the situation with regard to the viscosities of fused salts, water, and fused silica Experiments indicate that whereas water and fused NaCl have similar viscosities not far above the melting points of ice and solid salt, respectively, fused silica is a highly viscous liquid (Table 5.46). Here then is an interesting problem. 

We believe that the arbitrary 100 °C limit fixed for onium salts to be called ionic liquids introduces unnecessary limitations to what has to be considered as an ionic liquid. The distinction between RTILs including solutions of OSs in RTILs and OSs which melting points are above room temperature is far more relevant and easier to handle. Anyway, TSILs are undeniably TSOSs and as a consequence the onium salts generic terminology will be applied next. 

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