Ionic liquid melting properties

Ionic liquids once referred almost exclusively to molten salts, i.e., salts with melting points well above room temperature, although eutectic mixtures of salts can dramatically reduce melting points. Nowadays, there is a range of ionic compounds that are liquid at room temperature, and often much lower temperatures. Room temperature ionic liquids with properties that are conducive towards synthetic chemistry are described in Section 1.26.3. 

Physicochemical Properties of Ionic Liquids Melting Points and Phase Diagrams... 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

Thus, most ionic liquids are formed from cations that do not contain acidic protons. A summary of the applications and properties of ionic liquids may be found in a number of recent review articles [3]. The most common classes of cations are illustrated in Figure 2.1-1, although low melting point salts based on other cations, such as complex poly cationic amines [4] and heterocycle-containing drugs [5], have also been prepared. 

The presence of several anions in these ionic liquids has the effect of significantly decreasing the melting point. Considering that the formation of eutectic mixtures of molten salts is widely used to obtain lower melting points, it is surprising that little effort has been put into identifying the effects of mixtures of cations or anions on the physical properties of other ionic liquids [17]. 

Addition of co-solvents can also change the co-miscibility characteristics of ionic liquids. As an example, the hydrophobic [BMIM][PFg] salt can be completely dissolved in an aqueous ethanol mixture containing between 0.5 and 0.9 mole fraction of ethanol, whereas the ionic liquid itself is only partially miscible with pure water or pure ethanol [13]. The mixing of different salts can also result in systems with modified properties (e.g., conductivity, melting point). 

In a number of general properties, such as viscosity and thermal conductivity, melts differ little from solutions. Their surface tensions are two to three times higher than those of aqueous solutions. This leads to poorer wetting of many solids, including important electrode materials such as carbon and graphite, by the ionic liquids. 

Although chemically similar, the inorganic and organic chloroaluminate molten salts or ionic liquids, as some prefer to call them, differ greatly with respect to their melting temperatures and physical properties. Figures 1 and 2 show the phase diagrams... 

Ionic liquids are characterised by the following three definition criteria. They consist entirely out of ions, they have melting points below 100 °C and they exhibit no detectable vapour pressure below the temperature of their thermal decomposition. As a consequence of these properties most ions forming ionic liquids display low charge densities resulting in low intermolecular interaction. Figure 7.1 displays some of the most common ions used so far for the formation of ionic liquids. 

In 1996, Gratzel, Bonhote and coworkers published the synthesis and properties of ionic liquids with anions containing CF3- and other fluorinated alkyl groups [12]. These are usually highly hydrophobic melts with high stability (even under acidic conditions) against hydrolysis. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

Once the two salts are mixed in solution (acetone is a common solvent for this), the sodium chloride precipitates and is removed by filtration. The solvent is then removed under reduced pressure and, since salts have no vapour pressure, the ionic liquid remains in the flask. The problem with this reaction is that it is almost impossible to remove the last traces of chloride ions. The chloride not only influences the physical properties of the liquid such as melting point and viscosity, but is also a good nucleophile and can deactivate catalysts and affect reproducibility. A great deal of effort has been directed towards removal of the chloride contamination, including washes and chromatography, but none have proved to be completely effective [9], This has led to the development of some alternative synthetic routes. Simply exchanging Na[BF4]... 

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. 

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 readily can be seen by examining the phase diagrams for the hexafluorophosphate and tetrafluoroborate salts (see Figures 5.5 and 5.6, respectively). 

Ionic liquids are salts that are often defined as those with melting points < 100°C (1) many are liquids even at room temperature. These salts present a wide range of properties for applieations as new solvents and eatalysts. A large number of new ionic liquids have been reported reeently, and the seope of their potential applications has been expanding rapidly. For eatalytie applieations, ionie liquids can be viewed as salts that are stable in the liquid state under the proeess conditions. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

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