Physical Properties of Ionic Liquids

The physical properties of ionic liquids have been extensively studied and some trends are beginning to emerge. In particular, ionic liquids based on 1,3-dialkylimidazolium cations have been investigated in detail, partly due the their wide use as solvents to conduct synthesis and catalysis. The attraction of the imidazolium cation in synthetic applications is because the two substituent groups can be varied to modify the properties of the solvent. For example, Table 4.1 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been determined using the solvatochromic dyes Nile Red and Reichardt s dye [16, 17], Measurements with Nile Red do not give absolute values of polarity but provide a useful scale to estimate the relative polarity of the ionic liquids. Similar measurements have been made using a number of other solvatochromic dyes (dansylamide, pyrene, pyrenecarboxyaldehyde, and bromonapthalene) for [BMIM][PF6], and gave results consistent with those obtained with Nile Red. Values for Ej obtained for ionic liquids generally fall between the values of 0.6 and 1.0, as shown in Table 4.3. 

Comparative polarity studies have also been performed by absorption of ionic liquids on to gas chromatography columns followed by the elution of various compounds and lead to the conclusion that the polarity of [bmim][BF4] is similar to that of lower alcohols [18]. 

Miscibility is an important consideration when selecting solvents for use in biphasic systems. Table 4.4 shows the miscibility of three ionic liquids with water and some organic solvents. [bmim][PFe] was found to be miscible with organic solvents whose dielectric constant is higher than 7, but was not soluble in less polar solvents or in water. Basic [bmim][AlCl4] was found to react with protic solvents, and the acidic form also reacted with acetone, tetrahydrofuran and toluene. 

Organic solvent er [bmim] [PF6] [ bmim AICI4] Basic bmim AICI4 Acidic 

Organic solvent Sr [bmimliPFg] [bmim][AlCLt] Basic [bmimliAlCU] Acidic 

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Densities are perhaps the most straightforwardly determined and unambiguous physical property of ionic liquids. Given a quality analytical balance and good volumetric glassware the density of an ionic liquid can be measured gravimetrically (i.e., the sample can be weighed). 

The physical properties of ionic liquids can often be considerably improved through the judicious addition of co-solvents [55-58]. Flowever, surprisingly, this approach has been relatively underutilized. Flussey and co-workers investigated the effect of co-solvents on the physical properties of [EMIM]C1/A1C13 ionic liquids [55, 56]. They found significant increases in ionic conductivity upon the addition of a variety of co-solvents. Figure 3.6-5 displays representative data from this work. The magnitude of the conductivity increase depends both on the type and amount of the co-solvent [55, 56]. 

The field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

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. 

Seddon, K. R. Stark, A. Torres, M.-J. Influence of chloride, water, and organic solvents on tbe physical properties of ionic liquids. Pure Appl. Chem., 2000, 72(12), 2275-2287 Seddon, K. R. Stark, A. Torres, M. J. Viscosity and density of l-alkyl-3-methylimidazolium ionic liquids, in Clean Solvents Alternative Media for Chemical Reactions and Processing, Eds. Abraham, M. Moens, L. ACS Symposium Series, Vol. 819, American Chemical Society, Washington D.C., 2002, pp. 34-49. 

D.8.2. F Impurity. It has been observed that [MIMjSbFg produced a small amount of fiuoride ion over time because of its sensitivity to air. Furthermore, residual chloride impurities have been found to exert a large influence on the physical properties of ionic liquids 115). 

D.8.3. Cl Impurity. Trace amounts of chloride impurities, which may be present at levels between 0.1 and 0.5mol/kg, have significant effects on the physical properties of ionic liquids, such as viscosity and density. Increases in viscosity are of particular concern in biphasic processes because of the formation of emulsions that affect the interface between the two phases 88). 

Seddon, K.R., Stark, A., and Torres, M.J., Influence of chloride, water, and organic solvents on the physical properties of ionic liquids, Pure Appl. Chem., 72, 2275,2000. 

The physical properties of ionic liquids depend on the kind of cation and anion. Quaternary ammonium salts, which have been known about for more than a century, exhibit interesting and... 

Physical Properties of Ionic Liquids for Electrochemical Applications... 

Physical Properties of Ionic Liquids for Electrochemical Applications Table 3.4 Heat capacity and thermal conductivity of ionic liquids... 

In Chapter 1 we explain the motivation and basic concepts of electrodeposition from ionic liquids. In Chapter 2 an introduction to the principles of ionic liquids synthesis is provided as background for those who may be using these materials for the first time. While most of the ionic liquids discussed in this book are available from commercial sources it is important that the reader is aware of the synthetic methods so that impurity issues are clearly understood. Nonetheless, since a comprehensive summary is beyond the scope of this book the reader is referred for more details to the second edition of Ionic Liquids in Synthesis, edited by Peter Wasserscheid and Tom Welton. Chapter 3 summarizes the physical properties of ionic liquids, and in Chapter 4 selected electrodeposition results are presented. Chapter 4 also highlights some of the troublesome aspects of ionic liquid use. One might expect that with a decomposition potential down to -3 V vs. NHE all available elements could be deposited unfortunately, the situation is not as simple as that and the deposition of tantalum is discussed as an example of the issues. In Chapters 5 to 7 the electrodeposition of alloys is reviewed, together with the deposition of semiconductors and conducting polymers. The deposition of conducting polymers... 

From the perspective of a chemical engineer, who eventually will scale up any ionic liquid based process developed in the laboratory, whether a solvent melts at -30°C or -80°C makes very little difference. The viscosity of the solvent, on the other hand, is much more important, which for ionic liquids is usually high. Table 2.3 lists the viscosity data of a series of ionic liquids together with some molecular solvents for comparison purposes. Considerably more extensive series of physical properties of ionic liquids can be found elsewhere/21 221 the aim here is to illustrate general trends and principles. 

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