Physico-chemical Properties of ILs

In selecting a solvent for catalysis, both physical properties, such as melting points and viscosities, and chemical properties need to be considered. There have been several review articles covering these data in recent years [4], 

The existence of a useful liquid range is a prerequisite for a material being used as a solvent. Since I Ls are nonvolatile they do not have a boiling point. For that reason their upper limits are generated by their decomposition (see below). The lower limits are provided by their melting or glass transition temperatures. 

The accurate prediction of the melting points of salts remains an elusive aim. Lattice energies are relatively easy to calculate, but as shown in Table 1 they are poor predictors of melting points, even for the simplest salts. The principal reason for this is that the structure of the liquid form, which plays no part in determining the lattice energy of the salt, is as important in determining its melting point as the solid-state structure and can introduce a number of complications. 

As shown below, there are a number of factors that all together lead to the melting point of a salt. For different materials each factor contributes to a different degree. Further to this, many of the materials that form ILs are glass-forming. This means that the change of state can happen at different temperatures when heating or 

In order to reduce the melting of a salt it is necessary to reduce the attraction of the ions for each other. The principal attraction of ions for each other comes from Coulombic forces [see Eq. (1) where is the charge on the particle i, is the relative permittivity of the medium and r is the separation of the charges]. Hence, it is rare to find an IL with multiply-charged ions. 

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Very often, some issues remain to be addressed, such as mechanistic studies, kinetics, and the location of the reaction. The level of knowledge concerning the physico-chemical properties of ILs has been raised in recent years but is still to be improved. 

Although it is well known that anion modification leads to changes in the physico-chemical properties of ILs, all the works published so far showed no systematic effect of the anion, indicating that ILs toxicity is largely driven by the alkyl chain branching and hydrophobicity of the cation (Fig. 1). 

Aliquat 336 and quaternary phosphonium cation [QP] with 2-(methylthio)benzoate [MTBA] and [TS] have been apvplied as extracting agents for PH+ from chloride solution. The extraction efficiency tends to diminish with decreasing viscosity of IL in the following order [QP][MTBA] (100%) > [A336][rS] (85%) > [QP][TS] (76%) > [A336][MTBA] (40%) (Stojanovic et al., 2010). Thus, it proves that not only functionality appended to the anion but also physico-chemical properties of IL play an important role in extraction of metal ions. 

The reported experimental procedures in order to synthesize and purify different types of methylimidazolium ILs is significantly higher compared with other cation families. Depending of cation/anion combinations, the physico-chemical properties of ILs will be changed. Looking for the large number of possible combinations the development of novel... 

Comparing the results of entries 1-3 in Table 3, one can see that the reaction requires a pressure of about 100 bar of hydrogen to proceed to completion in the absence of CO2. In contrast, a partial pressure of 30 bar of H2 is sufficient in the biphasic IL/SCCO2 system. This striking difference results from the significant influence of the compressed CO2 on the physico-chemical properties of the IL phase. 

Preparation and Some Physico-Chemical Properties of 1,3-Dialkylimidazolium ILs... 

Impurities present in ILs may have an impact not only on physico-chemical properties of the liquid but also on catalytic performances when the liquid is used as a solvent. In order to obtain reproducible results, a great amount of attention has to be dedicated to a precise characterization of ILs and in particular to the degree of purity. 

Foreseeing the possibility of tuning the physico-chemical properties of these compounds by varying the nature of the anion and/or the cation, then taking into account the ionic state of the Dimersol catalyst, it was expected that ILs based on chloroalkyl aluminates would fit well. They do, indeed ... 

The effects of pressure and temperature on the physico-chemical properties of PILs were also examined.Different theoretical predictive models were developed on the subject. Noteworthy were theoretical investigations on the cation-anion interactions in PILs compared to ammonium ionic liquids, and an all-atomistic force field for a new class of halogen-free chelated orthoborate PILs. " A study was also reported on an ion contribution equation of state based on electrolyte perturbation theory for the calculation of ILs densities. ... 

While much work has been devoted to the wide range of applications of ILs, the basic understanding and study of their structure-property relationship is of equivalent importance but has lagged behind. More specifically, studies on how the structure of the ions in the IL influences their physical properties are rare. Knowledge of the structure-property relationship is important for assessing the suitability of ILs for specific applications as well as the design of new ILs. Very few works have systematically studied the qualitative and/or quantitative relationships between the structures of ILs and their fundamental properties[116], such as melting point, viscosity, density, surface tension, thermal and electrochemical stability, solvent properties, and speed of sound. At present, however, data for many other physico-chemical properties of ionic liquids are in short supply, or too unreliable to allow similar structure-property relationship studies. [117]... 

In spite of the explosion in studies on ionic liquids (ILs), there is only a small number of studies of their basic characteristics. There are limitless possibilities for the design of ILs by changing their component ion structures. However, the chance of succes s is not very great without accurate information on the structure-properties relationship. Physico-chemical property data for ILs are therefore very important for the present and future ofthe field of ILs. In this chapter, some basic properties of air-stable ILs have been summarized. Some are not directly related to electrochemistry but are very important and useful for a wide range of science and technology related to ILs. 

Ionic liquids (ILs) have been recognized since the early 1900s. They are molten salts with low melting points, usually below 100°C. They were introduced in organic synthesis as a new class of polar, non-molecular solvents in the late 1990s and have recently attracted interest in a variety of fields since solvents are at the heart of most chemical processes and ionic liquids show an exclusive and fascinating wide range of physico-chemical properties. 

All the physico-chemical properties strictly dependent upon the precise nature of the cation and anion constituting the IL and they can be changed or modulated by changing the anion or cation or modifying the nature of substituents on cation. Structurally, most of the ILs that have been investigated to date are based on imidazolium, ammonium and pyridinium cations, bearing alkyl chains, associated with polyatomic anions such as chloroaluminates, tetrafluoroborate, hexafluorophosphate and bis-triflimide. In Scheme 1 are reported the above mentioned cations and anions whose combination gives the most commonly employed ionic liquids. 

As already pointed out, of the various known ILs, those derived from the combination of the 1,3-dialkylimidazolium cation with various anions are the most popular and investigated class (Scheme 3.5-9). This is most probably due to their facility of synthesis stability and the possibility of fine-tuning their physico-chemical properties by the simple choice of the N-alkyl substituents and/or anions (Table 3.5-3). 

The model of non-homogeneous Hquids for ILs may be more helpful not only for the rationahzation of many of their properties but also in allowing the design of new IL materials with tailor-made properties. In order to develop this model a close look at the structure of pure ILs in the solid, liquid and gas phases is necessary, followed by analysis of the interaction of other substances with the ILs through changes in the physico-chemical properties. 

The main interest in ionic Hquids was first to offer green alternatives to volatile organic solvents. But not only this because of their unique set of physico-chemical properties they are very different from conventional organic solvents. They may give the opportunity to promote reactions that are not possible in other solvents. For example, they offer a nonaqueous environment to substrates and can be poorly miscible with organic compoimds (cf Section 5.2.1). But one would expect more than just a physical solvent new chemistry may be foreseen. Indeed, it has been proven that the nature of the ionic liquid may influence the outcome of chemical reactions [41]. In many cases, ILs contribute to improving reaction rates and... 

Physico-chemical properties, such as conductivity, viscosity and density of BMMImNs, BMMImBF4 and their mixtures, were reported in [11]. Both these salts have similar conductivities and viscosities with the density of the azide IL being about 10 % lower, in the range of the temperatures higher than 65 °C. The electrochemical window of the azide IL is by 1.5 V less than the fluoroborate IL due to the lower anodic stability of the N3 anion, though the cathodic decomposition potentials of both ILs vs. Ag/AgCl reference electrode are almost equal. 

The engineering modeling of IL environments is yet to emerge, and measurements of physico-chemical properties (such as viscosities, densities, gas solubilities, diffusion coefficients, toxicology, etc.) are only available for a very limited number of compounds. Moreover, new correlations need to be developed to account for, for example, the complex equilibrium behavior of ILs and traditional solvents. 

The most simple and efficient approach is based on gelation which is a simple method that allow a good compromise between the retention of the IL and its fluidity inside the polymeric network. These so called ion gels are simpler than solid polymer electrolytes and exhibit improved conductivities. In fact ion gels hold both the processability and mechanical properties of polymers, but with added physico-chemical properties and were primary developed as replacements for current solid-state polyelectrolytes in energy devices, such as dye-sensitized solar cells, supercapacitors, lithium ion batteries, and fuel cells. (Fernicola et al., 2006 Galinski et al., 2006 Le Bideau et al., 2011 Lu et al., 2002 Mazille et al., 2005 Stephan, 2006)... 

The purification process of ILs is a relevant step of synthetic methodologies because all of the physico-chemical properties reported in this chapter are moderate to high sensitive to the presence of impurities. 

ILs can be considered complex molecules containing cation and anion units with polar and apolar domains, which allow their use in a high range of tasks but at the same time restrict the physico-chemical properties prediction, simply by looking to the structure of the compounds. In order to circumvent this problem, the construction of models more or less complex and specific is required. 

Attending the recent applications of ILs, one of the most relevant parameters is related with the pnirity of ILs. Several studies have been reported about the significant impact of impurities such as halogen or water content, in some values of density, viscosity and conductivity of ILs. For similar ILs is possible to find discrepancies in terms of these physico-chemical properties. Also, the electrochemical and thermal stability and solubility parameters can be completely changed according with the final purity of ILs. 

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