Solubility of Ionic Liquids

3-dialkylimidazolium ILs reported to date are hygroscopic, and their miscibility with water is largely controlled by the nature of the anion. While those salts containing the nitrate, chloride and perchlorate anions are usually miscible with water in all proportions, those associated with hexafluorophosphate and bis(tri-fluoromethane) sulfonylamidate anions are almost completely immiscible with water [93]. Interestingly, the miscibility with water of those containing the tetraf lu-oroborate anion is temperature dependent (Fig. 3.5-7) [36]. It is also known that an increase of the N-alkyl chain lengths increases the hydrophobicity for a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ILs [94] The miscibility of water with ILs can be increased by the addition of short-chain alcohols [95] or diminished by the addihon of salts (salting-out effect) [96]. 

Extraction/Separation Processes Involving Ionic Liquids. 

It has been demonstrated that with the appropriate choice of membrane (hydro-phobic or hydrophilic polymers) the per-evaporation procedure can be efficiently applied for the quantitative and selective recovery of organic solutes, such as naphthalene, water, ethyl hexanoate and chlorobutane, from [C4CjIm]PF, IL [110]. It was also reported that the same ILs can be used as supported liquid membranes for the selective transport of secondary amines over tertiary amines with similar boiling points, and this was attributed to the higher hydrogen bond affinity of the secondary derivative with the imidazoHum cation [111,112]. 

As mentioned before, carbon dioxide is highly soluble in [C4CjIm]PFg, although the ionic liquid does not dissolve in carbon dioxide [103]. These properties can facilitate the extraction of solutes from the IL without contamination of the non-io- 

ILs-separation processes have also been appHed for actinides in aqueous solution by the incorporation of -octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide as the extractant. The portioning distribution ratios for Pu, Th and 

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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. 

Domariska, U. and Morawski, R, Influence of high pressure on solubility of ionic liquids experimental data and correlation. Green Chem., 9, 361, 2007. 

Determination of water solubility of ionic liquids Quanfitative ESI MS has been applied fo determine water solubility of several ILs [18]. The ion currenf (infensify) of the analyte signals was found to depend on the analyte concentration over a restricted range of concenfra-tions. Nevertheless, relative quantifications performed by the use of infernal standards deliver more reliable results. Isotopically labeled analogs of fhe... 

The water solubility of ionic liquids is very dependent on both the cation and anion. Generally, water solubility decreases with increasing the alkyl chain length on the cation. Seddon et al. [16] have discussed the metathesis reaction of the halide salt... 

Unlike polarity, the solubility of ionic liquids depends strongly on the structure of the associated anion. In fact, the miscibiUty of ionic liquids with traditional solvents is one of their most interesting features and is clearly observed during the anion metathesis of imidazolium derivatives. The starting chlorine salt, for example [bmim][Cl], is completely soluble in water. After exchange of the anion, [bmim][PF6] or [bmim][NTf2] salts are obtained as hydrophobic products and therefore form a separate layer. Table 2.7 summarizes the solubility of l-butyl-3-methylimidazolium salts as function of the anion. ... 

Decreasing the solubility of ionic liquids in water plays an important role in ionic liquid-based extraction. The anion has the primary effect on water miscibility, whilst the cation seems not to affect miscibility much (Seddon et al. 2000). The order of increasing hydrophobicity for the anions is Br , Cl < [BFJ < [PFe] < [NTfa] (Toh et al. 2006). Metal ions are poorly soluble in ILs except when coordinated with, or solvated by, hydrophobic molecules added to the IL phase, or with hydrophilic complexing anions that can facilitate metal ion transport into the IL phase. 

It can be noticed from Figure 11.4 that the mutual solubilities of ionic liquids and water are not symmetric. Generally, a much higher mole fraction of water is present in the ionic liquid phase than ionic liquid present in the water phase at the same temperature. The UCST is thus found at very low fractions of the ionic liquid. This asymmetry is also observed in (polymer + water) systems. Therefore, there are analogies between the phase diagrams of ionic liquid solutions and polymeric ones, although this may be due to different mechanisms. 

The mutual solubilities of ionic liquids and water are primarily defined by the type of anion and the cation alkyl side chain length of the ionic liquid. An increase in the alkyl chain length of the cation decreases the mutual solubility. Ionic liquids with bis(trifluoromethylsulfonyl)amide anions and hexafluorophosphate anions are more hydrophobic than ionic liquids with tetra-fluoroborate anions. However, even the most hydrophobic ionic liquids still... 

Shvedene, N. V., Borovskaya, S. V., Sviridov, V. V., Ismailova, E. R., and Pletnev, I. V. 2005. Measuring the solubilities of ionic liquids in water using ion-selective electrodes. Anal. Bioanal. Chem. 381,427-430. 

Table 4.2.1 Mutual solubility of ionic liquid and water for moderately hydrophobic ionic liquids suitable to ionic liquid salt bridge at 25 °C...

Ether Benzene Toluene IPE Cyclohexane 5.4 Solubility of ionic liquids in organic solvents at room temperature [34],... 

The solubility of ionic liquids may depend on the temperature of the solution. Freire at fit/. measured the solubility of imidazolium-, pyridinium-, pyrrolidinium-, and piper-idinium-based ionic liquids. In general, the solubility of each ionic liquid increased with an increase in temperature. For example, the solubility of 1-butyl-3-methylimidazolium hexafluorophosphate increased by about 74% over a 30-degree temperature increase (Fig. 

Acyloxyprolines can also be modified with ionic liquid-tags. The use of the proper ionic liquid-tag enables the use of these catalysts in the presence of water and their recovery and reuse. Indeed, the solubility of ionic liquid-anchored orga-nocatalysts can be tuned by an accurate choice of cations and anions groups present in their structure, and the solubility of ionic liquids can be readily tuned with different cations and anions. This approach allows phase separation to be realized from organic as well as aqueous media. Following the proper extraction procedure, it could be possible to separate the catalytic molecule from the product and to allow its reuse. 

Domanska U, Krolikowski M, Pobudkowska A, Bochenska P (2012) Solubility of ionic liquids in water and octan-l-ol and octan-l-ol/water, or 2-phenylethanol/water partition coefficients. J Chem Thermodyn 55 225-233... 

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