Ionic water-miscible

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL will form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

When water-miscible ionic liquids are used as solvents, and when the products are partly or totally soluble in these ionic liquids, the addition of polar solvents, such as water, in a separation step after the reaction can make the ionic liquid more hydrophilic and facilitate the separation of the products from the ionic liquid/water mixture (Table 5.3-2, case e). This concept has been developed by Union Carbide for the hydroformylation of higher alkenes catalyzed by Rh-sulfonated phosphine ligand in the N-methylpyrrolidone (NMP)/water system. Thanks to the presence of NMP, the reaction is performed in one homogeneous phase. After the reaction. 

When either the organic solvent or the ionic liquid is used as pure solvent, proper control over the water content, or rather the water activity, is of crucial importance, as a minimum amount is necessary to maintain the enzyme s activity. For ionic liquids, a reaction can be operated at constant water activity by use of the same methods as established for organic solvents [17]. [BMIM][PFg] or [BMIM][(CF3S02)2N], for example, may be used as pure solvents and in biphasic systems. Water-miscible ionic liquids, such as [BMIM][BF4] or [MMIM][MeS04], can be used in the second case. 

Husum et al. found that the hydrolytic activities of P-galactosidase from E. coli and the protease subtilisin in a 50 % aqueous solution of the water-miscible ionic liquid [BMIM][Bp4] were comparable to those in 50 % aqueous solutions of ethanol or acetonitrile (Entry 9) [37]. 

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

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. 

Fig. 3. Water miscibilities of representative ionic liquids. Reproduced with permission from Visser and Rogers (50).

In the synthesis of A-acetyllactosamin from lactose and A-acetylglucosamine with (3-galactosidase (289,290), the addition of 25 vol% of the water-miscible ionic liquid [MMIM][MeS04] to an aqueous system was found to effectively suppress the side reaction of secondary hydrolysis of the desired product. As a result, the product yield was increased from 30 to 60%. Product separation was improved, and the reuse of the enzymatic catalyst became possible. A kinetics investigation showed that the enzyme activity was not influenced by the presence of the ionic liquids. The enzyme was stable under the conditions employed, allowing its repeated use after filtration with a commercially available ultrafiltration membrane. 

When the water-miscible ionic liquid [MMIM][MeS04] was used as a neat medium for the enzymatic transformations, however, poorer performance was observed. For the kinetic resolution of mc-l-phenylethanol by transesterification with vinyl acetate with a set of different lipases dispersed in the pure ionic liquid, it was found that [MMIM][MeS04] was among the poorest media for the enzymes (291). It has been recognized that some water-miscible ionic liquids in the pure form are denaturants (27), but, when they are used in the presence of excess water, their tendency to... 

Water freezes to ice at 0°C expands by about 10% on freezing boils at 100°C vapor pressure at 0°, 20°, 50°, and 100°C are 4.6, 17.5, 92.5, and 760 torr, respectively dielectric constant 80.2 at 20°C and 76.6 at 30°C dipole moment in benzene at 25°C 1.76 critical temperature 373.99°C critical pressure 217.8 atm critical density 0.322 g/cm viscosity 0.01002 poise at 20°C surface tension 73 dynes/cm at 20°C dissolves ionic substances miscible with mineral acids, alkalies low molecular weight alcohols, aldehydes and ketones forms an azeotrope with several solvents immiscible with nonpolar solvents such as carbon tetrachloride, hexane, chloroform, benzene, toluene, and carbon disulfide. 

Tao, W., Pan, D., Liu, Q., Yao, S., Nie, Z., and Han, B., Optical and bioelec-trochemical characterization of water-miscible ionic liquids based composites of multiwalled carbon nanotubes. Electroanalysis, 18,1681-1688,2006. 

Gutowski, K.E., Broker, G.A., Willauer, H.D., Huddelston, R.R, Swatloski, J.D., Holbrey, J.D., Rogers, R.D., Gontrolling the aqueous miscibility of ionic liquids Aqueous biphasic systems of water-miscible ionic liquids and waterstructuring salts for recycle, metathesis and separations, /. Am. Chem. Soc., 125, 6632-6633, 2003. 

The miscibility of ionic liquids with water varies widely and unpredictably. [BMIm][BF4] and [BMIm][MeS04] are water-miscible, but [BMIm][PF6] and... 

The enzyme may be dissolved in a mixed aqueous-ionic liquid medium, which may be mono- or biphasic or it could be suspended or dissolved in an ionic liquid, with little or no water present. Alternatively, whole cells could be suspended in an ionic liquid, in the presence or absence of a water phase. Mixed aqueous-organic media are often used in biotransformations to increase the solubility of hydrophobic reactants and products. Similarly, mixed aqueous-ionic liquid media have been used for a variety of biotransformations, but in most cases there is no clear advantage over water-miscible organic solvents such as tert-butanol. 

The tolerance of lipases for anhydrous ionic liquid media discussed above was not universal. CaLB [82] was inactive in a range of ionic liquids that contained [MeS04], [N03], [AcO] or [lactate] anions [82]. All of these ionic liquids are water-miscible, and it is significant that the enzyme dissolves in these media because... 

Hong ES, Kwon OY, Ryu K (2008) Strong substrate-stabilizing effect of a water-miscible ionic liquid [BMIM][BF4] in the catalysis of horseradish peroxidase. Biotechnol Lett 30 529-533... 

More drastic changes in the CMC and N are observed when additives are present in the micelle-forming surfactant - water systems. The addition of ionic species (i.e. electrolytes) usually results in an increase in the aggregation number and a reduction in the CMC. Table III (and Table II) present some data which illustrate this effect. Depending upon the concentration, the presence of water miscible organic molecules can either enhance or inhibit micelle formation. 

In order to investigate the efficiency and selectivity of the enzymahc acylation of various polyhydroxylated natural compounds in ionic liquid media the transesterification of sahcin, hehcin, escuHn, and naringin, as well as sUybin catalyzed by immobilized CALB was investigated using vinyl butyrate as the acyl donor. Two ionic Hquids were used, the water-miscible 1-butyl-3-methyHmidazolium tetrafluo-roborate ([bmim]BF4) and l-butyl-3-methylimidazoHum hexafluorophosphate ([bmimJPFs), in which the solubility of water is hmited. 

Water-miscible liquids include polyethylene glycols (PEG) and non-ionic surfactants, such as the poly-sorbates. Low molecular weight grades of PEG (e.g., PEG 400) are used most commonly as they remain liquid at ambient temperatures. Small amounts (up to 5-10%) of other water-miscible liquids, such as propylene glycol, ethanol, and glycerin, can also be used. 

Schrekker H S, Stracke M P, Schrekker C M L, et al. Ether-functionalized imidazolium hexafluorophosphate ionic liquids for improved water miscibilities. Ind. Eng. Chem. Res. 2007. 46, 7389-7392. 

Water miscibility of a solvent is important for work-up considerations. Some solvents have such low water solubility that ready phase separations occur when reactions in this solvent are washed with inorganic aqueous solutions to remove impurities. The result is minimal loss of the product to the spent aqueous phase. With water-soluble solvents, e.g., THF and acetonitrile, the presence of either highly lipophilic or ionic compounds may allow for good phase separation during aqueous extractions. 

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