Solvent polarity, ionic liquids partitioning

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

Another nonreactive route to the characterization of solvent polarity is the study of the optical absorption and emission spectra of chromophores [188]. These spectra are sensitive to the molecular environment, and because different solvatochromic probes may have different capacities for specific interactions, it is possible to characterize the solvent environment in detail and to construct LFERs analogous to those described above. Studies of the spectra of solvatochromic probes in ionic liquids have in general been consistent with the results of partitioning studies described above [8-10, 69, 70, 198-200], though we will discuss one observed anomaly below [198]. 

When ILs are used in catalytic processes, reaction products, byproducts and residual reactants need to be separated from the solvent and the catalyst, which are recycled and re-used. The product solubility can be difficult to predict because distribution of products can depend on reactant conversion, residence time, and operating conditions. Consecutive reactions can provide byproducts with different polarities or functionalities. The existence of these byproducts can modify the partitioning of the desired product in the organic effluent/ionic liquid system (see also Section 5.2.1.3). 

Uptake of water and polar solvents by the column From mixed mobile phases, polar solvent(s) are preferentially adsorbed on the surface of polar adsorbents, sometimes giving rise to multilayer solvent adsorption on the adsorbent support. In such a case, the retention is contributed to by a liquid-liquid partition mechanism between the adsorbed liquid layer and the bulk mobile phase, in addition to the adsorption. Such a mixed-mode mechanism can be intentionally utilized for separation of strongly polar or even ionic compounds. 

Liquid-liquid extraction is a form of solvent extraction in which the solvents produce two immiscible liquid phases. The separation of analytes from the liquid matrix occurs when the analyte partitions from the matrix-liquid phase to the other. The partition of analytes between the two phases is based on their solubilities when equilibrium is reached. Usually, one of the phases is aqueous and the other is an immiscible organic solvent. Large, bulky hydrophobic molecules like to partition into an organic solvent, while polar and/or ionic compounds prefer the aqueous phase. 

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