11.3.1.1 IL Polarity

One of the key parameters in the biocompatibility of ILs with enzymes is their polar character, which is not to be confused with the non-water miscibility of some of them (Fig. 11.5).

The subject of ionic liquid polarity has been addressed using a variety of methodologies, including measuring the absorption maximum of a solvatochromic dye, eg. Nile Red or Reichardt’s dye, using a fluorescent probe, measuring the keto — enol equilibria (which are known to depend on the polarity of the medium), microwave dielectric spectroscopy measurements, etc. [79—82]. As a rule, the polar character of anions decreases as a function of the size/delocalization of the negative charge (e. g. [Cl] > [NO2] > [NO3] > [BF4] > [NTf2] > [PF6]), while, in the case of cations, polarity is mainly determined by the length of the alkyl chain groups. Simple chemical reasoning predicts that a polar medium will dissolve polar compounds, such as carbohydrates. By this measure, the ionic liquid [BMIM][BF4], which is hydrophilic, and [BMIM][PF6], which is hydrophobic, fail the polarity test, because both dissolve less than 0.5 g/L of glucose at room temperature [68]. The concept of solvent polarity with ILs is too elusive to serve as a basis to predict, for example, solubility or reaction rates. There are, moreover, indications that solvent-solute interactions of ionic liquids obey a dual interaction model (i. e. ionic liquids behave as non-polar solvents with non-polar solutes but display a polar character with polar solutes), even to the extent that ionic liquids should be regarded as nanostructured materials [8]. However, the IL polarity-enzyme activity correlation has not been established for many enzymatic reactions performed in ILs [15, 19, 83].

Yang and Pan [9] suggested that enzyme activity may be related more to the viscosity and less to the polarity of ionic liquids. Reaction rates have usually been compared in different ionic liquids when the same amount of water is present in the reaction system (e. g. 2 % v/v of water). Therefore, the higher reaction rates in more polar ionic liquids [25, 84] can be explained by the effect of viscosity. Under such conditions, the solvent of higher polarity would leave less water associated with the enzyme and more water remaining in the solvent: the former would result in a lower reaction rate, while the latter would lead to a reduction in the viscosity of the ionic liquid and, in turn, to an improvement in the mobility of the protein molecule.