Exotic ionic liquids

The anion in an ionic liquid can be varied, too. Many ionic liquids contain relatively simple inorganic anions, such as nitrate (NO3 ), tetrafluoroborate (BF4 ), or hexafluorophosphate (PFg ). Anions that are more exotic also are possible, such as the two shown below. Varying the anion provides another way of tuning the properties of an ionic liquid to match a desired application. 

As discussed below, ionic liquids often behave comparably to conventional polar organic solvents [6, 8, 10]. But the physics underlying solvation are entirely different. As noted above, ILs are characterized by considerable structural and dynamic inhomogeneity, and even simple concepts, such as the dipole moment, cannot be productively applied. We are therefore in the unusual position of needing to explain how an exotic microscopic environment produces conventional macroscopic behavior. To this end, we will review empirical characterizations of the ionic liquid environment, and then turn our attention to the underlying physics of solute-solvent interactions. 

Considering the many different applications of NMR spectroscopy in ionic liquids, it has become clear that this method is ready for the whole range of studies related to ionic liquid chemistry. These days, NMR in ILs is not exotic anymore but a routine technique for everyday chemistry. Virtually all standard and advanced NMR experiments can be run in ionic liquids very much like in ordinary conventional solvents. 

This graph gives a selection of 14 (out of approx. 360) usual solvents above the basis line and 7 exotic solvents (ionic liquids included) below. The 14 compounds include (from left to the right increasing solvent polarity) apolar, aprotic solvents (such as TMS, cyclohexene, or benzene), bipolar solvents (such as acetone, DMF, or DMSO), and eventually bipolar, protic solvents (cyclohexanol, ethanol, phenol, 2,2,2-trifluoroethanol). Using the Ej values numerous solvent-dependent processes may be correlated by far better than with the physical values of the solvents alone. This is true because the Ej values include specific cross interactions as well. 

This chapter explores the application of biocatalytic polymerization in exotic solvents. These solvents are often termed unconventional , in that they would not generally be considered as a polymerization media. However, their use over the previous decade has dramatically increased due to the international push for cleaner, greener reaction pathways in an effort to reduce volatile organic compounds (VOCs). The first solvents to be discussed (and by far the most fully investigated in the literature) are supercritical fluids. Within this field, supercritical C02 has been the most highly reported solvent. The second solvent class is ionic liquids. These have become increasingly popular over the last five years. Biphasic solvents will then be described and their application to biocatalytic polymerization. This section will be limited to biphasic solvents that are more unusual and, apart from a brief mention, will not encompass the broad field of emulsion polymerization in water. Finally, the use of fluorous solvents will be described. In all cases, the physical properties of the solvent imparts interesting,... 

Unfortunately, most polymers are insoluble in supercritical C02, and hence extraction from the ionic liquid by this method is difficult. However, if C02-soluble polymers were synthesized (for example, fluoropolymers and polysi-loxanes), then this method has the potential to be a very useful approach. Moreover, supercritical fluid-swollen ionic liquids offer a new solvent system that combines the viscosity-lowering properties of the supercritical fluid with the good solubilizing properties of the ionic liquid and may be a hybrid exotic solvent of the future. 

The use of exotic media for biocatalytic polymerization has ranged from the extensive, and often fundamental, studies using supercritical C02, to more exploratory and recent reports for media such as ionic liquids and fhroro-solvents. In all cases, however, intriguing results have lead to further investigation. As increasing pressure is exerted upon scientists in both academia and industry alike to develop and commercialize greener reaction systems, it is expected that biocatalysis and reactions in these and other exotic solvents will continue to be of considerable interest into the future, as they have been over the previous decades. 

These solvents are of significant use to the inorganic chemist, but also available are more exotic solvents such as liquid NH3, liquid SO2, H2SO4, Brp3 and ionic liquids such as [pyBu][AlCl4], 9.4 (see Section 9.12). 

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