172
L. Brown et al.
7.4 Ionic Liquids
Ionic liquids have some distinct advantages when compared to organic solvents [9c,
27]. Although they are salts, they have low melting points (conventionally < 100 °C),
or in some cases even below room temperature. They have interesting characteristics
in terms of solubility; they behave either like polar solvents (with the ability to dissolve a huge range of organic compounds, polymers, inorganic molecules, and salts)
[28] or like nonpolar solvents (dissolving molecules, such as hexane and benzene)
[10b]. Because ionic liquids are salts, most of them have negligible vapor pressure
[29] resulting in their non-flammability, hence ionic liquids were coined as “green
solvents”. However, caution must be applied here, as ionic liquids are not intrinsically
green, and some may be either toxic or non-biodegradable, or both [30].
Currently, ionic liquids are used in different areas of chemistry and chemical
engineering and in a number of industrial processes mainly due to their “designer”
properties. Ionic liquids can be tuned by the choice of the anion and cation allowing
them to be optimized for a particular application. An example of design is to vary
the structure of an ionic liquid such that it phase-separates from the compound of
interest, making product isolation straightforward [31]. Another approach is to make
their building blocks (cation or the anion, or both) either acidic or basic (either Lewis
or Brønsted) [32]. In this way, the ionic liquid–liquid interactions and distribution
ratios can be modified and tuned to a particular end use [33].
A serious problem of ionic liquids is their high viscosities, especially for their
application in chromatography [34]. However, this disadvantage can be overcome
by specially designing and constructing CCC machines to eliminate pressure bottlenecks. Examples of the machinery suitable for ionic liquid–liquid chromatography
(ILLC) from AECS-QuikPrep Ltd. (Fig. 7.3) can be found elsewhere [15, 26]. The
aim in ILLC research is to develop a generic methodology for separation of practically all soluble organic and inorganic target compounds. The techniques used are
based on the principles of conventional HPLLC, CCC, CPC, HPCCC, or HPCPC
[35]. Given that industry uses several hundred organic solvents, and it is estimated
that there are more than one million simple ionic liquids attainable, the permutations
of possible mobile and stationary phases are immense [36]. In contrast, conventional
LLC only uses about ten different molecular solvents, which are mixed in various
proportions to produce biphasic systems.
7.5 Ionic Liquid–Liquid Chromatography (ILLC)
For ILLC, with a J-type centrifuge, the instrument that is applied must be able to
withstand high working pressures (minimum 50 bar) for extended periods of time.
For this reason, the coils are made from either stainless steel or, in the case of highly
L. Brown et al.
7.4 Ionic Liquids
Ionic liquids have some distinct advantages when compared to organic solvents [9c,
27]. Although they are salts, they have low melting points (conventionally < 100 °C),
or in some cases even below room temperature. They have interesting characteristics
in terms of solubility; they behave either like polar solvents (with the ability to dissolve a huge range of organic compounds, polymers, inorganic molecules, and salts)
[28] or like nonpolar solvents (dissolving molecules, such as hexane and benzene)
[10b]. Because ionic liquids are salts, most of them have negligible vapor pressure
[29] resulting in their non-flammability, hence ionic liquids were coined as “green
solvents”. However, caution must be applied here, as ionic liquids are not intrinsically
green, and some may be either toxic or non-biodegradable, or both [30].
Currently, ionic liquids are used in different areas of chemistry and chemical
engineering and in a number of industrial processes mainly due to their “designer”
properties. Ionic liquids can be tuned by the choice of the anion and cation allowing
them to be optimized for a particular application. An example of design is to vary
the structure of an ionic liquid such that it phase-separates from the compound of
interest, making product isolation straightforward [31]. Another approach is to make
their building blocks (cation or the anion, or both) either acidic or basic (either Lewis
or Brønsted) [32]. In this way, the ionic liquid–liquid interactions and distribution
ratios can be modified and tuned to a particular end use [33].
A serious problem of ionic liquids is their high viscosities, especially for their
application in chromatography [34]. However, this disadvantage can be overcome
by specially designing and constructing CCC machines to eliminate pressure bottlenecks. Examples of the machinery suitable for ionic liquid–liquid chromatography
(ILLC) from AECS-QuikPrep Ltd. (Fig. 7.3) can be found elsewhere [15, 26]. The
aim in ILLC research is to develop a generic methodology for separation of practically all soluble organic and inorganic target compounds. The techniques used are
based on the principles of conventional HPLLC, CCC, CPC, HPCCC, or HPCPC
[35]. Given that industry uses several hundred organic solvents, and it is estimated
that there are more than one million simple ionic liquids attainable, the permutations
of possible mobile and stationary phases are immense [36]. In contrast, conventional
LLC only uses about ten different molecular solvents, which are mixed in various
proportions to produce biphasic systems.
7.5 Ionic Liquid–Liquid Chromatography (ILLC)
For ILLC, with a J-type centrifuge, the instrument that is applied must be able to
withstand high working pressures (minimum 50 bar) for extended periods of time.
For this reason, the coils are made from either stainless steel or, in the case of highly