7 Ionic Liquid–Liquid Chromatography: A Novel Separation Method
183
7.8 Solvent Engineering and Ionic Liquid–Liquid
Chromatography
The chemical structure of ionic liquids can be designed to give a particular set of
physicochemical properties, which can be optimized for a particular end use [9c].
Factors, such as viscosity, hydrophobicity, hydrophilicity, density, acidity or basicity, surface or interfacial tensions, and corrosivity, can be adjusted, which permits
considerable control over phase behavior and therefore separation performance. The
distribution ratios of compounds between an ionic liquid-containing phase and a second immiscible phase (which may be either water, an organic solvent, or other ionic
liquid) can be manipulated by altering the chemical makeup of the ionic liquid. For
example, if a solute, dissolved in a two-phase hexane-ionic liquid solvent system has
a solubility that is too low in the ionic liquid phase, then the structure of the ionic
liquid cation or anion can be changed to increase the solubility in the ionic phase.
The use of longer alkyl chains attached to the charged parts of the anion or cation can
be used to improve solubility in the ionic phase. This great flexibility makes ILLC
a general-purpose separation technology, useful for the separations of a very wide
range of compounds including compounds that were previously thought to be too
difficult to separate. We call this approach solvent engineering, a complement to the
chemical engineering required in the design of ILLC instrumentation.
7.9 Conclusions
The use of molecular solvents as biphasic eluents has been shown to be excellent
for a huge variety of applications in laboratory-scale preparations of up to a kilo or
multi-kilo targets. However, the complexity of molecular-solvent biphasic eluents
with three, four, or on occasion five solvents becomes less favored in larger-scale
processes. For these larger-scale process applications of 10s of kilos to many tons per
annum, the unique properties of ionic liquids can provide many advantages, which
we have discussed. This can make the additional effort of developing ionic liquid
eluents well worthwhile. The use of ionic liquids in countercurrent chromatography
instruments was initially found to be problematic due to high backpressures and
consequently low mobile-phase flow rates. With careful design of the fluid flow
paths and by eliminating pressure bottlenecks, this problem has now been solved.
Currently, there are very few papers in the literature describing the use of solvent
systems containing a high percentage of ionic liquids in CCC or LLC. Control over
the structure and design of ionic liquids and ionic liquid-containing solvent systems
(solvent engineering) has enabled us to select or alter solute distribution ratios of
solutes that are to be separated.
The testing of these new ionic liquid solvent systems was performed in a customdesigned ILLC instrument capable of operating with higher viscosity solvent systems and at considerably higher backpressures (70 bar) than are usually found in
183
7.8 Solvent Engineering and Ionic Liquid–Liquid
Chromatography
The chemical structure of ionic liquids can be designed to give a particular set of
physicochemical properties, which can be optimized for a particular end use [9c].
Factors, such as viscosity, hydrophobicity, hydrophilicity, density, acidity or basicity, surface or interfacial tensions, and corrosivity, can be adjusted, which permits
considerable control over phase behavior and therefore separation performance. The
distribution ratios of compounds between an ionic liquid-containing phase and a second immiscible phase (which may be either water, an organic solvent, or other ionic
liquid) can be manipulated by altering the chemical makeup of the ionic liquid. For
example, if a solute, dissolved in a two-phase hexane-ionic liquid solvent system has
a solubility that is too low in the ionic liquid phase, then the structure of the ionic
liquid cation or anion can be changed to increase the solubility in the ionic phase.
The use of longer alkyl chains attached to the charged parts of the anion or cation can
be used to improve solubility in the ionic phase. This great flexibility makes ILLC
a general-purpose separation technology, useful for the separations of a very wide
range of compounds including compounds that were previously thought to be too
difficult to separate. We call this approach solvent engineering, a complement to the
chemical engineering required in the design of ILLC instrumentation.
7.9 Conclusions
The use of molecular solvents as biphasic eluents has been shown to be excellent
for a huge variety of applications in laboratory-scale preparations of up to a kilo or
multi-kilo targets. However, the complexity of molecular-solvent biphasic eluents
with three, four, or on occasion five solvents becomes less favored in larger-scale
processes. For these larger-scale process applications of 10s of kilos to many tons per
annum, the unique properties of ionic liquids can provide many advantages, which
we have discussed. This can make the additional effort of developing ionic liquid
eluents well worthwhile. The use of ionic liquids in countercurrent chromatography
instruments was initially found to be problematic due to high backpressures and
consequently low mobile-phase flow rates. With careful design of the fluid flow
paths and by eliminating pressure bottlenecks, this problem has now been solved.
Currently, there are very few papers in the literature describing the use of solvent
systems containing a high percentage of ionic liquids in CCC or LLC. Control over
the structure and design of ionic liquids and ionic liquid-containing solvent systems
(solvent engineering) has enabled us to select or alter solute distribution ratios of
solutes that are to be separated.
The testing of these new ionic liquid solvent systems was performed in a customdesigned ILLC instrument capable of operating with higher viscosity solvent systems and at considerably higher backpressures (70 bar) than are usually found in