Solvent properties of supercritical fluids.
A crucial aspect of carrying out reactions in supercritical carbon dioxide is solubility. Pure supercritical carbon dioxide is a relatively non-polar solvent, but has some limited affinity with polar molecules due to its large molecular quadrupole. Modifiers can often be added (e.g. MeOH) to improve the solubility of polar molecules. Alternatively, when reactions involve more than one reagent, less polar reagents can in effect act as modifiers enhancing the solubility of more polar reagents avoiding the need to resort to additional co-solvents.
Another approach widely used to enhance solubility in supercritical carbon dioxide is to introduce fluorinated substitutents, often onto a ligand or counterion for orgamometallic catalysis. However, expense of reagents can be a limiting factor, necessitating recycling.
There are also a number of practical advantages associated with the use of supercritical carbon dioxide as a solvent. Product isolation to total dryness is achieved by simple evaporation. This could prove to be particularly useful in the final steps of pharmaceutical syntheses where even trace amounts of solvent residues are considered problematic. There are also two very useful complementary routes to particle formation with SCFs and supercritical carbon dioxide in particular, rapid expansion of supercritical solutions (RESS) and supercritical anti-solvent precipitation (SASP).
One of the main differences between supercritical fluids and conventional solvents is their compressibility. Conventional solvents in the liquid phase require very large pressures to change the density, whereas for supercritical fluids, very significant changes in density and hence solvating properties can be achieved by comparatively small pressure and/or temperature changes, particularly around the critical point (Figure 2). This provides an infinite range of solvent properties, which can in some cases, be tuned to significantly affect the outcome of a reaction. Note that in general, supercritical fluids are considerably less dense than conventional solvents. This can lead to problems of solubility in some cases, but also means they are considerably less viscous than conventional solvents which leads to a significantly greater diffusivity. This can result in significantly faster reaction rates if diffusion is rate limiting.
Figure 2. Variation of carbon dioxide density with pressure.
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