Supercritical Fluids

It is now 170 years since Baron Charles Cagniard de la Tour discovered that, above a certain temperature, single substances do not condense or evaporate, but exist only as fluids. In the following decades the 'critical point' was characterized, with its parameters: the critical temperature and pressure. In recent years fluids have been widely exploited at conditions above, but not too far removed from, their critical temperatures and pressures. The term 'supercritical fluids' has been coined to describe these media. Their value lies in the fact that they can have properties intermediate between those we associate with gases and liquids, and also that the properties can be controlled by pressure as well as temperature. Consequently, supercritical fluids can often provide optimum conditions for both experiments and processes. Equally important, especially as regulations become tougher, is that supercritical fluids offer environmental advantages. This is mainly because carbon dioxide and water are available as solvents. The disadvantages of supercritical fluids are that high pressures and sometimes temperatures are involved, and, in the case of water, there are corrosion problems. As the technology to overcome them is available, these disadvantages become cost and convenience factors to weigh against potential advantages. Consequently, supercritical fluids are being exploited in specialized areas. Amongst these is supercritical fluid extraction (SFE), on both an industrial and analytical scale.

Substances used as supercritical fluids include hydrocarbons, such as propane and ethene, water and ammonia, fluorinated hydrocarbons and even xenon. However, one compound, carbon dioxide, has so far been the most widely used in extraction, because of its convenient critical temperature, cheapness, chemical stability, non-flammability, stability in radioactive applications and non-toxicity. Large amounts of carbon dioxide released accidentally could constitute a working hazard, given its tendency to blanket the ground, but hazard detectors are available. It is an environmentally friendly substitute for other organic solvents. The carbon dioxide that is used is obtained in large quantities as a by-product of fermentation, combustion and ammonia synthesis and would be released into the atmosphere sooner rather than later, if it were not used as a supercritical fluid. Its polar character as a solvent is intermediate between a truly non-polar solvent such as hexane and weakly polar solvents. Because the molecule is non-polar it is often classified as a non-polar solvent, but it has some limited affinity with polar solutes because of its large molecular quadrupole. It has a particular affinity for fluorinated compounds and is useful for working with fluorinated metal complexes and fluoropolymers.

To increase the affinity of carbon dioxide to a variety of solutes, substances are added as 'modifiers' or 'entrainers'. The characteristics they impart include increased or decreased polarity, aromaticity, chiral-ity, and the ability to further complex metal-organic compounds. For example, methanol is added to increase polarity, aliphatic hydrocarbons to decrease it, toluene to impart aromaticity, [R]-2-butanol to add chirality, and tributyl phosphate to enhance the sol-vation of metal complexes. They are often added in 5% or 10% amounts by volume, but sometimes much more, say 50%. They can have significant effects when added in small quantities and in these cases it may be the effect on surface processes rather than solvent character which is important. For example, the modifier may be effective in extraction by adsorbing onto surface sites, preventing the readsorption of a compound being extracted.

Because supercritical fluid have properties intermediate between those of gases and liquids to an extent controlled by pressure, optimum conditions can be sought for extraction. The medium can be adjusted for compounds to be sufficiently soluble to be removed, while at the same time the viscosity and diffusion coefficients can be high enough to bring about relatively rapid mass transport. Table 1 shows typical values for the density and viscosity of a gas, supercritical fluid and liquid, taking carbon dioxide as an example. Density is more than half that of the liquid, giving rise to reasonable solubility. Moreover, by controlling the solvent density SFE can, to some extent, be made selective. In contrast, however, the viscosity of a supercritical fluid is much closer to that of a gas than that of a liquid. Thus pressure drop through a supercritical extraction cell is less than for the equivalent liquid process. Diffusion coefficients, also shown in Table 1 for naphthalene in carbon dioxide, are higher in a supercritical fluid than in a liquid. They are approximately inversely related to the fluid density. The advantage shown in the table is seen not to be so great and the main diffusional advantage lies in the fact that typical supercritical solvents have smaller molecules than typical liquid solvents. The diffusion coefficient for naphthalene in

Table 1 The density, p, and viscosity, rç, of carbon dioxide and the diffusion coefficient for naphthalene in carbon dioxide, D, under gas, supercritical and liquid conditions p/kgm~3 rç/pPas D/m2s~1

Supercritical, 313 K, 100 bar 632 17 1.4x10~8

Liquid, 300 K, 500 bar 1029 133 8.7 x10~9

a typical liquid would be closer to 1 x 10~9m2 Thus diffusion coefficients in supercritical fluid experiments and processes are typically an order of magnitude higher than in a liquid medium. This has the advantage of faster transport in the narrow passages typical in an extraction.

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