Choice of Supercritical Fluids

Many fluids have been investigated for use in supercritical fluid chromatography (SFC) and extraction (SFE). Ikble 8-1 lists the critical points of temperature and pressure for some of them. 

The physical and chemical properties of a supercritical fluid depend upon temperature and pressure and are of paramount importance when designing a supercritical fluid chromatograph or extractor. Excessively high pressure or temperature require- 

8 Scaling-up of Supercritical Fluid Chromatography to Large-Scale Applications 

The most popular and safe supercritical fluid is undoubtedly carbon dioxide for use in SFC and SFE. Thermally labile materials chromatographed in it are unlikely to be affected because its critical temperature is only slightly above room temperature. The critical pressure of carbon dioxide (73.8 Bar) is not difficult to maintain with modern high performance liquid chromatography (HPLC) solvent delivery systems. 

Carbon dioxide has the additional advantages of being non-flammable, chemically inert, odour free, available in a state of high purity at a low cost and does not present a solvent disposal problem. 

---

Enzymes can express activity in supercritical and near-supercritical fluids, such as carbon dioxide, freons (CHF3), hydrocarbons (ethane, ethylene, propane) or inorganic compounds (SFe, N2O). The choice of supercritical fluids is often... 

The choice of supercritical fluid for use in combination with molecular sieves is thus influenced by the preceding considerations. ... 

These characteristics make the use of supercritical fluids very attractive for membrane preparation, considering such advantages as the reduced risk of structural collapse, complete removal of traces of solvent without additional treatments, easy recovery of solvent and non-solvent, and modularity of the solvent power and diffusivity depending on the temperature and pressure used. The choice of supercritical fluid usually depends on the operating conditions and compatibility with selected materials. Carbon dioxide and its mixtures with ethanol are the most commonly used supercritical fluids. 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Environmental applications of SFE appear to be the most widespread in the literature. A typical example is the comparison of extraction efficiency for 2,3,7,8 -tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from sediment samples using supercritical fluid extraction and five individual mobile phases with Soxhlet extraction was made (101). The mobile phases, carbon dioxide, nitrous oxide, pure and modified with 2% methanol as well as sulfur hexafluoride were examined. Pure nitrous oxide, modified carbon dioxide and modified nitrous oxide systems gave the recoveries in the acceptable range of 80 to 100%. Carbon dioxide and sulfur hexafluoride showed recoveries of less than 50% under identical conditions. Classical Soxhlet recoveries by comparison illustrated the poorest precision with average extraction efficiencies of less than 65%. Mobile phase choice, still as yet a major question in the science of supercritical fluid extraction, seems to be dependent upon several factors polarity of the solute of interest, stearic interactions, as well as those between the matrix and the mobile phase. Physical parameters of the solute of interest, as suggested by King, must also be considered. Presently, the science behind the extraction of analytes of interest from complex matrices is not completely understood. 

In addition to density, diffusivity of the supercritical fluids is higher than that of liquid solvents, and can be easily varied. For typical conditions, diffusivity in supercritical fluids is of the order of lO cm /sec as compared to 10 for gases and 10 for liquids. Typical viscosity of supercritical fluids is of the order of 10 g/cm/sec, similar to that of gases, and about 100-fold lower than that of liquids. High diffusivity and low viscosity provide rapid equilibration of the fluid to the mixture to be extracted, hence extraction can be achieved close to the thermodynamic limits. However, the main extraction benefit of supercritical fluids is their adjustable density that provides adjustable solvent strength. The compounds of choice can be dissolved/extracted in the supercritical fluid at high pressure and then this fluid mixture is carried to another vessel where simple lowering of the pressure... 

Depending upon the choice of temperature and pressure, the behaviour of a supercritical fluid can sometimes looks like a dense gas and sometimes like a liquid. For these reasons, use of supercritical fluids as mobile phases in chromatography presents certain advantages. 

Supercritical water and CO2 are substances that are compatible with various applications and processed materials. However, several other supercritical fluids can be equally used such as methanol or ethanol. The final choice of the fluid depends on the specific application and additional factors such as safety, flammability, phase behavior and solubility at the operating conditions, the price of the fluid, and the related storage and processing costs. Due to this unique property, supercritical water is essentially used to treat toxic wastewater and/ or process forestry and agricultural wastes/residues. Therefore, this chapter will focus only on supercritical CO2. 

The use of supercritical fluids represents a way to replace conventional solvents. Recall that a supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid, (Section 11.4) Water and carbon dioxide are the two most popular choices as su-... 

The use of supercritical fluids represents a way to replace conventional solvents. Recall that a supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid, qqo (Section 11.4) Water and carbon dioxide are the two most popular choices as supercritical fluid solvents. One recently developed industrial process, for example, replaces chlorofluorocarbon solvents with liquid or supercritical CO2 in the production of polytetrafluoroethylene ([CF2CF2] , sold as Teflon ). Though CO2 is a greenhouse gas, no new CO2 need be manufactured for use as a supercritical fluid solvent. 

The use of supercritical fluids became a powerful approach in green chemistry. Both CO and hydrocarbons can serve a supercritical medium, but the preferable choice is the use of the substrate of the catalytic reaction itself as a sc-fluid. 

Watei has an unusually high (374°C) ctitical tempeiatuie owing to its polarity. At supercritical conditions water can dissolve gases such as O2 and nonpolar organic compounds as well as salts. This phenomenon is of interest for oxidation of toxic wastewater (see Waste treatments, hazardous waste). Many of the other more commonly used supercritical fluids are Hsted in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific appHcation, however, is likely to depend on additional factors such as safety, flammabiUty, phase behavior, solubiUty, and expense. 

Supercritical fluid extraction (SFE) is a technique in which a supercritical fluid [formed when the critical temperature Tf) and critical pressure Pf) for the fluid are exceeded simultaneously] is used as an extraction solvent instead of an organic solvent. By far the most common choice of a supercritical fluid is carbon dioxide (CO2) because CO2 has a low critical temperature (re = 31.1 °C), is inexpensive, and is safe." SFE has the advantage of lower viscosity and improved diffusion coefficients relative to traditional organic solvents. Also, if supercritical CO2 is used as the extraction solvent, the solvent (CO2) can easily be removed by bringing the extract to atmospheric pressure. Supercritical CO2 itself is a very nonpolar solvent that may not have broad applicability as an extraction solvent. To overcome this problem, modifiers such as methanol can be used to increase the polarity of the SFE extraction solvent. Another problem associated with SFE using CO2 is the co-extraction of lipids and other nonpolar interferents. To overcome this problem, a combination of SFE with SPE can be used. Stolker et al." provided a review of several SFE/SPE methods described in the literature. 

Sampling methods in SFC are far more restricted than in the case of GC (Section 4.3 and Table 7.5). Not surprisingly, supercritical fluid extraction is an obvious choice. 

---