In supercritical fluids

Jinno, K., Hyphenated Techniques in Supercritical Fluid Chromatography and Extraction, Elsevier, Amsterdam, 1992. 

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

The equilibrium between neutral a and zwitterionic b forms in the case of nicotinic 6 and isonicotinic 7 acids has been studied by Halle in mixtures of DMSO and water (from 0 to 100%) (Scheme 4). The position of the equilibrium is very sensitive to the composition of the solvent and for more than 80% of DMSO, the a form essentially dominates the equilibrium in solution (96CJC613). An analysis of their data shows a perfect linear relationship (r = 1) between the In Kt of the two acids and moderate linear relationships between In Kt and the percentage of DMSO. Johnston has studied the equilibrium 2-hydroxypyridine/2-pyridone in supercritical fluids (propane at 393 K and 1,1-difluoroethane at 403 K) (89JPC4297). The equilibrium constant Kt (pyridone/hydroxypyridine) increases four-fold for a pressure increase of 40 bar in 1,1-difluoroethane. 

T. Hartmann, E. Schwabe, T. Scheper, Enzyme catalysis in supercritical fluids in R. Patel, Stereoselective Biocatalysis, Marcel Dekker, 2000, 799. 

Studies of reversed micelles dispersed in supercritical fluids have shown their ability to solubihze hydrophihc substances, including biomolecules and dyes, opening the door to many new applications [60,61]. In particular, solutions of reversed micelles in liquid and supercritical carbon dioxide have been suggested as novel media for processes generating a minimum amount of waste and with a low energy requirement [62]. 

Eggers, R., Supercritical fluid extraction (SFE) of oilseeds/lipids in natural products, in Supercritical Fluid Technology in Oil and Lipid Chemistry, King, J.W. and List, G.R., Eds., AOCS Press, Champaign, IL. 1996, 35. 

Surfactants and Colloids in Supercritical Fluids Because very few nonvolatile molecules are soluble in CO2, many types of hydrophilic or lipophilic species may be dispersed in the form of polymer latexes (e.g., polystyrene), microemulsions, macroemulsions, and inorganic suspensions of metals and metal oxides (Shah et al., op. cit.). The environmentally benign, nontoxic, and nonflammable fluids water and CO2 are the two most abundant and inexpensive solvents on earth. Fluorocarbon and hydrocarbon-based surfactants have been used to form reverse micelles, water-in-C02... 

Current interest in supercritical fluid extraction as a sample preparation technique for chromatographic analysis is intense, in spite of it receiving very little tion until the mid-1980s. Although neglected by analytical cl Hsts, during the... 

C. Berger, in Supercritical Fluid Chromatography with Packed Columns (K. Anton and C. Berger, eds), M. Dekker, New York, NY (1997), pp. 301-48. 

H.H. Hill and D.A. Atkinson, in Hyphenated Techniques in Supercritical Fluid Chromatography and Extraction (K. Jinno, ed.), Elsevier, Amsterdam (1992), pp. 1-8. 

A.A. Clifford, in Supercritical Fluid Extraction and Chromatography (M.S. Verrall, ed.), John Wiley Sons, Ltd, Chichester (1996), pp. 241-57. 

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