Near-critical and supercritical fluids

Biocatalysis and Microemulsions in Near-critical and Supercritical Fluids... 

High-pressure processes have been widely applied in the polymer industry. Near-critical and supercritical fluids (SCFs) are in particular used owing to their easily tunable density, which enhances the control of polymer solubility and their good separability from polymer material [1], SCF solvents (e.g. scC02) offer a potential advantage for separation process. The solubility of different polymeric material in SCFs can be systematically varied by changing operating conditions. Several... 

Oag, R.M., PI. King, C.J. Mellor, M.W. George, J. Ke and M. Poliakoff, Probing the Vapor-Liquid Phase Behaviors of Near-Critical and Supercritical Fluids Using a Shear Mode Piezoelectric Sensor, Analytical Chemistry, 75, 479-485 (2003). 

Crooks RM, Bard AJ. Electrochemistry in near-critical and supercritical fluids, nitrogen heterocycles nitrobenzene, and solvated electrons in ammonia at temperatures to 150°C. J Phys Chem 1987 91(5) 1274. 

Cabrera, C. R. and Bard A. J. (1989) Electrochemistry in near-critical and supercritical fluids. Part 8. Methyl viologen, decamethylferrocene, Os (bpy)2+ and ferrocene in acetonitrile and the effect of pressure on diffusion coefficients under supercritical conditions. J. Electroanal. Chem. 273, 147-160. 

Liu, C., Snyder, S. R. and Bard, A. J. (1997) Electrochemistry in near-critical and supercritical fluids. 9. Improved apparatus for water systems (23°C-385°C). The oxidation of hydroquinone and iodide. J. Phys. Chem. 101, 1180-1185. 

Dynamic light scattering has been utilized to directly measure droplet diffusion coefficients in near-critical and supercritical fluids. Blitz et al.f and Fulton et measured the effect of pressure on... 

Another class of extreme environments being used for synthetic applications are near critical and supercritical fluids. A number of reviews have looked at reactions in supercritical water.Brill and coworkers have developed experimental devices in which to perform infrared and Raman... 

Structure of Reverse Micelle and Microemulsion Phases in Near-Critical and Supercritical Fluid as Determined from Dynamic Light-Scattering Studies... 

Fulton JL. Structure and reactions in microemulsions formed in near-critical and supercritical fluids. In Kumar P, ed. Microemulsions Fundamentals and Applied Aspects. New York Marcel Dekker, 1999 629-650. 

Before explaining the thermodynamic framework of the propane deasphalting process, we present several paragraphs from the 1936 paper by Wilson, Keith, and Haylett to highlight what was known fifty years ago about near-critical and supercritical fluid processing. 

The underlying thermodynamic principles for this process are identical to those exhibited for the propane deasphalting process. This process is yet another example of the use of the unique solvent characteristics of near-critical and supercritical fluids developed some forty years ago. 

E. Kiran, Polymer miscibility and kinetics of pressure-induced phase separation in near-critical and supercritical fluids. Chapter 6 in Supercritical Fluids. Fundamentals and Applications, E. Kiran, P. G. Debenedetti and C. J. Peters, Eds., Kluwer Academic Publishers, Dordrecht, The Netherlands (2000). 

Since the discovery of microemulsion phases in supercritical fluids in the mid-1980s [1] and their subsequent characterization [2-16], there has been much interest in exploiting the unusual properties of the supercritical fluid phase in applications of these systems. One such application is as a new type of solvent for chemical reactions. In the following sections, I discuss the properties of these systems for reactions, review the progress so far, and analyze the future potential. As a prelude to these discussions, I begin with a brief overview of what is known about the molecular structure of microemulsions in near-critical and supercritical fluids. The details of the primary and secondary molecular structures of various types of microemulsion phases can dramatically affect the reactivity in these systems. 

Table 1 Critical Parameters of Some Near-Critical and Supercritical Fluids That Have Moderate Critical Points Suitable for Microemulsion Formation...

Light scattering [37,38] was one of the first methods used to derive microemulsion structure in near-critical and supercritical fluids [2,4,39-42]. This technique provided the first direct evidence that microemulsion droplets existed in a supercritical fluid. 

After the initial DLS studies were complete, it became apparent that the very strong interdroplet attractive interactions in near-critical and supercritical fluids limited the standard DLS technique to systems of higher dilution or to high fluid densities. Thus, small-angle scattering techniques were later used to better resolve the full dimensional scale range of these microemulsions over a wider range of conditions. 

The high difflisivities and the low viscosities of supercritical and near-critical fluids are well established. Studies have shown that the favorable mass transport properties of dense gases are retained in microemulsions formed in these systems, and multiple benefits are expected for chemical reactions. The diffusion rates of microemulsion droplets in near-critical and supercritical fluids are up to 10 times greater than in liquids. Conventional liquid microemulsion systems often contain nanometer-sized droplets whose diffusion rates are 10-100 times lower than those of molecularly dispersed species. The higher diffusion rates of these droplets in near-critical and supercritical fluids offset the transport limiting effect due to large droplet size. 

Few results have been published in this rich area, but as the advantages and properties of these systems become known we expect to see more work in this area. A fundamental study of reaction mechanisms in a near-critical microemulsion was carried out by Zhang et al. [43]. Further, microemulsions in near-critical and supercritical fluids have been used in particle synthesis, hydrogenation, and polymerization reactions [41,67,68]. These diverse areas are discussed in this section. 

Microemulsions tremendously expand the potential applications of supercritical fluids as reaction media for chemical reactions. By themselves, near-critical and supercritical fluids are much weaker solvents than the typical organic liquid. However, microemulsions create a highly polar region that is capable of solvating polar catalysts, reactants, or products. The unique aspects of the near-critical or supercritical continuous phase offer many advantages over their liquid-phase analogs. 

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