Supercritical fluids early studies

Although several other early studies were conducted using Supercritical Fluids (SFs), also termed at that time as dense gases , this field did not receive the attention it deserved during the first half century after its discovery. Even then, the small number of investigators dedicated to further explore the scientific and technological potential of supercritical fluids concentrated then efforts on industrial rather than analytical applications (9, 10). 

Vibrational spectroscopy, too, has been used to study supercritical fluid systems. Buback reviewed (59) this area however, much of his discussions are on fluid systems that are well removed from ambient conditions or difficult to handle easily (e.g., H20, HC1). In an early report, Hyatt (21) used IR absorbance spectroscopy to determine the influence of several solvent systems, including COz, on the vibrational frequencies ( ) of solute molecules. Specifically, he studied the vc=o of acetone and cyclohexanone and vs.H of pyrrole. The goal of this work was to determine the suitability of supercritical fluids as reaction solvent. Hyatt concluded that the ketones experienced an environment similar to nonpolar hydrocarbons in COz and that there were no differences between liquid and supercritical CO2. In contrast, the pyrrole studies indicated that the solvent strength of CO2 was between ether and ethyl acetate. This apparent anomalous result was a manifestation of the, albeit weak, degree of pyrrole hydrogen bonding to CO2. 

Since the early days of SFC, there always has been a desire to extend the useful range of the technique to more polar molecules. A similar type of desire exists in SFE. The hope for achieving efficient extractions of polar molecules from polar as well as non-polar substrates can only be realized with the use of more polar primary supercritical fluids or by the use of modifiers. Many of the more primary supercritical fluids that exists namely, ammonia or water, are not effectively usable in the analytical laboratory due to instrumental as well as safety restrictions, therefore, the need to do more research on the use of modifiers in SFE is greatly necessitated. Based upon the limited study that was done within the scope of this chapter, a few conclusions can be drawn. These conclusions are summarized in Figure 16. 

Most of the early work involving microemulsions in supercritical fluids utilized the supercritical alkanes, ethane and propane, with the surfactant AOT. Table 1 gives a summary of the surfactant systems that have been studied in supercritical hydrocarbon solvents. More recently, there has been some success with the formation of... 

Supercritical fluids were identified in early 1989 as a possible CFC replacement for certain niche applications at Honeywell Space Systems. At that time there was very limited data on either the application of supercritical fluids for precision cleaning or compatibility studies with materials common to the aerospace industry. 

The first reported observation of the occurrence of a supercritical phase was made by Baron Cagniard de la Tour in 1822 [3]. He noted visually that the gas/liquid boundary disappeared when heating each of them in a closed glass container increased the temperature of certain materials. From these early experiments, the critical point of a substance was first discovered. The first workers to demonstrate the solvating power of supercritical fluids for solids were Hannay and Hogarth in 1879 [4]. They studied the solubility of cobalt(II) chloride, iron(III) chloride, potassium bromide, and... 

As is by now evident to the reader, the phenomenon of solubility in supercritical fluids is not new. Since 1879 (or 1861, if we include the high-pressure, near-critical liquid carbon dioxide studies of Gore), solubility, phase, and spectroscopic studies have been performed on a large number of solute-SCF mixtures. They were made for their inherent scientific and technical interest and value. And they received a resurgence of interest with the work of Diepen, Scheffer, and coworkers in the late 1940s and early 1950s. 

Several other chemists were active in the field of supercritical fluid reactions at the beginning of the twentieth century. For example, Briner studied the decomposition and reactivity of supercritical fluids such as scNO and scCO (e.g., eq 1.1-6) [119-122]. He also investigated the system N2-H2 and reported that N2 and H2 do not combine at room temperature and 900 bar [122]. At the Chemical Institute of the University of Berlin, Arthur Stabler studied the reactions of alkyl halides such as chloroethane with SCNH3 (eq 1.1-7) [123]. One of the earliest attempts to utilize SCFs for the selective synAesis of low molecular weight organic products dates to the early 1940s, when Patat at the University of Innsbruck studied the hydrolysis of aniline under supercritical conditions (eq 1.1-8) [124]. 

Although currently under extensive study, supercritical fluids are the only example of an alternative solvent under investigation for Green Chemistry. Hatton (67) at MIT has reported early results of the use of amphiphilic star polymers as solvents in synthesis. In addition to their innocuous nature, due in part to their size, they also have the advantage of minimizing the need for intensive separations that can require additional solvent use. 

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