Solvent Properties of Supercritical Fluids

Supercritical fluids (SCFs) are solvent systems sufficiently different to offer interesting new opportunities for catalytic reactions. Our research groups at Nottingham University and MPI fur Kohlenforschung have performed a wide spectrum of reactions using these fluids [1-6], This chapter is not intended to be comprehensive, but rather it aims to give a general introduction to the variety of work which can be performed by the use of these solvents. 

An SCF can be defined as a substance heated above its critical temperatiue (Tc), and which is also compressed above its critical pressure (pc) (see Fig. 12.1) [13]. At the critical point, the fluid phase boundary between liquid and gaseous phases disappears, and the properties of the new single supercriticar phase are best described as a combination of those of the liquid and gaseous phase. 

---

The unusual solvent properties of supercritical fluids (SCFs) have been known for over a century (1). Just above the critical temperature, Tc, forces of molecular attraction are balanced by kinetic energy and fluid properties, including solvent power, exhibit a substantial pressure dependence. Many complex organic materials are soluble at moderate pressures (80 to 100 atmospheres) and SCF solvent power increases dramatically when the pressure is increased to 300 atmospheres. The pressure responsive range of solvent properties thus attainable provides a tool for investigating the fundamental nature of molecular interactions and is also being exploited in important areas of applied research (2,3). 

The unique solvent properties of supercritical fluids hold many opportunities for new and innovative applications in catalysis. Heterogeneous and homogeneous... 

The solvent properties of supercritical fluids are particularly interesting. Liquid water, for example, has a dielectric constant of around 80, whereas the value for supercritical water is around 2. At this low value it no longer acts as a polar solvent and many organic compounds can be dissolved in and crystallized from it. The potential exploitation of supercritical fluids, especially H2O and CO2, in crystallization processes is discussed in section 7.1.4. 

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

A supercritical fluid is a state of matter achieved by high temperature and extremely high pressure, exceeding the so-called critical temperature and pressure for that substance. The solvent properties of a supercritical fluid are much improved over the normal solvent properties of that fluid. 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

As its name suggests, supercritical fluid extraction (SEE) relies on the solubilizing properties of supercritical fluids. The lower viscosities and higher diffusion rates of supercritical fluids, when compared with those of liquids, make them ideal for the extraction of diffusion-controlled matrices, such as plant tissues. Advantages of the method are lower solvent consumption, controllable selectivity, and less thermal or chemical degradation than methods such as Soxhlet extraction. Numerous applications in the extraction of natural products have been reported, with supercritical carbon dioxide being the most widely used extraction solvent. However, to allow for the extraction of polar compounds such as flavonoids, polar solvents (like methanol) have to be added as modifiers. There is consequently a substantial reduction in selectivity. This explains why there are relatively few applications to polyphenols in the literature. Even with pressures of up to 689 bar and 20% modifier (usually methanol) in the extraction fluid, yields of polyphenolic compounds remain low, as shown for marigold Calendula officinalis, Asteraceae) and chamomile Matricaria recutita, Asteraceae). " ... 

The ability to fine-tune the solubility properties of supercritical fluids makes them useful in a variety of applications. This property enhances the solvent selectivity and... 

Carbon dioxide has a conveniently low critical point (31 °C, 7.39 MPa), and supercritical CO2 has become the most widely used fluid where supercritical solvent properties are required, as it is also inexpensive and nontoxic. The solvent powers of supercritical fluids generally increase with increasing density, which can be regulated at will by varying the pressure. The absence of a gas-liquid interface and associated surface tension in a supercritical fluid enables the fluid to penetrate porous solids freely, and also to... 

In a dynamic extraction system, the supercritical fluid is pumped only once through the container with the sample to the receiver. In the receiver, the liquid is vaporized, leaving concentrated analytes that are then dissolved in a small volume of the solvent. Such extracts are analyzed to determine selected analytes. This manner of extraction is effective if the analytes are well soluble in the solvent and the sample matrix is penetrable. Apart from the aforementioned possibility of fractionated extraction, SFE has many other advantages accruing from the special properties of supercritical fluids ... 

The wide variety of possible solvent-solute interactions requires that any scale used to quantify solvent properties will be complex. Unfortunately, no universally accepted scale of solvating power has been devised. It does not seem reasonable to develop an entirely new scale for supercritical fluid solvents, especially since it is desirable to compare the solvent behavior of supercritical fluids with that of liquid solvents. 

The physical properties of supercritical fluids tend to lie between those of gases and liquids. The increased density relative to a gas, and the decreased viscosity relative to a liquid, allow supercritical fluids to be used as excellent solvents in many laboratory and industrial applications (19-25). Also, some notable solvation peculiarities of supercritical fluids have been discovered. For example, supercritical water can dissolve nonpolar oils because the dielectric constant of supercritical water decreases drastically near the critical point (26). 

The general properties of supercritical fluids make them an attractive alternative to liquid solvents in column operations where transport effects come into play. If supercritical CO2 is employed as the solvent, this advantage is further supplemented by the non-flammable, non-toxic nature of the fluid, and the relative ease of solvent recovery. Supercritical solvents also offer the potential to greatly enhance thermally driven separations through dramatic changes in component solubility, adsorptive characteristics, and thermal conductivity near the critical region. 

Supercritical fluid extraction (SFE) utilizes the properties of supercritical fluids for extraction of analytes from solid samples. A supercritical fluid (SCF) is a substance above its critical temperature and pressure, when it is between the typical gas and liquid state. Low viscosity and near-zero surface tension and heat of vaporization allow SCFs to penetrate into solids more rapidly than liquid solvents, which leads to more favorable mass transfer. The density of an SCF is close to the liquid density. 

The supercritical fluid mefhod is a relafively new method, which can minimize the use of organic solvents and harsh manufacturing conditions taking advantage of two distinctive properties of supercritical fluids (i.e., high compressibility and liquid-like density). This method can be broadly divided into two parts rapid expansion of supercritical solutions (RESS), which utilizes the supercritical fluid (e.g., carbon dioxide) as a solvent for the polymer, " and supercritical antisolvent crystallization (SAS), using the fluid as an antisolvent that causes polymer precipitation. Recent reviews of the supercritical technology for particle production are available in the literature. ... 

Several years ago we reported initial observations of reverse micelles and microemulsions in supercritical fluid solvents (JL) These studies suggested the possibility of creating a previously unsuspected broad range of organized molecular assemblies in dense gas solvents. Such systems are of interest due to potential applications which exploit the readily variable properties of supercritical fluids as well as the unique solvent environments of reverse micelles and microemulsions. These initial studies showed that even gram quantities of proteins, such as Cytochrome-c (Mwt. 12,842 dalton) could be solvated in a liter of supercritical ethane or propane due to the microemulsion solvent environment, something which is not achievable with "conventional"... 

Some properties of supercritical fluids can be moni-fored (manipulafed) continuously by adjusting the temperature and pressure or density of the fluid. Dielecfric consfanf is such a property and the solvent s dielectric constant can influence the rate of the reaction. 

---