Properties of supercritical fluids

All these examples show the challenges occurring in view of kinetic modeling of reactions in supercritical fluids, especially in supercritical water. On the other hand the studies presented here give an impression of the opportunities of kinetic modeling after overcoming the lack of fundamental studies concerning solvent effects on elementary reactions steps. 

Supercritical fluids are defined as a fluid at a pressure above the critical pressure and a temperature above the critical temperature. Below the critical point, the vapor-the pressure curve separates the Hquid and gaseous phase. The vapor pressure ends up at the critical point. Beyond the critical point, the density of the fluids can be varied continuously from liquid-hke to gas-like values without phase transition. This variability of density corresponds to diversity of properties. Supercritical fluids are tunable solvents [26] for which the properties can be adjusted as a function of temperature and pressure. This chapter focuses on the utilization of supercritical CO2 and water. The properties of these two supercritical fluids will now be introduced. 

The critical point of CO2 is at 31.1 °C and 7.38MPa. Supercritical CO2 possess properties that are between those of the liquid and the gaseous state. It is used as a nonpolar solvent and exhibits a static dielectric constant generally between 1.1 and 1.5, depending upon density. 

In the case of water, the changes in properties from ambient via the critical point (Tc = 373 C, p - = 22.1 MPa, p - = 320kg/m ) up to supercritical conditions 

The ionic product increases slightly with temperature up to around in 

Symbols used p, density ii, viscosity D, diffusion coefficient. 

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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],... 

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. 

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... 

Tab. 12.3 Comparison of properties of supercritical fluids and those of gases and liquids [22 b]...

The combination of the high selectivity of chromatographic interactions and the unique properties of supercritical fluids leads to promising applications in the pharmaceutical field. A review on the topic has been presented by Perrut [10]. 

A gaseous pure component can be defined as supercritical when its state is determined by values of temperature T and pressure P that are above its critical parameters (Tc and Pc). In the proximity of its critical point, a pure supercritical fluid (or a dense gas as it is alternatively known) has a very high isothermal compressibility, and this makes possible to change significantly the density of the fluid with relatively limited modifications of T and P. On the other hand, it has been shown that the thermodynamic and transport properties of supercritical fluids can be tuned simply by changing the density of the medium. This is particularly interesting for... 

TABLE 1. Properties of Supercritical Fluids vs. Gases and Liquids(13)... 

This combination of the aforementioned properties of supercritical fluids accounts for the fact that they penetrate the sample matrix like a gas and at the same time dissolve analytes like liquids. 

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 ... 

Supercritical fluid extraction (SFE) utilizes the unique properties of supercritical fluids to facilitate the extraction of organics from solid samples. Analytical scale SFE can be configured to operate on- or off-line. In the online configuration, SFE is coupled directly to an analytical instrument, such as a gas chromatograph, SFC, or high-performance liquid chromatograph. This offers the potential for automation, but the extract is limited to analysis by the dedicated instrument. Off-line SFE, as its name implies, is a stand-alone extraction method independent of the analytical technique to be used. Off-line SFE is more flexible and easier to perform than the online methods. It allows the analyst to focus on the extraction per se, and the extract is available for analysis by different methods. This chapter focuses on off-line SFE. 

It is well known that the properties of supercritical fluids are sensitive to pressure, and thus pressure may drastically influence the catalytic activity or the product selectivity when a reaction takes place in supercritical conditions. The favorable pressure for the Wacker oxidation of styrene is around 16 MPa of total pressure including 3 MPa 02, at which the selectivity toward acetophenone reaches 92 %, while under a total pressure of 9 MPa, the selectivity for acetophenone is lower (86 %). However, C02 with a higher pressure of over 20 MPa might retard the interaction between the substrate and the catalyst, and might cause a low concentration of substrate in the vicinity of the catalyst, thus resulting in a relatively low yield [38]. 

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). 

Franck, E.U. "Thermophysical Properties of Supercritical Fluids with Special Consideration of Aqueous Systems", Fluid Phase Equilib. 1983, 10(2-3), 211-22. 

Scientists have been aware of the novel properties of supercritical fluids for more than a century. Early investigators were fascinated by the schizophrenic behavior of this gaslike, liquidlike state, and it is disconcerting but coincidental that research activities also split into two disparate areas. Fundamental interactions in simple systems were meticulously investigated to correlate and predict phase behavior. At the other extreme, scientists applied supercritical fluids with Edisonian zeal, seeking miraculous solutions to complex problems in extraction and fractionation. 

The properties of supercritical fluids are generally different from those of regular fluids. For example, supercritical water is relatively nonpolar and acidic. Further, the properties of a supercritical fluid, such as its density and viscosity, change with changing pressure and temperature, dramatically as the critical point is approached. Thus, carbon dioxide is not listed in Table 6.1 because it has no liquid phase at terran atmospheric pressure. Carbon dioxide has a critical temperature of 304.2 K and pressure of 73.8 atm, however. It is therefore a supercritical fluid above that pressure, and may even exist as a potential biosolvent for rocky planets having the approximate mass of Earth (or Venus). 

The properties of supercritical fluids fall between those of gases and liquids, as shown in Table 1. Thus the mobile phase in SFC has a viscosity... 

Fluids are highly compressible along near-critical isotherms (L01-1.2 Tc) and display properties ranging from gas-like to Liquid-Like with relatively small pressure variations around the critical pressure. The liquid-like densities and better-than-liquid transport properties of supercritical fluids (SCFs) have been exploited for the in situ extraction of coke-forming compounds from porous catalysts [1-6], For i-hexene reaction on a low activity, macroporous a catalyst, Tiltschcr el al. [1] demonstrated that reactor operation at supercritical... 

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