Supercritical fluids, properties polarity

Several research groups have used pyrene as a fluorescent probe in the study of supercritical fluid properties (2,3,40-48). In particular, the density dependence of the Py scale has been examined systematically in a number of supercritical fluids such as CO2 (2,3,40-43,45,46), ethylene (40,41,47), fluoro-form (3,40,41,43,47), and C02-fluoroform mixtures (43). The Py values obtained in various supercritical fluids correlate well with the polarity or polarizability parameters of the fluids (3,40,41,43,47). For example, Brennecke et al. (40) found that the Py values obtained in fluoroform were consistently larger than those obtained in CO2, which were, in turn, consistently larger than those found in ethylene over the entire density region examined. In addition, the Py values obtained in the liquid-like region (reduced density 1.8) indicate that the local polarity of fluoroform is comparable to that of liquid methanol, CO2 with xylenes, and ethane with simple aliphatic hydrocabons (15,16). 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

A variety of equations-of-state have been applied to supercritical fluids, ranging from simple cubic equations like the Peng-Robinson equation-of-state to the Statistical Associating Fluid Theoiy. All are able to model nonpolar systems fairly successfully, but most are increasingly chaUenged as the polarity of the components increases. The key is to calculate the solute-fluid molecular interaction parameter from the pure-component properties. Often the standard approach (i.e. corresponding states based on critical properties) is of limited accuracy due to the vastly different critical temperatures of the solutes (if known) and the solvents other properties of the solute... 

Supercritical fluid extraction (SEE) is another modern separation technology usually employed to extract lipophilic compounds such as cranberry seed oil, lycopene, coumarins, and other seed oils. Anthocyanins generally and glycosylated anthocyanins in particular were considered unsuitable for SEE due to their hydrophilic properties, since SEE is applicable for non-polar analytes. However, a small amount of methanol was applied as co-solvent to increase CO2 polarity in anthocyanin extraction from grape pomace. New applications of SEE for anthocyanin purification have been reported for cosmetic applications from red fruits. ... 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

Extraction by supercritical fluids, in particular carbon dioxide and propane, is currently being investigated as a means of controlling the size and shape of particles for inhalation. Supercritical fluids are liquids above their critical pressure and temperature [28]. Under these conditions the molecules exhibit the flow, polarity, and solvency properties common of liquids but have the diffusivities and reactivities characteristic of gases. 

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

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). " ... 

In some aspects, supercritical fluids, which represent a state between the gaseous and liquid phases, have properties resembling those of non-polar solvents in being adequate for biotransformations of hydrophobic compounds. Although the use of supercritical fluids is not restricted to hydrolases, the use of this class of enzymes, especially lipases, dominates [3, 4]. Esters represent the main flavour compounds produced by this process [5]. 

Adding an organic solvent (such as methanol or acetone) to the supercritical fluid can modify its solvating properties. Since the polarity of C02 in its supercritical state (at 100 atm and 35 °C) is comparable to that of hexane, it can be altered by introducing a modifier. Nonetheless, isolating analyte from the matrix requires knowledge about the solubility and the transfer rate of solute in the solvent as well as chemical and physical interactions between matrix and solvent (Fig. 20.6). 

Steady-State Fluorescence. The fluorescence characteristics of PRODAN are extremely sensitive to the physicochemical properties of the solvent (38). As benchmarks, the steady-state emission spectra for PRODAN in several liquid solvents are presented in Figure 1. It is evident that the PRODAN emission spectrum red shifts with increasing solvent polarity. This red shift is a result of the dielectric properties of the surrounding solvent and the large excited-state dipole moment (ca. 20 Debye units) of PRODAN (38). It is the sensitivity of the PRODAN fluorescence that will be used here to investigate the local solvent composition in binary supercritical fluids. 

The physical properties of the supercritical fluid differ from those of the bulk liquid. One of the most notable changes is the lower dielectric constant of polar solvents such as water which allows the accumulation of low-polarity solutes at this interface. This explains the crucial role of the hydro-phobicity of solutes during reactions in the solution. Thermolysis as well as radical abstraction reactions occur in this region. A temperature of approximately 800 K was determined for the interfacial region surrounding the... 

Several liquids and gases can be brought into the supercritical phase. Different solvents can be selected as extraction media for use in analytical-scale SFE. Carbon dioxide is most commonly used as an SFE medium because of its desirable properties and easy handling it is relatively inexpensive and commercially available at a purity grade acceptable for most analytical applications. Another advantage of carbon dioxide is that the polarity can easily be adjusted by adding modifiers such as methanol to the supercritical fluid or the extraction vessel. 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

A number of other important potential applications of a micellar phase in supercritical fluids may utilize the unique properties of the supercritical fluid phase. For instance, polar catalyst or enzymes could be molecularly dispersed in a nonpolar gas phase via micelles, opening a new class of gas phase reactions. Because diffusivities of reactants or products are high in the supercritical fluid continuous phase, high transport rates to and from active sites in the catalyst-containing micelle may increase reaction rates for those reactions which are diffusion limited. 

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