Properties of supercritical fluids and their uses as solvents

Properties of supercritical fluids and their uses as solvents 

Although SCCO2 is a clean alternative to organic solvents for a range of extraction processes, it is non-polar. While the 

The technique of supercritical fiuid chromatography (SFC) is similar to high-performance liquid chromatography (HPLC) but has major advantages over the latter separation is more rapid, and the use of organic solvents is minimized. 

The pharmaceutical industry applies SFC to the separation of chiral and natural products. 

One area that is rich for development is the use of SCCO2 as a cleaning solvent. It has already been introduced for the dry-cleaning of clothes, and this application should become more widespread in future years. Supercritical CO2 is also used to clean optical and electronics components, as well as heavy-duty valves, tanks and pipes. 

Although SCCO2 is a clean alternative to organic solvents for a range of extraction processes, it is non-polar. While the behaviour of SCCO2 does not parallel a typical non-polar organic solvent, its ability to extract polar compounds is 

In the examples given above, supercritical CO2 is used in what is termed clean technology with drastic reductions in the use of organic solvents, and the twenty-first century should see an increase in the use of supercritical fluids in commercial processes. 

Above its critical temperature and critical pressure, an element 

Above the critical temperature, T cnticab Ih can no longer be liquefied, no matter how high the pressure is increased. If a sample is observed as the critical point is reached, the meniscus at the liquid-gas interface disappears, signifying that there is no longer a distinction between the two phases. At temperatures and pressures above the critical temperature and pressure (i.e. above the critical point), a substance becomes a supercritical fluid. 

Compound or element Critical temperature / K Critical pressure / MPa 

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SC-CO2 as a Reaction Solvent. The supercritical state is achieved when a substance is taken above its critical temperature and pressure. The bulk properties of a supercritical fluid are intermediate between those of a gas and a liquid. Because of the unique properties of supercritical fluids, analytical methods based upon their use... 

The central theme of this review has been to provide an overview of how supercritical fluids might serve as green solvent replacements in a wide range of chemical synthesis applications. While the evaluation of their suitability is incomplete, several examples from ongoing research were discussed to illustrate the potential of using the unique properties of supercritical fluids involving CO2 and H2O to achieve acceptable conversions and selectivities for practical processes. 

The properties of supercritical fluids that make them useful chromatographic mobile phases also make their use as extraction solvents an attractive option for polymer analysis. The low viscosities and high solute diffusivities allow efficient mass transfer during extraction, and the relatively low extraction temperatures reduce the risk of analyte degradation. Thus, the extraction of polymer additives from polymer matrices by using supercritical fluids has many advantages over conventional liquid solvent extraction, with the potential of higher recoveries and shorter analysis times. The further combination... 

The interest in supercritical fluids and their usage in various industrial processes has been on the rise in the past few decades. Their applications are widely ranged and vary from the food and pharmaceutical to material and waste industry. Special attention has been paid to their use in spraying processes as they are proving to be suitable for powder production [1-3]. An extensive research more recently shows that they meet the requirements for the production of carrier materials for active substances, extending their application to the production of microcapsules and composite materials [4]. Their advantages lie mainly in the production of solvent free powders, adjustable particle sizes, and lower process temperatures for thermo-labile substances. Moreover, the product properties can be easily adjusted by variation of the process parameters. 

The use of supercritical fluids as mobile phases in chromatography can offer several advantages because their properties are between those of liquids and those of gases. In particular, the viscosity of a supercritical fluid is almost that of a gas (50 times lower than that of a solvent) while its solvation properties (governed by the distribution coefficients K) are similar to those of a nonpolar solvent such as benzene. 

Based on its ability to enhance solvating power by increasing fluid density, supercritical fluid extraction offers an attractive alternative for fractionation of fats and oils. It works by the phenomena of selective distillation and simultaneous extraction, as has been shown by many researchers [3-5]. While the use of supercritical fluids in the extraction of numerous biomaterials has been reported, its commercialization has been limited to the decaffeination of coffee and tea and to the extraction of flavors from hops and spices. The chemical complexity of most food ingredients and their tendency to react and degrade at elevated temperatures, emphasize the difficulties of supercritical solvent selection. Carbon dioxide is the preferred supercritical solvent (its properties have previously been cited [6]). 

In addition to fluorous solvents and ionie liquids, supercritical fluids sc-fluids, scf s), sueh as supercritical carbon dioxide (se-C02), constitute a third class of neoteric solvents that can be used as reaction media. Although sc-fluids have been known for a long time and have been advantageously used as eluants in extraction and chromatography processes (see Sections A.6 and A.7 in the Appendix), their application as reaction media for chemical processes has become more popular only during the last decade. Some of their physical properties and the supercritical conditions necessary for their existence have already been described in Section 3.2 (see Figure 3-2 and Table 3-4) see also references [209, 211-220, 224-230] to Chapter 3 for reviews on sc-fluids and their applications (particularly for SC-CO2 and SC-H2O). 

Supercritical fluids (SCFs) have long fascinated chemists and over the last 30 years this interest has accelerated. There is even a journal dedicated to the subject— the Journal of Supercritical Fluids. These fluids have many fascinating and unusual properties that make them useful media for separations and spectroscopic studies as well as for reactions and synthesis. So what is an SCF Substances enter the SCF phase above their critical pressures P and temperatures (Tc) (Figure 4.1). Some substances have readily accessible critical points, for example for carbon dioxide is 304 K (31 °C) and is 72.8 atm, whereas other substances need more extreme conditions. For example for water is 647 K (374 °C) and P is 218 atm. The most useful SCFs to green chemists are water and carbon dioxide, which are renewable and non-flammable. However, critical data for some other substances are provided for comparison in Table 4.1. In addition to reactions in the supercritical phase, water has interesting properties in the near critical region and carbon dioxide can also be a useful solvent in the liquid phase. Collectively, carbon dioxide under pressurized conditions (liquid or supercritical) is sometimes referred to as dense phase carbon dioxide. 

Although modifiers are added to supercritical fluids to increase their polarity, they can also impart decreased polarity, aromaticity, chirality and the ability to further complex organometallic compounds. Just as carbon dioxide is the most popular substance for use as a supercritical fluid, it is also that to which modifiers are most frequently added. This is so because modifiers are seen as the means for enabling the use of CO, in situations where it may not be the best solvent. For example, methanol is added to supercritical CO, to increase its polarity, aliphatic hydrocarbons to decrease it, toluene to impart aromaticity, [/ ]-2-butanol to add chirality and tributyl phosphate to enhance the solvation of metal complexes. The amount of modifier to be added depends on the properties of the extractant and those of the analyte and matrix usually, it ranges from a few... 

Supercritical solvents have generated an increased interest in the last few decades. One reason is that their solvent properties vary considerable with temperature and density. They are tunable solvents [1] and for each purpose - separations or reactions - the optimal properties can be adjusted (see, for example, [1-8]). Usually, supercritical fluids are used as a tool to get homogeneous mixtures. In a homogeneous phase, for example, oxidations are extraordinarily fast and complete. The usually improved heat and mass transfer is a further advantage. Supercritical fluids show their good solvent properties only in the supercritical state. Therefore separation after reaction or extraction is very simply achieved by reducing temperature and pressure. This enables very sustainable processes (for example [1, 9]). Here supercritical carbon dioxide and water are of special interest, because they are cheap, nontoxic or of very low toxicity, in the case of carbon dioxide and nonexplosive. 

The use of supercritical fluid extraction (SEE) as an extraction technique is related to the unique properties of the supercritical fluid. These fluids have a low viscosity, high diffusion coefficients, low toxicity, and low flammability, all clearly superior to the organic solvents used in SPE extraction. The most common fluid used is carbon dioxide. SEE extractions of sediment samples have shown recoveries of >95% for all the individual PCBs. The separation of PCDDs from PCBs and chlorinated benzenes is difficult because of their similar solubility. An interesting development is the use of fat retainers. Samples, mixed in different weight ratios with, e.g., silica/silver nitrate 10% or basic alumina, can be placed in 7 ml extraction cells. The analytes are recovered by elution with 1.5-1.8 ml of hexane. With the correct fat-silica ratios and SEE conditions, no additional cleanup procedure is necessary for GC with an electron-capture detector (ECD). One drawback of SEE may be that the methods developed are valid for a specific matrix, but as soon as, e.g., the fat content of a biota sample or the type of lipids changes, the method has to be adapted. SEE is relatively complicated compared to other extraction techniques. In addition, the cell volumes are small, which limits the sample intake, and, with that, the detection limits. Einally, some reliable types of SEE equipment have recently been withdrawn from the market. This will have a substantial negative effect on the use of SEE in the near future. 

Recently, there has been an increasing interest in the use of supercritical fluid extraction (SEE) with carbon dioxide (CO2) as a solvent. This process uses the properties of gases above their critical points to extract selective soluble components from a raw material. Carbon dioxide is an ideal solvent for the extraction of natural products because it is nontoxic, nonexplosive, readily available, and easy to remove from extracted products [3,6]. SFE has the abihty to use low temperatures, leading to less deterioration of the thermally labile components in the extract. In addition, SFE is typically carried out in the absence of air which also ensures minimal alteration of the active ingredients and preservation of the curative properties [46, 47]. SC CO2 is generally efficient in the purification and fractionation of hydrophobic compounds, such as flavonoids and cinnamic acid derivatives from plant matrixes [49]. 

Reacting lipophilic substrates with hydrophilic compounds, as in the case of most transesteriflcation reactions, is one of the major difficulties in lipase-catalyzed reactions. Several parameters need to be considered to overcome this immiscibility problem. One commonly proposed strategy is the use of a nonaqueous medium. In this chapter, the advantages of using nonaqueous media in biochemical synthesis reactions, over aqueous and solvent-free systems, are discussed. The use of hydrophobic solvents is also discussed, followed by a presentation of the alternatives that can overcome the limitations of solvents. The focus of this chapter is mainly on the use of supercritical fluids (SCFs) as a green alternative reaction medium. The chapter also discusses ionic liquids (ILs) as another alternative. These solvents and the factors affecting their physical properties and their effect on the activity and stability of lipase are also discussed. 

Supercritical fluids (SCFs) offer the potential for a controlled solution environment because of the tunability of their properties by small changes in temperature and pressure. Indeed, near-critical water and supercritical water are obvious candidates as solvents in nanoparticle formation because water is the most commonly used solvent in conventional synthesis of inorganic particles. However, other solvents, such as carbon dioxide, can also be used. Several methods that take advantage of SCF behavior are described below. Not all have been employed in the production of magnetic nanoparticles. However, they represent a natural bridge between methods that are carried out mainly in the liquid state and those that are carried out in the gaseous state. 

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