The Supercritical State

Supercritical fluid technology has been widely used in extraction and purification processes in the food and pharmaceuticals industryPl 1 1 and for techniques such as supercritical fluid chromatography. Recently, there has been a significant increase in interest of the use of sub- as well as supercritical (SC) carbon dioxide as a substitute for chlorofluorocarbons (CFCs) for a variety of specific and specialized applications t in which the choices of enviromnentally acceptable alternatives are quite limited. 

Today most chemists are familiar with the critical point of a substance as defined by the critical temperature and pressure, and Pc respectively, but often their knowledge goes no further. The behavior of gases close to the critical point is so far removed from ideal that, the topic is usually ignored. Hence, many scientists are unaware that near-critical fluids display unusual and intriguing properties which can lead to new chemistry with exciting applications. 

A substance is said to be in the gaseous state when heated to temperatures beyond its critical point. However, the physical properties of a substance near the critical point are intermediate between those of normal gases and liquids, and it is appropriate to consider such supercritical fluid as a fourth state of matter. For applications such as cleaning, extraction and chromatographic purposes, supercritical fluid often has more desirable transport properties than a liquid and orders of magnitude better solvent properties than a gas. Typical physical properties of a gas, a liquid, and a supercritical fluid are compared in Table 1. The data show the order of magnitude and one can note that the viscosity of a supercritical fluid is generally comparable to that of a gas while its diffusivity lies between that of a gas and a liquid. 

The simplest applications of thermodynamics to chemically significant systems involve the phase transitions that pure substances undergo. The phase of a substance is a form of matter that is uniform throughout in chemical compoation and phyacal state. The word phase comes from the Gredc word for )pearance. Thus, we speak of the solid, liquid, and gas phases of a substance, and of different solid phases distingui ed by thdr ciystal structures (such as white and black phosphorus), h phase transition, spontaneous conversion of one phase to another, occurs at a characteristic temperature for a ven pressure. Thus, at 1 atm, ice is the stable phase of water below 0 C, but above 0°C the liquid is more stable. The difference indicates that, below 0°C, the chemical potential of ice is lower than that of liquid water, //(solid) //(liquid) (Fig. 1), and that above OX, //(liquid) //(solid). The transition temperature is the temperature at which the chemical potentials coincide and //(solid) = //(liquid). 

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Supercritical Mixtures Dehenedetti-Reid showed that conven-tionaf correlations based on the Stokes-Einstein relation (for hquid phase) tend to overpredict diffusivities in the supercritical state. Nevertheless, they observed that the Stokes-Einstein group D g l/T was constant. Thus, although no general correlation ap es, only one data point is necessaiy to examine variations of fluid viscosity and/or temperature effects. They explored certain combinations of aromatic solids in SFg and COg. 

In principle, the sample transfer from the Supercritical state is relatively easily adaptable to other systems, due to the high volatility of the fluid at atmospheric pressure, particularly for carbon dioxide which is the most frequently used fluid. 

The use of solvents above their normal conditions of temperature and pressure, up to and including the supercritical state, expands the range of analytical methods exploiting an overall spectrum of solubility, polarity and volatility properties of solvents and mobile phases. The fundamentals and applications of SCFs have been reviewed [243] and described in numerous books [248,251-256],... 

On the other hand, solvents usually show a decrease in dielectric constant with temperature. Efficiency of microwave absorption diminishes with temperature rise and can lead to poor matching of the microwave load, particularly as fluids approach the supercritical state. Solvents and reaction temperatures should be selected with these considerations in mind, as excess input microwave energy can lead to arcing. If allowed to continue unchecked, arcing could result in vessel rupture or perhaps an explosion, if flammable compounds are involved. Therefore it is important in microwave-assisted organic reactions, that the forward and reverse power can be monitored and the energy input be reduced (or the load matching device adjusted) if the reflected power becomes appreciable. 

Supercritical fluid chromatography employs supercritical fluid instead of gas or liquid to achieve separations. Supercritical fluids generally exist at conditions above atmospheric pressure and at an elevated temperature. As a fluid, the supercritical state generally exhibits properties that are intermediate to the properties of either a gas or a liqiud. Chapter 16 discusses various advantages of SFC over GC and HPLC and also provides some interesting applications. 

The discovery of supercritical fluids occurred in 1879, when Thomas Andrews actually described the supercritical state and used the term critical point. A supercritical fluid is a material above its critical point. It is not a gas, or a liquid, although it is sometimes referred to as a dense gas. It is a separate state of matter defined as all matter by both its temperature and pressure. Designation of common states in liquids, solids and gases, assume standard pressure and temperature conditions, or STP, which is atmospheric pressure and 0°C. Supercritical fluids generally exist at conditions above atmospheric pressure and at an elevated temperature. Figure 16.1 shows the typical phase diagram for carbon dioxide, the most commonly used supercritical fluid [1]. 

In condensed phases, the noncoincidence effect between IR and Raman spectra provides insights into the intermolecular coupling [170, 171]. The combination of IR and Raman spectroscopy is also useful in the study of alcohol clusters in the supercritical state [25]. 

Liquefied or Supercritical Cases as Solvents for Electrolytes For very special applications, where the increased efforts for low temperature and/or pressurized cells are acceptable, liquefied gases, for example, sulfur dioxide or ammonia, can be interesting solvents for electrolytes (see e.g. [3a]). Supercritical fluids show remarkable properties that are very different from other solvents. Detrimental to electrochemistry is that especially the dielectric constant in the supercritical state becomes low. For supercritical carbon dioxide, no supporting electrolyte with sufficient conductivity is known. 

The critical point of CO2 is readily accessible at 304.1282 K and 7.3773 MPa. The supercritical state of CO2 is characterized by properties that are partly... 

In the past, the majority of high-pressure homogeneous catalytic reactions were conducted in batch systems, which may cause problems in scale-up for SCFs because of the higher pressures needed for achieving the supercritical state. Therefore, continuous processing has also been investigated in the last years. It would be preferable for industrial-scale SCF reactions, because it involves smaller and, hence, safer equipment [144-150]. In addition, capital costs are likely to be lower than in batch systems. 

Another way to produce biphenyl derivates using flow was described by Leeke et al. [34] where they performed a Pd catalyzed Suzuki-Miyaura synthesis in the presence of a base. First experiments were carried out in toluene/methanol solvent. A reaction mixture was passed through the encapsulated Pd filled column bed length 14.5 cm (some cases 10 cm) x 25.4 mm id. 45 g of PdEnCat. Base concentration, temperature and flow rate were optimized and at optimum parameters (0.05 M base concentration, 100°C and 9.9 mL/min) the conversion was 74%. Then the reaction was performed under supercritical conditions using supercritical CO2 at high pressure and temperature. After optimizing the concentration of base, flow rate, pressure and temperature, the highest conversion rate (81%) was observed at 166 bar and 100°C where the reactant mixture was monophasic in the supercritical state. This system is able to produce 0.06 g/min of the desired product. 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

Although CO2 is highly soluble in ionic liquids, there is no appreciable ionic liquid solubility in the CO2 phase in the supercritical state (29). 

The liquid state exists only below the critical point pressure and above the triple point pressure. When a vapor below the triple point pressure is cooled down, we encounter a discontinuous and abrupt phase change to solid but, above the critical point pressure, a cooled vapor turns into the supercritical state where the properties of the fluid... 

The supercritical state of CO2 is relatively easily attained (Tc = 31°C, Pc = 74 bar). This feature of SC-CO2 implies that the cost (e.g., energy, apparatus, and materials) will not be prohibitive with the conversion to SC-CO2. 

The process involves the use of supercritical fluids rather than liquids as solvents. A fluid is in the supercritical state when its pressure and temperature exceed the pl ical properties which defines its critical point. Carbon dioxide is by far the most widely used supercritical solvent. Many other selected fluids have potential use for SFE technologies. 

The physico-chemical effect of high pressure, especially in the supercritical state, to enhance the solubility and phase conditions of the components involved. Supercritical hydrogenation, or enzymatic syntheses are offer new steps with high pressure. Supercritical water oxidation at high pressure represents an efficient method for the decontamination of wastes. 

To access the supercritical fluid state, we must have conditions in excess of the critical temperature and pressure. Given the rating of the autoclave, ammonia would not be suitable because one could not access the supercritical state as a result of the pressure limitation. Methylamine would not be suitable for a room temperature extraction because its Tc is too high. Either methane or tetrafluoromethane would be suitable for this application. 

The critical state is achieved when a substance is taken above its critical temperature and pressure (Tc, Pc). Above this point on a phase diagram, the gas and liquid phases become indistinguishable. The physical properties of the supercritical state (e.g., density, viscosity, solubility parameter, etc.) are intermediate between those of a gas and a liquid, and vary considerably as a function of temperature and pressure. 

The interest in sc C02 specifically is related to the fact that C02 is nontoxic and naturally occurring. The critical parameters of C02 are moderate (Tc = 31 °C, Pc = 74 bar), which means that the supercritical state can be achieved without a disproportionate expenditure of energy. For these two reasons, there is a great deal of interest in sc C02 as a solvent for chemical... 

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