Definition of a Supercritical Fluid

The critical point on a phase diagram designates the pressure (pc) and temperature (Tq) at which the vapor and liquid phases of a substance become indistinguishable. By definition, a supercritical fluid (SCF) is above pc and Tc- Generally, the physical properties of an SCF (density, viscosity, and dielectric constant) are intermediate between those of a liquid and a gas, and these properties vary dramatically as a function of temperature and pressure [1,2]. Because of these unique features, there is enormous interest in the use of SCFs as solvents for chemical reactions [3-6]. 

Another reason for the increased interest in SCFs as reaction solvents is that many common SCFs are essentially nontoxic and environmentally benign (e.g. H2O and CO2) [7]. By virtue of the fact that these are naturally occurring substances, it is unlikely that their use in chemical synthesis and manufacture will result in unanticipated environmental damage. Also, from the standpoint of public perception, there is less likelihood of opposition to the use of these naturally occurring (and recognizable) substances. 

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Thermodynamic Properties The variation in solvent strength of a supercritical fluid From gaslike to hquidlike values may oe described qualitatively in terms of the density, p, or the solubihty parameter, 6 (square root of the cohesive energy density). It is shown For gaseous, hquid, and SCF CO9 as a function of pressure in Fig. 22-17 according to the rigorous thermodynamic definition ... 

Now consider the mixture of a supercritical fluid or a gas with a polymer. The Sanchez-Lacombe equation-of-state treatment is described by the equations below, where the subscripts 1 and 2 refer to the gas and the polymer, respectively. The weight and the volume fractions of the gas (wj and Oj) and the polymer (W2 and O2) in the mixture add up to 1. The definition of the... 

Heterogeneously catalyzed reactions in SCFs must, by definition, involved at least two separate phases, the reaction mixture and the solid catalyst, and frequently may contain more. There has been considerable argument as to whether a singlephase reaction mixture is needed to exploit the advantages of a supercritical fluid, but these arguments really lie out of the scope of this chapter [25]. Suffice it to say... 

FIGURE 24.1 Definition of the supercritical fluid region in the phase diagram of a pure substance. 

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (the stationary phase), while the other (the mobile phase) moves in a definite direction. A mobile phase is described as a fluid which percolates through or along the stationary bed in a definite direction . It may be a liquid, a gas or a supercritical fluid, while the stationary phase may be a solid, a gel or a liquid. If a liquid, it may be distributed on a solid, which may or may not contribute to the separation process. ... 

The mobile phase is the phase which moves in a definite direction. It may be a liquid (LC and CEC), a gas (GC), or a supercritical fluid (supercritical-fluid chromatography, SFC). The mobile phase consists of the sample being separated/analyzed and the solvent that moves the sample through the column. In the case of HPLC the mobile phase consists of a non-polar solvent(s) such as hexane in normal phase or polar solvents in reverse phase chromotagraphy and the sample being separated. The mobile phase moves through the chromatography column (the stationary phase) where the sample interacts with the stationary phase and is separated. 

Solvents are substances that are liquid (or fluid in the case of supercritical fluids) under the conditions of application and in which other substances can dissolve, and from which they can be recovered unchanged on removal of the solvent. So many substances conform to this definition—practically all those that can be liquefied under some conditions—that it is not very helpful, unless the word application is stressed, meaning that the solvents and the solutes in them ought to be applicable for some purpose. This leaves a host of organic and many inorganic substances that are liquid at or near ambient conditions, which could be considered to be solvents under the present definition. Of these, a limited number are selected (Marcus, 1998). 

The aim of this Chapter is the development of an uniform model for predicting diffusion coefficients in gases and condensed phases, including plastic materials. The starting point is a macroscopic system of identical particles (molecules or atoms) in the critical state. At and above the critical temperature, Tc, the system has a single phase which is, by definition, a gas or supercritical fluid. The critical temperature is a measure of the intensity of interactions between the particles of the system and consequently is a function of the mass and structure of a particle. The derivation of equations for self-diffusion coefficients begins with the gaseous state at pressures p below the critical pressure pc. A reference state of a hypothetical gas will be defined, for which the unit value D = 1 m2/s is obtained at p = 1 Pa and a reference temperature, Tr. Only two specific parameters, Tc, and the critical molar volume, VL, of the mono-... 

Let us now focus upon the critical temperature and consider a few of the definitions that can describe this invariant point. It is important to note that the critical point is defined by the temperature only the value of the critical pressure appears to have a lesser or secondary significance. The critical (or supercritical) fluid region exists at all pressures at or above the critical temperature for a pure substance. Above this critical temperature, there exists only one phase, completely independent of the pressure. That is, no matter how high (or how low) you cause the pressure to be, the one phase wiU not condense to a hquid. 

With fluids, we think of the pump as the source of pressure as well as the flow rate determining device. However, with supercritical fluids (in contrast to t3q)ical liquids), a pump needs a control point downstream to hmit the passage of molecules per unit time. This restriction then "holds-back" the previously unlimited flow of molecules to a definite, but not always pre-determined level. Ideally then, the restrictor serves to restrict the flow until the density of molecules distributed from the pump through the extraction region right up to the final restriction point in space is such that the operating density desired in the extraction zone is achieved. This is much easier to state in words than it is to achieve in actual experimental practice. This is especially true if you wish to achieve an experimental set of parameters and hold those values over a finite period of time (ranging from minutes to hours) and do it with the statistical precision and accuracy that are necessary to attain the final quantitative analytical results. 

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