Supercritical fluids hquid phase

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

Numerous high pressure Hquid chromatographic techniques have been reported for specific sample forms vegetable oHs (55,56), animal feeds (57,58), seta (59,60), plasma (61,62), foods (63,64), and tissues (63). Some of the methods requite a saponification step to remove fats, to release tocopherols from ceHs, and/or to free tocopherols from their esters. AH requite an extraction step to remove the tocopherols from the sample matrix. The methods include both normal and reverse-phase hplc with either uv absorbance or fluorescence detection. AppHcation of supercritical fluid (qv) chromatography has been reported for analysis of tocopherols in marine oHs (65). 

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. 

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. 

The third instrumental approach is the use of supercritical fluid extraction (SEE). A supercritical fluid is a substance at a temperature and pressure above the critical point for the substance. (You may want to review phase diagrams and the critical point on the phase diagram in your general chemistry text.) Supercritical fluids are more dense and viscous than the gas phase of the substance but not as dense and viscous as the hquid phase. The relatively high density (compared with the gas phase) of a supercritical fluid allows these fluids to dissolve nonvolatile organic molecules. Carbon dioxide, CO2, has a critical temperature of 31.3°C and a critical pressure of 72.9 atm this temperature and pressure are readily attainable, making supercritical CO2 easy to form. Supercritical CO2 dissolves many organic compormds, so it can replace a variety of common solvents supercritical... 

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. 

Both adsorption from a supercritical fluid to an adsorbent and desorption from an adsorbent find applications in supercritical fluid processing.The extrapolation of classical sorption theory to supercritical conditions has merits. The supercritical conditions are believed to necessitate monolayer coverage and density dependent isotherms. Considerable success has been observed by flic authors in working with an equation of state based upon the Tofli isoterm. It is also important to note that the retrograde behavior observed for vapor-hquid phase equilibrium is experimentally observed and predicted for sorptive systems. 

When the temperature exceeds the critical temperature and the pressure exceeds the critical pressure, the liquid and gas phases are indistinguishable from each other, and the substance is in a state called a supercritical fluid. A supercritical fluid expands to fill its container (like a gas), but the molecules are still quite closely spaced (Hke a Hquid). 

When a fluid is in its supercritical region, the phase boundary between the liquid and gaseous phases disappears and the two distinct phases converge into a single supercritical phase. The properties of a fluid in this supercritical region are generally described as a combination of those of a Hquid and a gas, and have therefore an intermediate value between liquids and gases, as can be seen in Table 18.1 [23]. 

Diffusivity values are higher in the supercritical phase than in the Hquid phase, so that species wiU diffuse faster through a supercritical fluid (SCF) than through a Hquid, implying faster solubility of solids in SCFs than in more normal liquids, and that SCFs wiU be more efl dent at penetrating through microporous materials thereby increasing the rate of mass transport... 

Ashraf-Khorassani, M. Nazem, N. Taylor, L.T. Feasibihty of supercritical fluid extraction with on-hne coupling reverse-phase hquid chromatography for quantitative analysis of polymer additives. J. Chromatogr. A, 2003, 995, 227-232. 

The data discussed earher concerned the lateral mobility in the hquid region of the Langmuir films spanning a range of MM As of ca. 50-100 A /molecule. Our inability to probe surfactants mobilities at lower surface concentration was related to the liquid/gas (L/G) phase transition, as discussed in Sect. I.5.3.2.2. A far more complete picture regarding surfactants lateral mobility was obtained for the 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (Tempo) derivatives, which are supercritical fluids at room temperature and thus do not undergo a L/G phase transition [49, 64,65]. (See the structure of G le Tempo.)... 

In an ahemative approach (Figure 6.14.9b) the catalyst is dissolved in a solvent other than SCCO2 and forms a hquid-hquid biphasic reaction system with the supercritical fluid. Here, the SCCO2 serves as mobile phase to transport the reactants and products through the reactor. In this case, the catalyst phase should stay in the reactor at all times during operation to avoid entrainment of the catalyst from the reactor. 

The physico-chemical properties of supercritical fluids (SCFs) are often described as a mixture of gas and liquid properties. Densities and solvent power are more hquid-hke, whereas transfer properties such as diSusivity are more similar to the gas phase (Table 25.2). Thus, SCFs offer features that might be beneficial for overcoming intrinsic problems of liquid-phase processes. 

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