Supercritical fluids critical temperatures

The investigation of high-critical-temperature supercritical fluids is a more challenging task. One of the significant difficulties associated with these studies is probe-molecule thermal stability many molecular probes commonly used with ambient supercritical fluids decompose at the temperatures required by these high-critical-temperature fluids. Fortunately, pyrene can be employed for such tasks. Several reports have been made of the use of pyrene as a molecular probe to investigate solute-solvent interactions in high-critical-temperamre supercritical fluids (e.g., pentane, hexane, heptane, octane, cyclohexane, meth-cyclohexane, benzene, toluene, and water) (44,48,49). In supercritical hexane... 

Crooks RM, Bard AJ. Electrochemistry in near-critical and supercritical fluids, nitrogen heterocycles nitrobenzene, and solvated electrons in ammonia at temperatures to 150°C. J Phys Chem 1987 91(5) 1274. 

Critical temperatures for fluids used in chromatography vary widely, from about 30°C to more than 200°C. Lower critical temperatures are advantageous in chromatography from several standpoints. For this reason, much of the work to date has focused on the supercritical fluids shown in Table 33-2. Note that these temperatures and the pressures at these temperatures are well within the operating conditions of ordinary high-performance liquid chromatography (HPLC). 

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. 

A supercritical fluid is defined as one that is above its thermodynamic critical point, as identified by the critical pressure (p ) and critical temperature (Tc). Supercritical fluid behavior can be peculiar because of the variation of theimophysical properties such as density and specific heat near and at the critical point. Supercritical fluids have some properties similar to liquids (e.g., density), and some properties that are comparable to those of gases (e.g., viscosity). Thus, they cannot be considered either a liquid or a gas. 

Among these various phase separation techniques, pressure-induced phase separation is particularly important since pressure changes can be brought about uniformly and very fast throughout the bulk of a solution. This would not be so in other techniques due to for example heat (in TIPS) and mass transfer (in SIPS) limitations. The technique therefore opens up new opportunities for formation of microstructured materials with potentially more uniform morphologies. It is also important to recognize temperature, solvent, reaction, or field-induced phase separation may all be carried out at elevated pressures if so desired, as such all modes of phase separation methods are of interest when working with near-critical or supercritical fluid systems. 

Consider the phase diagram for a binary polymer-solvent mixture in which the solvent is roughly the size of one monomer unit (segment). Starting from a one phase mixture, a decrease in temperature can lead to separation into polymer-rich and polymer-lean phases at the upper critical solution temperature (UCST) phase boundary. In general a polymer solution also phase separates as temperature is increased to the lower critical solution temperature (LCST) phase boundary [40]. In near critical and supercritical fluids, the driving force for phase separation at the LCST phase boundary is the difference in the compressibility of polymer and solvent, which becomes large... 

Supercritical fluids have already been shown to be useful in the production of particles, including those of metal oxides and other metal species. The near-critical and supercritical fluid environment therefore offers a means for the control of size and composition of mixed oxide nanoparticles. Supercritical fluids also have the potential for providing facile separation of impurities from desired species, such as CoFc204. The role of processing variables such as temperature, pressure, pH, residence time, and relative solubility has been discussed briefly in this work. However, a complete understanding of the reaction and subsequent particle formation will require much further work. Nevertheless, the examples discussed in this work clearly demonstrate the potential of near-critical and supercritical fluids in this area. 

The term fluid includes not only liquids but also anything that flows such as powdered coal or sand slurries in water and of course airflow. Experimentally, the critical temperature for carbon dioxide is 30.98°C, the critical pressure is 73.75 bar, and the critical volume is 94 cm /mol [1]. Above the critical temperature, the fluid is called supercritical fluid. We can compare that to the value estimated from the van der Waals equation using data from Table 1.3. The result is within 0.25° of the experimental value. 

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

Supercritical Fluid Extraction. Supercritical fluid (SCF) extraction is a process in which elevated pressure and temperature conditions are used to make a substance exceed a critical point. Once above this critical point, the gas (CO2 is commonly used) exhibits unique solvating properties. The advantages of SCF extraction in foods are that there is no solvent residue in the extracted products, the process can be performed at low temperature, oxygen is excluded, and there is minimal protein degradation (49). One area in which SCF extraction of Hpids from meats maybe appHed is in the production of low fat dried meat ingredients for further processed items. Its apphcation in fresh meat is less successful because the fresh meat contains relatively high levels of moisture (50). 

Eig. 1. Schematic pressure—temperature diagram for a pure material showing the supercritical fluid region, where is the pure component critical point... 

Erequenfly, the term compressed fluid, a more general expression than supercritical fluid, is used. A compressed fluid can be either a supercritical fluid, a near-critical fluid, an expanded Hquid, or a highly compressed gas, depending on temperature, pressure, and composition. 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

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