| A pure supercritical fluid (SCF) is any compound at a temperature and pressure above the critical point. Above the critical temperature of a compound the pure, gaseous component cannot be liquefied regardless of the pressure applied. The critical pressure is the vapor pressure of the gas at the critical temperature. In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described as intermediate to the two extremes.This phase retains solvent power approximating liquids as well as the transport properties common to gases. A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs is presented in table 14:
Table 14. Comparison of physical and transport properties of gases, liquids and SCFs
Property |
Density (kg/m3 ) |
Viscosity (cP) |
Diffusivity (mm2 /s) |
Gas |
1 |
0.01 |
1-10 |
SCF |
100-800 |
0.05-0.1 |
0.01-0.1 |
Liquid |
1000 |
0.5-1.0 |
0.001 |
By increasing the pressure of the gas above the critical point (Figure 45), it is possible to give liquid-like densities and solvating strengths.
Figure 45 : Phase diagram of CO 2
Near the critical point, the density of the gas will increase rapidly with increasing pressure, here, the solubility of many compounds is several orders of magnitude greater than predicted from the classical thermodynamics of ideal gases. As the average distance between molecules decreases, non-ideal gas behavior will begin to govern the interactions between the solvent and the sample accounting for a tremendous enhancement in solubility. In supercritical region, solvating strength is a direct function of density, which in turn is dependent on system pressure (at constant temperature). Solvating strengths can be fine-tuned by minutely adjusting the pressure and/or temperature, using the solvent anywhere in the range of ideal gas to nearly pure liquid. Because of the non-compressibility of liquids, this phenomenon is unique to supercritical fluids. It is even possible, by adding small quantities of co-solvent, to custom design a supercritical fluid for a specific application.
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