|A supercritical fluid exhibits physicochemical properties intermediate between those of a liquid and a gas. In addition to its relatively high, liquid-like density at high pressure, which affords good solvent power, mass transfer relative to a liquid is rapid in SFs.) For pressures between 50 and 500 atm, diffusivity of supercritical CO 2 varies between 10"4 and 10~13 cm2/sec. Similarly the viscosities of supercritical fluids mirror their diffusivities being 10-100 times lower than liquids. Table 7 compares some of these important parameters for both the gaseous, SF, and liquid states. Table 8 draws a more specific comparison of supercritical CO 2 with the properties of traditional organic liquid solvents.
As was the case for density, values for viscosity and diffusivity are dependent on temperature and pressure. The viscosity and diffusivity of the SF approach those of a liquid as pressure is increased. Whereas an increase in temperature
Table 7 Orders of Magnitude of Physical Data For Gaseous, Supercritical Fluid, and Liquid Slates
Table 8 Comparison of Physical Properties of Supercritical CO, with Liquid Solvents at 25°C
-200 atm, 55°C. leads to an increase in viscosity of a gas, the opposite is true in the case of SFs. Diffusivity, on the other hand, will increase with an increase in temperature. As evidenced by Figures 6 and 7, changes in viscosity and diffusivity are most pronounced in the region about the critical point. Even at high pressures (300-400 atm), viscosity and diffusivity of SFs differ by 1-2 orders of magnitude from normal liquids. A review of these important points follows:
Fig. 6. Diffusivity of CO 2 versus temperature at various pressures.
Fig. 7. Viscosity behavior of CO 2 at various temperatures and pressures.
• Fixed density, temperature f , diffusivity f , viscosity |
• Density f , fixed temperature, diffusivity j , viscosity |
The properties of gas-like diffusivity and viscosity, coupled with liquid-like density, combined with the pressure-dependent solvating power of SFs have provided the impetus for applying SF technology to analytical separation problems. Finally, the low (essentially zero) value of surface tension of SFs allows better penetration into the sample matrix relative to liquid solvents.
The effects of pressure on dielectric constant can also be pronounced for selected SFs. For example, fluoroform (CHF3) exhibits a large change in dielectric constant (1.5-7.0 units) over a pressure range of 800-4000 psi. Such a large change is, however, not universal. Figure 8 shows that the dependence of dielectric constant on pressure for ethane, propane, fluoroform, and sulfur hexafluo-ride at 50°C varies considerably. The change for fluoroform is striking.
Fig. 8. Effect of pressure on dielectric constant for ethane (curve 4), propane (curve 3), fluoroform (curve 2), and sulfur hexafluoride (curve 1) at 50°C.
It has been stated that at 600 psi the physical properties of CHF3 resemble those of propane; above 4000 psi CHF3 is similar to methylene chloride.
Other properties of SFs that vary widely over a broad range of temperatures and pressures around the critical point are; thermal conductivity, partial molar volume (i.e., the change in volume of a system with the addition of one of its components), and heat capacity. The decrease in partial molar volume can be rather large in the vicinity of the critical point, which means that solvent molecules move toward and cluster around the solute molecules very tightly (vide infra). At high densities there is less clustering because of molecular repulsion. At the critical point of water (374°C, 221 bar), the heat capacity of water approaches infinity. The generality of this behavior to other fluids is not known.