Supercritical state, characteristic values

Supercritical fluids possess characteristics that make them interesting for use as polymerization media. A supercritical fluid exists at temperatures and pressures above its critical values. In the supercritical state, the fluid exhibits physical and transport properties intermediate between the gaseous and liquid state. This is illustrated in Table 2. SCFs have liquid-like densities, but gas-like diffusivities. These intermediate properties can provide advantages over liquid-based processes. In particular, the higher diffusivities of SCFs reduce mass transfer limitations in diffusion-controlled processes. Additionally, lower energy is required for processing the supercritical fluid because its viscosity is lower than that of most liquids, and because the need to vaporize large quantities of liquid is avoided. 

Supercritical fluids make ideal solvents because their density is only about 30% that of a normal fluid, a factor sufficient to provide for good solvent capability, but low enough for high diffusivity and rapid mass transfer. A sample of gas is supercritical whenever its temperature and pressure are above thar critical values, but in practice the operating temperatures are not far above 7. A simple way to predict the solvent characteristics of a low-boiling substance in its supercritical state is to compare the boiling point (Ti,) and critical temperature (TJ of substance. The Guldberg s rule ... 

Characteristic values for the gaseous, liquid, and supercritical state are listed in Table 12.1 [4]. Supercritical values for these features take place between liquids and gases. This holds trne for properties of fluids, which are decisive for mass and heat transfer (diffnsivity, viscosity, thermal conductivity, heat capacity) [5]. 

TABLE 12.1 Characteristic Values of Gas, Liquid, and Supercritical State [4]... 

By setting the appropriate pressure and temperature conditions one can tune solvation properties to affect reaction rates and chemical equilibria. For example, as illustrated in Figure 15.1, by changing state parameters one can reduce the static dielectric constant of water to a value characteristic for low-polar solvents. Supercritical water (SW) may substitute toxic organic solvents, such as acetone ( 25 = 20.7) or benzene ( 25 = 2.3). In contrast to ambient water, supercritical fluid is a poor solvent for ionic species but is well miscible with hydrocarbons and gases. 

The K scale of solvent polarity/polarizability is based on the correlation between the experimentally observed absorption or emission shifts (vmax values) of various nitroaromatic probe molecules and the ability of the solvent to stabilize the probe s excited state via dielectric solute-solvent interactions (18). Since tt values are known for many commonly used liquid solvents, the k scale allows comparison of the solvation strength of supercritical fluids and normal liquid solvents. Several research groups have utilized the k probes to investigate solvent characteristics for a series of supercritical fluids (19-34). For example, Hyatt (19) employed two nitroaromatic dyes and the penta-ferf-butyl variation of the Riechardt dye (18) to determine the k values in liquid and supercritical CO2 (0.7 reduced density at 41°C). The experimental results were also used to calculate the t(30) solvent polarity scale (19), which is similar to the n scale. ... 

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