# Eight Physical Properties of Supercritical Fluids

## Density

Under normal temperature and pressure, the density of the liquid is 0.6~1.6g/cm³, and the density of the supercritical fluid is 0.2~0.5g/cm³. It can be seen that supercritical fluid has a density close to that of liquid. But the density of the two has a different dependence on temperature and pressure. This is due to the compressibility of supercritical fluid, so compared with normal liquid, its density has a greater correlation with temperature and pressure. For example: at 400 ℃, the pressure changes in the range of 0.22~2.5kPa, and the density of water can be reduced from 0.1g/cm³ to 0.84g/cm³.

## Viscosity

In the standard state, the viscosity of the liquid is 0.2 ~ 0.3Pa.s, the viscosity of gas is 0.01 ~ 0.03Pa.s, the viscosity of SCF is 0.01 ~ 0.03Pa.s, it can be seen that the viscosity of SCF is close to that of gas. Temperature and density are the main factors affecting viscosity.

Both the collision between molecules and the collision during the free translation of molecules can cause momentum transfer, and the combined effect of momentum transfer caused by these two collisions can reflect the fluid viscosity.
The temperature and density will affect the momentum transfer mode, thereby changing the fluid viscosity. The changes in supercritical fluid and liquid viscosity are different under the influence of temperature and density.

• Generally, the viscosity of a liquid decreases with increasing temperature;
• Under the condition of high density, the viscosity of supercritical fluid decreases with the increase in temperature;
• The results are reversed under low-density conditions.

## Diffusion coefficient

The diffusion coefficient of SCF is between gas and liquid, which is 10 to 100 times that of liquid at room temperature.

The diffusion coefficient is pressure and temperature dependent. However, the variation law and expression of the diffusion coefficient of normal fluid and SCF are different. Generally, the diffusion coefficient of a normal fluid increases with the pressure drop and is inversely proportional to the viscosity. The diffusion coefficient can be estimated according to the Stokes-Einstein relationship. The diffusion coefficient of a supercritical fluid increases with pressure. When the density is very high, the S-E relationship can be used, and it is known that a small pressure change can cause a large change in the diffusion coefficient, and the diffusion coefficient is inversely proportional to the viscosity.

## Surface Tension

Generally, all liquids have surface tension, but in the supercritical state, the surface tension of each fluid is approximately 0. This is because in the non-supercritical state, as the system approaches the critical point, the fluid two-phase interface gradually thickens and diffuses each other; when the critical point is reached, the two fluids will lose their respective characteristics and become homogeneous; when the system reaches the supercritical state, As the degree of interfacial diffusion increases, the interfacial tension gradually decreases until it disappears completely.

According to the experiment, as the two-phase temperature T in equilibrium reaches the critical temperature Tc, the interface thickness tends to 0 (C=0.61~0.67) according to the C power of (Tc~T), and the interfacial tension R according to (Tc~T) The L power tends to 0 (L=1 .22~ 1 .34).

## Dielectric constant

The dielectric constant of supercritical fluids is different from that of normal fluids. For example, the dielectric constant of methanol is 32.6 in the standard state, while in the supercritical state (such as 250°C, 20Mpa), its dielectric constant drops to 7.2.

The change of dielectric constant is related to density and temperature, it increases with the increase of density and decreases with the increase of temperature. And affected by the number of hydrogen bonds. For example, water has strong hydrogen bonds under normal conditions, so the dielectric constant is relatively large. As the temperature and pressure increase, the number of hydrogen bonds decreases, resulting in a significant decrease in the dielectric constant.

## Solubility properties

With respect to different solutes, supercritical fluids have different solubility under different temperature and pressure conditions. The solubility of supercritical fluid is related to its polarity and dielectric constant, so the solubility of supercritical fluid is significantly different from that of the normal liquid.

Generally, the supercritical temperature and pressure conditions close to the liquid density state are adopted, and its solubility is the highest, which is about 100 times that of the normal temperature and pressure conditions. For example, the solubility of water in supercritical and normal conditions is very different.

Compared with the normal state of supercritical methanol, the number of hydrogen bonds is reduced from 1.93 to less than 0.7, and the dielectric constant is also reduced, so its solubility properties undergo the following changes:

Under normal temperature and pressure, methanol and oil are incompatible with each other;
In the supercritical state, methanol and oil can be completely miscible.

In addition, the supercritical state solubility of polar solvents and non-polar solvents is selective for solutes. For example, supercritical CO2 solvents are non-polar when used alone, and generally have good solubility for fat-soluble substances with small molecular weights. The properties of the supercritical fluid can be improved by adding other solvents, that is, entraining agents, such as macromolecular polar solutes, which can dissolve the solutes when the supercritical CO2 fluid is injected into the polar solvent that entrains ethanol at the same time.

## Selectivity

Supercritical fluid has the property of selectively extracting different substances. There are often more than two different compound components in the same plant. Generally, a certain component is extracted separately, which requires selective extraction. Supercritical fluid can complete the separate separation of different components under different conditions such as temperature, pressure, and Co-solvent extract.

Different types of solvents have selectivity for solutes with different properties:

• Esters, ethers, and ketones are suitable for extraction with non-polar solvents;
• Glycosides, alkalis, sugars, and other solutes are suitable for extraction with polar solvents.

Supercritical CO2 cannot extract water at normal temperatures. When the temperature rises, the solubility increases and water can be extracted. When solutes have significant differences in molecular weight, vapor pressure, and polarity, fractional extraction can be performed.

## Thermal conductivity

Near the critical point, the thermal conductivity of a substance is very sensitive to changes in temperature and pressure.

• Under supercritical conditions, if the pressure is constant, as the temperature increases, the thermal conductivity first decreases to a minimum value, and then increases;
• If the temperature is constant, the thermal conductivity increases with increasing pressure. For convective heat transfer, including forced convection and natural convection, when the temperature and pressure are high, natural convection is easy to produce.

For example, when supercritical CO2 is at 38°C, only a temperature difference of 3°C can cause natural convection.