The basic principle of supercritical extraction
1. Extractant
The extractant used in supercritical extraction is supercritical fluid.
Supercritical fluid is a state of matter between gas and liquid that is neither gas nor liquid. This substance can only exist when its temperature and pressure exceed the critical point.
The density of supercritical fluid is higher, similar to liquid, and its viscosity is closer to gas. Therefore, supercritical fluid is a very ideal extractant.
2. The solvent strength of supercritical fluid depends on the temperature and pressure of extraction
Using this feature, only by changing the pressure and temperature of the extractant fluid, different components in the sample can be extracted successively according to their solubility in the fluid.
(1) The weakly polar substances are first extracted under low pressure. With the increase of pressure, the substances with larger polarity and large molecular weight have basic properties. Therefore, the supercritical extraction of different extraction components is carried out under the program boost pressure. Play the role of separation.
(2) The temperature change is reflected in two factors that affect the density of the extractant and the vapor pressure of the solute. In the low temperature region (still above the critical temperature), the increase in temperature reduces the density of the fluid, while the vapor pressure of the solute does not increase much. Therefore, the extraction The increase in the solubility of the solvent can make the solute separate from the fluid extractant. When the temperature further rises to the high temperature zone, although the density of the extractant is further reduced, the vapor pressure of the solute increases and the volatility increases, so the extraction rate will not decrease. Instead, there is an increasing trend.
(3) In addition to pressure and temperature, adding a small amount of other solvents to the supercritical fluid can also change its ability to dissolve solutes. Its mechanism of action has not yet been fully understood. Usually the addition amount does not exceed 10%, and polar solvents such as methanol and isopropanol are mostly used. Adding a small amount of polar solvent can further expand the scope of application of supercritical extraction technology to more polar compounds.