Matter exists in three forms: gas, liquid and solid, and can be transformed into phase under different pressures and temperatures. When the temperature is higher than a certain value, any large pressure can not convert the pure substance from gas phase to liquid phase, and the temperature at this time is called the critical temperature Tc; At the critical temperature, the minimum pressure at which the gas can be liquefied is called the critical pressure Pc.
The solvent strength of supercritical fluid depends on the extraction temperature and pressure. With this feature, it is only necessary to change the pressure and temperature of the extractant fluid to extract different components of the sample successively according to their solubility in the fluid. Under low pressure, substances with weak polarity are extracted first. With the increase of pressure, substances with large polarity and large molecular weight and their basic properties are extracted. Therefore, supercritical extraction is carried out under program pressure to obtain different extraction components, which can also play a role in separation.
The change of temperature is reflected in two factors that affect the density of the extractant and the vapor pressure of the solute. In the low temperature area (still above the critical temperature), the increase of temperature reduces the density of the fluid, while the increase of the vapor pressure of the solute is not much. Therefore, the increase of temperature during the dissolution of the extractant can separate the solute from the fluid extractant. When the temperature further increases to the high temperature area, although the density of the extractant further decreases, the vapor pressure of the solute increases, With the increase of volatility, the extraction rate will not decrease but will increase.
In addition to pressure and temperature, a small amount of other solvents can also change the solubility of supercritical fluids to solutes. The mechanism of action has not been fully understood yet. Generally, the amount added shall not exceed 10%, and most of them are polar solvents such as methanol and isopropanol. Adding a small amount of polar solvent can further expand the application scope of supercritical fluid extraction technology to the compounds with larger polarity.
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