definition
Supercritical fluid extraction (SPE) is a novel extraction technique that utilizes gas under supercritical conditions as an extractant to extract and separate certain components from liquids or solids.
Gas under supercritical conditions, also known as supercritical fluid (SF), is a substance that exists in fluid form above the critical temperature (Tc) and critical pressure (Pc). Usually, there are carbon dioxide (CO2), nitrogen (N2), nitrous oxide (N2O), ethylene (C2H4), trifluoromethane (CHF3), etc.
Basic principles
The basic principle of supercritical fluid extraction is that when gas is in a supercritical state, it becomes a single phase state with properties between liquid and gas, with a density similar to that of liquid, a viscosity higher than that of gas but significantly lower than that of liquid, and a diffusion coefficient of 10-100 times that of liquid. Therefore, it has good permeability and strong solubility for materials, and can extract certain components from materials. And the density and dielectric constant of supercritical fluid increase with the increase of pressure in the closed system, and the polarity increases. By using program boosting, different polarity components can be extracted separately. After the extraction is completed, the temperature or pressure of the system is changed to make the supercritical fluid turn into ordinary gas and escape. The extracted components in the material can be completely or almost completely precipitated, achieving the purpose of extraction and separation.
The four states of matter (solid, liquid, gas, and supercritical) (as well as other forms) change with its temperature and pressure. Taking CO2 as an example, at the triple point, the temperature T (tr) at which solid, liquid, and gas coexist is -56.4 ℃ (217K), and the pressure P (tr) is 5.2 × 105Pa. The vapor pressure line of CO2 terminates at the critical point C (Tc=31.3 ℃, Pc=7.38 × 106Pa, ρ c=0.47 g/cm3). Beyond the critical point, the interface between the liquid and gas phases disappears, becoming a supercritical fluid (SF). The diffusion coefficient of SF (10-1~10-4cm2/s) is one order of magnitude higher than that of general liquids (10-2~10-5cm2/s), while its viscosity (10-2~10-4N · s/m2) is one order of magnitude lower than that of general liquids (10-1~10-3N · s/m2). Compared with liquid-liquid extraction systems, SF systems have faster mass transfer and extraction speeds. Therefore, it can effectively penetrate into the gaps of solid samples for extraction and separation. The density of SF changes with temperature and pressure, leading to changes in its solubility parameter. At lower densities, the solubility parameter of SF-CO2 is close to that of hexane; At higher densities, it can approach chloroform. Therefore, controlling the density (temperature and pressure) of SF can achieve the required solvent strength. This ability allows SF to freely change the solvent strength and adapt to different solutes. Generally speaking, SF can effectively dissolve non-polar solids and selectively extract solutes based on their polarity, which has a wide range of applications in the fields of separation and analytical chemistry.
CO2 has a lower critical temperature and pressure, and is inexpensive, non-toxic, and has lower activity. Therefore, SF-CO2 is often used to extract non-polar and slightly polar substances.
In the supercritical state, fluids have the dual characteristics of both gas and liquid phases, with high diffusion coefficient and low viscosity comparable to gases, as well as density similar to liquids and good solubility for substances. Its density is highly sensitive to temperature and pressure changes, and is proportional to its solubility within a certain pressure range. Therefore, the solubility of a substance can be changed by controlling temperature and pressure. Supercritical fluid has been used for the extraction, synthesis, analysis, and processing of drugs.