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What is a supercritical fluid?

Supercritical fluid

supercritical fluid or better known as FSC , is any substance that is under some conditions of pressure and temperature that become higher than its critical point and that behaves as “a hybrid between a liquid and a gas”, this wants say that it can diffuse as a gas, a process known as effusion , and can then dissolve substances as a liquid or as a solvent. FSC or supercritical fluids are characterized by having a wide range of densities that they can adopt. Above critical conditions, small changes in pressure and temperature produce large changes in density. Supercritical fluids are formed when a substance is under conditions of pressure and temperature above its critical point.

Supercritical fluid 

What is a supercritical fluid?

This type of fluids refers to any type of substances that are found under some specific pressure and temperature conditions and that manage to become higher when they reach their critical point acting in a similar way to a hybrid between a liquid and a gas acting as a type of liquid solvent .

  • History of supercritical fluid
  • Properties of a supercritical fluid
  • Supercritical Fluid Extraction (SFE)
  • Supercritical fluid applications
  • Supercritical CO2

History of  supercritical fluid

It was the Irish scientist named Thomas Andrews who, during the years from 1813 to 1885, in the studies he carried out with CO2 under pressure by subjecting them to different degrees of temperatures, recognized in the year 1869, for the first time, the existence of the critical point which is located at 31.1 ° C. He was able to observe in his experiment that at this point the boundary between gas and liquid disappeared. Andrews then suggested that a different critical temperature existed for each gas that existed. In 1879, Hannay and Hogart measured the solubility of different solids in supercritical fluids.

Properties of a  supercritical fluid

In general and scientific terms, a supercritical fluid has characteristics and properties between those of a gas and those of a liquid . There is no gas-to-liquid interface. Its isothermal compressibility becomes positive and its coefficient of expansion is infinite. If the critical density remains constant and in the same way as the density, the heat capacity goes to a constant volume and tends to infinity. The density above the critical point depends mainly on pressure and temperature, but nevertheless, in some cases, it is closer to that of liquid components than to that of gases. the densityit can increase when the pressure at constant temperature also increases, and the temperature decreases at constant pressure. Its viscosity is much lower than that of liquids, and this makes it have very positive hydrodynamic properties . Its low surface tension also allows high penetrability through porous solids.

Supercritical Fluid Extraction  (SFE)

FSC or supercritical fluids have several advantages in extraction processes , since by being able to behave as a liquid it facilitates the dissolution of solutes, while, at the same time, its behavior as a gas allows an easy separation of the matrix. This directs us towards a faster, more efficient and selective extraction process, as opposed to having to carry out a liquid-liquid extraction. In the same way, some ” green solvents ” such as CO2 can be used, thus avoiding the use of the usual organic solvents for liquid-liquid extractions. The instrumentation required to perform supercritical fluid extraction is primarily based on aa high pressure pump , an extraction cell , a zone in which fluid decompression occurs , and a suitable analyte collection system . Temperatures vary between 30 and 150 ° C and the size of the extraction cells can vary considerably. The extraction can be carried out statically or dynamically . In static form, the extraction cell undergoes pressurization. In the dynamic model, the fluid is allowed to flow continuously through extraction cells.

Supercritical fluid applications 

The first applications of supercritical fluids occurred in the industrial field in the late 1970s. The area for the analysis of pesticides has been one of the areas where the SFE technique has aroused the greatest interest. The incredible properties of these fluids have already been fully exploited in the extraction of certain pesticides from soils through quick, simple and cheap analyzes, with minimal environmental impact and with many possibilities for automation and miniaturization.

Supercritical CO2

One more advantage of the SFE technique occurs when the fluid that is used has the property of being a gas at atmospheric pressure , such as CO2 . In the case of carbon dioxide, in addition to being a gas at atmospheric pressure, it also has practical advantages since it is inert, non-toxic, cheap and can acquire a high degree of purity. Its only disadvantage compared to other fluids is its nonpolar character , which means that it may not be suitable enough to extract high priority analytes.

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