A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid.
While we typically think of fluids as either a liquid or a gas, by manipulating pressure and temperature we can take a fluid (carbon dioxide, in our case) past its “critical” phase-change point to create a “supercritical” fluid — a unique state that exhibits the properties of both liquid and gas.
These properties make supercritical fluids extremely useful in extracting certain substances — flavors, active ingredients, essential oils — while leaving others behind. In its supercritical state, CO2 can diffuse through solids like a gas, and dissolve materials like a liquid. By precisely controlling variables like temperature, pressure and flow rate we can fine-tune the extraction process, customizing it to best suit the compounds we’re extracting, whether it’s caffeine from coffee or nicotine from tobacco.
Supercritical Fluid Extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids. SFE can be used as a sample preparation step for analytical purposes, or on a larger scale to either strip unwanted material from a product (e.g. decaffeination) or collect a desired product (e.g. essential oils).
Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation, respectively.
Carbon dioxide (CO2) is the most common supercritical fluid, sometimes modified by co-solvents such as ethanol or methanol. Extraction conditions for supercritical CO2 are above the critical temperature of 31°C and critical pressure of 74 bar. However, the addition of modifiers may slightly alter this. The discussion below will mainly refer to extraction with CO2, except where specified.
The Critical Point (C) is marked at the end of the gas-liquid equilibrium curve, and the shaded area indicates the supercritical fluid region. It can be shown that by using a combination of isobaric changes in temperature with isothermal changes in pressure, it is possible to convert a pure component from a liquid to a gas (and vice versa) via the supercritical region without incurring a phase transition.
The behavior of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behavior approaches that of the liquid phase while penetration into a solid matrix is facilitated by the gas-like transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than conventional extraction processes. Furthermore, the extraction conditions can be controlled to effect a selected separation. Supercritical fluid extraction is known to be dependent on the density of the fluid that in turn can be manipulated through control of the system pressure and temperature. The dissolving power of a SCF increases with isothermal increase in density or an Isopycnic (constant density) increase in temperature. In practical terms, this means that a SCF can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extracted material is negligible.
The basic principle of SCF extraction is that the solubility of a given compound (solute) in a solvent varies with both temperature and pressure. At ambient conditions (25°C and 1 bar) the solubility of a solute in a gas is usually related directly to the vapor pressure of the solute and is generally negligible. In a SCF, however, solute solubilities of up to 10 orders of magnitude greater than those predicted by ideal gas law behavior have been reported.
The dissolution of solutes in supercritical fluids results from a combination of vapor pressure and solute-solvent interaction effects. The impact of this is that the solubility of a solid solute in a supercritical fluid is not a simple function of pressure.
Although the solubility of volatile solids in SCFs is higher than in an ideal gas, it is often desirable to increase the solubility further in order to reduce the solvent required for processing. The solubility of components in SCFs can be enhanced by the addition of a substance referred to as an entrainer, or co-solvent. The Volatility of this additional component is usually intermediate to that of the SCF and the solute. The addition of a co-solvent provides a further dimension to the range of solvent properties in a given system by influencing the chemical nature of the fluid.
Co-solvents also provide a mechanism by which the extraction selectivity can be manipulated. The commercial potential of a particular application of SCF technology can be significantly improved through the use of co-solvents. A factor that must be taken into consideration when using co-solvents, however, is that even the presence of small amounts of an additional component to a primary SCF can change the critical properties of the resulting mixture considerably.