Supercritical fluid
A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous solids like a gas, overcoming the mass transfer limitations that slow liquid transport through such materials. SCFs are superior to gases in their ability to dissolve materials like liquids or solids. Near the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned".
Supercritical fluids occur in the atmospheres of the gas giants Jupiter and Saturn, the terrestrial planet Venus, and probably in those of the ice giants Uranus and Neptune. Supercritical water is found on Earth, such as the water issuing from black smokers, a type of hydrothermal vent. SCFs are used as a substitute for organic solvents in a range of industrial and laboratory processes, most commonly carbon dioxide for decaffeination and water for steam boilers for power generation. Some substances are soluble in the supercritical state of a solvent but insoluble in the gaseous or liquid state—or vice versa. This can be used to extract a substance and transport it elsewhere in solution before depositing it in the desired place by allowing or inducing a phase transition in the solvent.
Properties
Supercritical fluids generally have properties between those of a gas and a liquid. In Table 1, the critical properties are shown for some substances that are commonly used as supercritical fluids.| Solvent | Molecular mass | Critical temperature | Critical pressure | Critical density |
| Carbon dioxide | 44.01 | 304.1 | 7.38 | 0.469 |
| Water | 18.015 | 647.096 | 22.064 | 0.322 |
| Methane | 16.04 | 190.4 | 4.60 | 0.162 |
| Ethane | 30.07 | 305.3 | 4.87 | 0.203 |
| Propane | 44.09 | 369.8 | 4.25 | 0.217 |
| Ethylene | 28.05 | 282.4 | 5.04 | 0.215 |
| Propylene | 42.08 | 364.9 | 4.60 | 0.232 |
| Methanol | 32.04 | 512.6 | 8.09 | 0.272 |
| Ethanol | 46.07 | 513.9 | 6.14 | 0.276 |
| Acetone | 58.08 | 508.1 | 4.70 | 0.278 |
| Nitrous oxide | 44.013 | 306.57 | 7.35 | 0.452 |
Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.
| Density | Viscosity | Diffusivity | |
| Gases | 1 | 10 | 1–10 |
| Supercritical fluids | 100–1000 | 50–100 | 0.01–0.1 |
| Liquids | 1000 | 500–1000 | 0.001 |
Also, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary. By changing the pressure and temperature of the fluid, the properties can be "tuned" to be more liquid-like or more gas-like. One of the most important properties is the solubility of material in the fluid. Solubility in a supercritical fluid tends to increase with density of the fluid. Since density increases with pressure, solubility tends to increase with pressure. The relationship with temperature is a little more complicated. At constant density, solubility will increase with temperature. However, close to the critical point, the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again.
Mixtures
Typically, supercritical fluids are completely miscible with each other, so that a binary mixture forms a single gaseous phase if the critical point of the mixture is exceeded. However, exceptions are known in systems where one component is much more volatile than the other, which in some cases form two immiscible gas phases at high pressure and temperatures above the component critical points. This behavior has been found in systems such as N2-NH3, NH3-CH4, SO2-N2 and n-butane-H2O.The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
where denotes the mole fraction of component.
For greater accuracy, the critical point can be calculated using equations of state, such as the Peng–Robinson, or group-contribution methods. Other properties, such as density, can also be calculated using equations of state.
Phase diagram
Figures 1 and 2 show two-dimensional projections of a phase diagram. In the pressure-temperature phase diagram the boiling curve separates the gas and liquid region and ends in the critical point, where the liquid and gas phases disappear to become a single supercritical phase.The appearance of a single phase can also be observed in the density-pressure phase diagram for carbon dioxide. At well below the critical temperature, e.g., 280 K, as the pressure increases, the gas compresses and eventually condenses into a much denser liquid, resulting in the discontinuity in the line. The system consists of 2 phases in equilibrium, a dense liquid and a low density gas. As the critical temperature is approached, the density of the gas at equilibrium becomes higher, and that of the liquid lower. At the critical point, there is no difference in density, and the two phases become one fluid phase. Thus, above the critical temperature a gas cannot be liquefied by pressure. At slightly above the critical temperature, in the vicinity of the critical pressure, the line is almost vertical. A small increase in pressure causes a large increase in the density of the supercritical phase. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave more like an ideal gas, with a more linear density/pressure relationship, as can be seen in Figure 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure.
Many pressurized gases are actually supercritical fluids. For example, nitrogen has a critical point of and. Therefore, nitrogen in a gas cylinder above this pressure is actually a supercritical fluid. These are more often known as permanent gases. At room temperature, they are well above their critical temperature, and therefore behave as a nearly ideal gas, similar to CO2 at 400 K above. However, they cannot be liquified by mechanical pressure unless cooled below their critical temperature, requiring gravitational pressure such as within gas giants to produce a liquid or solid at high temperatures. Above the critical temperature, elevated pressures can increase the density enough that the SCF exhibits liquid-like density and behaviour. At very high pressures, an SCF can be compressed into a solid because the melting curve extends to the right of the critical point in the P/T phase diagram. While the pressure required to compress supercritical CO2 into a solid can be, depending on the temperature, as low as 570 MPa, that required to solidify supercritical water is 14,000 MPa.
The Fisher–Widom line, the Widom line, or the Frenkel line are thermodynamic concepts that allow to distinguish liquid-like and gas-like states within the supercritical fluid.
History
In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.In recent years, a significant effort has been devoted to investigation of various properties of supercritical fluids. Supercritical fluids have found application in a variety of fields, ranging from the extraction of floral fragrance from flowers to applications in food science such as creating decaffeinated coffee, functional food ingredients, pharmaceuticals, cosmetics, polymers, powders, bio- and functional materials, nano-systems, natural products, biotechnology, fossil and bio-fuels, microelectronics, energy and environment. Much of the excitement and interest of the past decade is due to the enormous progress made in increasing the power of relevant experimental tools. The development of new experimental methods and improvement of existing ones continues to play an important role in this field, with recent research focusing on dynamic properties of fluids.