Supercritical Fluids
Learning Objective
- Discuss the properties of supercritical fluids.
Key Points
- Supercritical fluids have properties between those of a gas and a liquid.
- A supercritical fluid can effuse through solids like a gas and dissolve materials like a liquid.
- All supercritical fluids are completely miscible with each other, so for a mixture a single phase can be guaranteed, if the critical point of the mixture is exceeded.
Terms
- critical pressureThe pressure beyond which no phase boundaries exist for a given substance.
- supercritical fluidAny substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
- critical temperatureThe temperature beyond which no phase boundaries exist for a given substance.
- critical pointAlso known as a critical state, this point occurs under conditions (such as specific values of temperature, pressure, or composition) at which no phase boundaries exist.
Properties of Supercritical Fluids
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. This can be rationalized by thinking that at high enough temperatures (above the critical temperature) the kinetic energy of the molecules is high enough to overcome any intermolecular forces that would condense the sample into the liquid phase. On the other hand, high enough pressures (above the critical pressure) would not allow a sample to stay in the pure gaseous state. Therefore, a balance between these two tendencies is achieved and the substance exists in a state between a gas and a liquid.
In general terms, supercritical fluids have properties between those of a gas and a liquid. The critical properties of some substances used as solvents and as supercritical fluids are shown in Table 1. Table 2 shows density, diffusivity, and viscosity for typical liquids, gases, and supercritical fluids.

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.
All supercritical fluids are completely miscible with each other; therefore a single phase for a mixture can be guaranteed if the critical point is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
Tc(mix) = (mole fraction of A) x Tc(A) + (mole fraction of B) x Tc(B) 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.
Case Study: Carbon Dioxide
In the pressure-temperature phase diagram of CO2, the boiling 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. At well below the critical temperature, (e.g., 280 K), as the pressure increases, the gas compresses and eventually (at just over 40 bar) condenses into a much denser liquid, resulting in the discontinuity in the line (vertical dotted line). The system consists of 2 phases in equilibrium, a dense liquid and a low density gas.
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Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"Boundless."
http://www.boundless.com//chemistry/definition/critical-pressure
Boundless LearningCC BY-SA 3.0.
"Boundless."
http://www.boundless.com//chemistry/definition/critical-temperature
Boundless LearningCC BY-SA 3.0.
"Carbon dioxide pressure-temperature phase diagram."
https://en.wikipedia.org/wiki/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg
Wikimedia CommonsCC0 1.0 Universal.
"Phase Diagram For Pure Substance."
http://commons.wikimedia.org/wiki/File:Phase_diagram_for_pure_substance.JPG
Wikimedia CommonsCC BY-SA 3.0.
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