Properties
In general terms, supercritical fluids have properties between those of a gas and a liquid. In Table 1, the critical properties are shown for some components, which are commonly used as supercritical fluids.
| Solvent | Molecular weight | Critical temperature | Critical pressure | Critical density |
|---|---|---|---|---|
| g/mol | K | MPa (atm) | g/cm3 | |
| Carbon dioxide (CO2) | 44.01 | 304.1 | 7.38 (72.8) | 0.469 |
| Water (H2O) (acc. IAPWS) | 18.015 | 647.096 | 22.064 (217.755) | 0.322 |
| Methane (CH4) | 16.04 | 190.4 | 4.60 (45.4) | 0.162 |
| Ethane (C2H6) | 30.07 | 305.3 | 4.87 (48.1) | 0.203 |
| Propane (C3H8) | 44.09 | 369.8 | 4.25 (41.9) | 0.217 |
| Ethylene (C2H4) | 28.05 | 282.4 | 5.04 (49.7) | 0.215 |
| Propylene (C3H6) | 42.08 | 364.9 | 4.60 (45.4) | 0.232 |
| Methanol (CH3OH) | 32.04 | 512.6 | 8.09 (79.8) | 0.272 |
| Ethanol (C2H5OH) | 46.07 | 513.9 | 6.14 (60.6) | 0.276 |
| Acetone (C3H6O) | 58.08 | 508.1 | 4.70 (46.4) | 0.278 |
Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.
| Density (kg/m3) | Viscosity (µPa∙s) | Diffusivity (mm²/s) | |
|---|---|---|---|
| Gases | 1 | 10 | 1–10 |
| Supercritical Fluids | 100–1000 | 50–100 | 0.01–0.1 |
| Liquids | 1000 | 500–1000 | 0.001 |
In addition, 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- 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 (at constant temperature). 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.
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. 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 A) x TcA + (mole fraction B) x TcB.
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.
Read more about this topic: Supercritical Fluid
Famous quotes containing the word properties:
“A drop of water has the properties of the sea, but cannot exhibit a storm. There is beauty of a concert, as well as of a flute; strength of a host, as well as of a hero.”
—Ralph Waldo Emerson (1803–1882)
“The reason why men enter into society, is the preservation of their property; and the end why they choose and authorize a legislative, is, that there may be laws made, and rules set, as guards and fences to the properties of all the members of the society: to limit the power, and moderate the dominion, of every part and member of the society.”
—John Locke (1632–1704)