Editing Supercritical fluid (section)

==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.

{|class="wikitable" style="text-align:center"
|+Table 1. Critical properties of various solvents <ref>{{cite book |last1=Reid |first1=Robert C. |last2=Sherwood |first2=Thomas Kilgore |last3=Prasnitz |first3=J. M |last4=Poling |first4=Bruce E. |title=The Properties of Gases and Liquids |date=1987 |publisher=McGraw-Hill |isbn=9780070517998 |edition=4th |url=https://books.google.com/books?id=AcRTAAAAMAAJ}}</ref>
|-
! rowspan="2" | Solvent !! Molecular mass !! Critical temperature !! Critical pressure !! Critical density
|-
! g/mol !! [[Kelvin|K]] !! [[Pascal (unit)|MPa]] ([[Atmosphere (unit)|atm]]) !! g/cm<sup>3</sup>
|-
| [[Carbon dioxide]] (CO<sub>2</sub>)
| 44.01 || 304.1 || 7.38 (72.8) || 0.469
|-
| [[water (molecule)|Water]] (H<sub>2</sub>O)<sup>†</sup>
| 18.015 || 647.096 || 22.064 (217.755) || 0.322
|-
| [[Methane]] (CH<sub>4</sub>)
| 16.04 || 190.4 || 4.60 (45.4) || 0.162
|-
| [[Ethane]] (C<sub>2</sub>H<sub>6</sub>)
| 30.07 || 305.3 || 4.87 (48.1) || 0.203
|-
| [[Propane]] (C<sub>3</sub>H<sub>8</sub>)
| 44.09 || 369.8 || 4.25 (41.9) || 0.217
|-
| [[Ethylene]] (C<sub>2</sub>H<sub>4</sub>)
| 28.05 || 282.4 || 5.04 (49.7) || 0.215
|-
| [[Propylene]] (C<sub>3</sub>H<sub>6</sub>)
| 42.08 || 364.9 || 4.60 (45.4) || 0.232
|-
| [[Methanol]] (CH<sub>3</sub>OH)
| 32.04 || 512.6 || 8.09 (79.8) || 0.272
|-
| [[Ethanol]] (C<sub>2</sub>H<sub>5</sub>OH)
| 46.07 || 513.9 || 6.14 (60.6) || 0.276
|-
| [[Acetone]] (C<sub>3</sub>H<sub>6</sub>O)
| 58.08 || 508.1 || 4.70 (46.4) || 0.278
|-
| [[Nitrous oxide]] (N<sub>2</sub>O)
| 44.013 || 306.57 || 7.35 (72.5) || 0.452
|}

<small>†Source: International Association for Properties of Water and Steam ([http://www.iapws.org IAPWS])</small><ref>{{cite web|url=http://www.iapws.org/|title=International Association for the Properties of Water and Steam|website=www.iapws.org|access-date=2020-01-20}}</ref>

Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.

{|class="wikitable" style="text-align:center"
|+Table 2. Comparison of gases, supercritical fluids, and liquids<ref>{{cite web |url=http://sfe.kkft.bme.hu/en/current-research.html |title=What is a supercritical fluid? |access-date=2014-06-26 |author=Edit Székely |publisher=Budapest University of Technology and Economics |archive-url=https://web.archive.org/web/20160108021936/http://sfe.kkft.bme.hu/en/current-research.html |archive-date=2016-01-08 |url-status=dead }}</ref>
!  !! Density (kg/m<sup>3</sup>) !! Viscosity ([[Dynamic viscosity|μPa·s]]) !! Diffusivity (mm<sup>2</sup>/s)
|-
! 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 (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.<ref>{{cite web |url=http://eng.ege.edu.tr/~otles/SupercriticalFluidsScienceAndTechnology/Wc488d76f2c655.htm|title= Supercritical Fluid Extraction, Density Considerations|access-date=2007-11-20 }}</ref>

===Mixtures===
Typically, supercritical fluids are completely [[Miscibility|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 for example in the systems N<sub>2</sub>-NH<sub>3</sub>, NH<sub>3</sub>-CH<sub>4</sub>, SO<sub>2</sub>-N<sub>2</sub> and n-butane-H<sub>2</sub>O.<ref>{{cite journal |last1=Gordon |first1=R. P. |title=A Supercritical Phase Separation |journal=Journal of Chemical Education |date=1972 |volume=49 |issue=4 |pages=249–252 |doi=10.1021/ed049p249 }}</ref>

The critical point of a binary mixture can be estimated as the [[arithmetic mean]] of the critical temperatures and pressures of the two components,
{{block indent|1=''T''<sub>c(mix)</sub> = ''χ''<sub>''A''</sub> × ''T''<sub>c(''A'')</sub> + ''χ''<sub>''B''</sub> × ''T''<sub>c(''B'')</sub>}}
where ''χ''<sub>''i''</sub> denotes the [[mole fraction]] of component ''i''.

For greater accuracy, the critical point can be calculated using [[equations of state]], such as the [[Peng–Robinson equation of state|Peng–Robinson]], or [[group-contribution method]]s. Other properties, such as density, can also be calculated using equations of state.<ref>{{cite web | url = http://www.criticalprocesses.com/Calculation%20of%20density,%20enthalpy%20and%20entropy%20of%20carbon%20dioxide.htm | title = Calculation of Thermodynamic Properties of CO<sub>2</sub> using Peng–Robinson equation of state | access-date = 2007-11-20 | author = A. A. Clifford | date = 2007-12-04 | publisher = Critical Processes Ltd | url-status = dead | archive-url = https://web.archive.org/web/20080505235710/http://www.criticalprocesses.com/Calculation%20of%20density,%20enthalpy%20and%20entropy%20of%20carbon%20dioxide.htm | archive-date = 2008-05-05 }}</ref>