A supercritical fluid (SCF) is a state of matter that occurs when a substance is at a temperature and pressure above its critical point. In this state, there is no distinction between the liquid and gas phases. The SCF has unique properties, combining aspects of both gases (like low viscosity and high diffusivity) and liquids (like high density and solvent power).
Properties of Supercritical Fluids
| Property | Gas | Supercritical Fluid | Liquid |
|---|---|---|---|
| Density (g/cm3) | ~0.001 | 0.2–0.8 | ~1 |
| Viscosity (g·cm-1·s-1) | 10-4 | 10-3 | 10-2 |
| Diffusion Coefficient (cm2/s) | 10-1 | 10-3 | 10-6 |
| Surface Tension | None | None | Present |
| Solubility (for solids/liquids) | Low | Tunable, Efficient | High |
Examples and Critical Data
| Substance | Critical Temperature (°C) | Critical Pressure (MPa) |
|---|---|---|
| CO2 | 31.0 | 7.38 |
| H2O | 374 | 22.1 |
| NH3 | 132.4 | 11.3 |
Applications
- Extraction of caffeine from coffee (using supercritical CO2).
- Supercritical fluid chromatography (SFC) for analytical separations.
- Nanomaterial synthesis and cleaning of delicate electronics.
- Green solvent for pharmaceuticals, food, and perfume industries.
Supercritical fluids are valued for their tunable properties and environmental friendliness, especially as alternatives to traditional organic solvents.
Test Your Knowledge
1. A supercritical fluid exists when a substance is at a temperature and pressure:
a) Below its triple point.
b) Above its critical point.
c) Between its boiling point and critical point.
d) Below its critical temperature but above its critical pressure.
2. Which of the following properties is characteristic of a Supercritical Fluid (SCF)?
a) High surface tension, low density.
b) Low diffusivity, high viscosity.
c) Tunable density and solvent power, zero surface tension.
d) Fixed properties regardless of pressure changes.
3. Compared to a liquid, a Supercritical Fluid typically has:
a) Lower density but much higher viscosity.
b) Higher surface tension and lower diffusivity.
c) Lower viscosity and higher diffusion coefficient.
d) Similar viscosity and similar diffusion coefficient.
4. What is the primary industrial application mentioned that utilizes Supercritical CO2?
a) Petroleum refining.
b) Decaffeination of coffee.
c) Water purification via reverse osmosis.
d) Steel production.
5. What is the approximate critical temperature (Tc) for Carbon Dioxide (CO2)?
a) 100°C
b) 374°C
c) 0°C
d) 31°C
Answers:
1. b) Above its critical point.
2. c) Tunable density and solvent power, zero surface tension.
3. c) Lower viscosity and higher diffusion coefficient. (This combination allows SCFs to penetrate porous materials quickly.)
4. b) Decaffeination of coffee.
5. d) 31°C