What are supercritical fluids like supercritical CO2, and why are they used as green solvents?

In one line

A supercritical fluid is a substance above its critical temperature and critical pressure, where it has liquid-like density (dissolving power) with gas-like viscosity and diffusivity, and supercritical CO₂ is a non-toxic, non-flammable replacement for organic solvents.

Explained simply

Normally, squeezing a gas turns it into liquid. But above a certain temperature (critical temperature) no amount of pressure makes a separate liquid; the gas and liquid become one fluid. This "in-between" state flows easily like a gas, gets into tiny pores, yet dissolves things like a liquid. With CO₂, you simply release pressure at the end, the CO₂ turns to gas and leaves no solvent residue.

How it works

1. Critical point: CO₂: T_c = 31.1 °C, P_c ≈ 73.8 bar (about 72.8 atm). Water: T_c = 374 °C, P_c ≈ 221 bar.
2. Above both, the substance is supercritical: no liquid–gas boundary or surface tension.
3. Solvent power can be tuned by changing pressure and temperature (density); small pressure changes cause big changes in solubility, which allows selective extraction.
4. After use, lowering pressure turns CO₂ back to gas; the extracted product separates, and CO₂ is recycled.

Characteristics of supercritical CO₂:

• Non-toxic, non-flammable, chemically inert, cheap and abundant (often recovered as a by-product).
• Low critical temperature: suitable for heat-sensitive substances (food, flavours, drugs).
• Low viscosity and high diffusivity: fast mass transfer and penetration.
• Leaves no solvent residue; easy separation.
• Limitation: non-polar, so it dissolves polar substances poorly unless a co-solvent (e.g., small amount of ethanol) is added; requires high-pressure equipment.

Key formula

No formula. Condition: \( T > T_c \) and \( P > P_c \).

Quick example

Applications:

• Decaffeination of coffee and tea: scCO₂ extracts caffeine selectively instead of dichloromethane.
• Extraction of hop flavour, essential oils, spices, and natural products.
• Dry cleaning: replaces perchloroethylene (a suspected carcinogen).
• Reaction medium for polymerisation and hydrogenation; supercritical fluid chromatography.
• Supercritical water oxidation for destroying hazardous waste.

Common mistakes

• Assuming scCO₂ processes add new CO₂ to the air. The CO₂ used is usually recovered as a by-product of other industries and is recycled within the process.
• Giving only one condition (temperature or pressure); both must exceed critical values.
• Saying supercritical fluids are liquids. They are a single fluid phase with mixed properties.

How MU asks it

"What are supercritical fluids? Give characteristics and applications of supercritical CO₂" (5 marks). For full marks: definition with a phase diagram showing the critical point and supercritical region, critical constants of CO₂, 4–5 characteristics, 3–4 applications, and why it is green.