Discuss super critical solvents with its applications.

A supercritical fluid (SCF) is a substance held above its critical temperature (T_c) and critical pressure (P_c). There it exists as a single phase with a liquid-like density and dissolving power and a gas-like low viscosity and high diffusivity, with zero surface tension. Its solvent power can be tuned by changing P and T.

The most common is supercritical CO₂ (T_c = 31.1 °C, P_c = 72.8 atm). Supercritical water (T_c = 374 °C, P_c = 218 atm) is also used.

Characteristics that make them green: non-toxic, non-flammable, cheap and abundant, recyclable. The solvent separates from the product simply by releasing the pressure, so no solvent residue is left.

Applications

• Decaffeination of coffee and tea.
• Extraction of essential oils, flavours, fragrances, hops and natural pigments.
• Dry cleaning of clothes in place of perchloroethylene.
• A reaction medium for polymerisation, hydrogenation and oxidation.
• Cleaning of precision parts and electronics; producing fine particles of drugs.
• Supercritical water oxidation to destroy hazardous waste.