What are ionic liquids and why are they called green, designer solvents?

In one line

Ionic liquids are salts that are liquid below 100 °C (many at room temperature), made of bulky organic cations and inorganic or organic anions, and their negligible vapour pressure and tunable properties make them useful green solvents.

Explained simply

Table salt (NaCl) melts at 801 °C because small Na⁺ and Cl⁻ ions pack tightly. If you replace them with big, lopsided ions, they cannot pack neatly, so the salt stays liquid even at room temperature. Because it is made only of ions, it hardly evaporates, so it does not pollute the air the way volatile solvents like benzene or chloroform do.

How it works

1. Cations (bulky, organic): 1-alkyl-3-methylimidazolium (e.g., [BMIM]⁺, 1-butyl-3-methylimidazolium), N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium.
2. Anions: Cl⁻, Br⁻, BF₄⁻, PF₆⁻, CH₃COO⁻, (CF₃SO₂)₂N⁻.
3. Large size, low symmetry and charge delocalisation weaken the lattice energy, lowering the melting point.
4. Changing the cation, its alkyl chain length or the anion changes polarity, water miscibility, viscosity and solubility of solutes, hence "designer solvents".

Characteristics:

• Negligible vapour pressure: no evaporation loss, no VOC emission.
• Non-flammable and thermally stable over a wide liquid range (often up to 300 °C or more).
• Dissolve a wide range of organic, inorganic and polymeric materials (even cellulose).
• Good ionic conductivity and wide electrochemical window.
• Recyclable: products are separated by extraction or distillation and the ionic liquid is reused.
• Limitations: costly, often viscous, and some are toxic or poorly biodegradable, so each one's greenness must be checked.

Key formula

No formula. Example: [BMIM][BF₄] = 1-butyl-3-methylimidazolium tetrafluoroborate.

Quick example

Applications:

• Reaction media for Diels–Alder, Friedel–Crafts and hydrogenation reactions, often acting as both solvent and catalyst.
• Dissolving cellulose (e.g., [BMIM]Cl) for making fibres and processing biomass.
• Electrolytes in batteries, supercapacitors, solar cells and electroplating.
• Extraction and separation (metal ions, desulphurisation of fuels), CO₂ capture.
• Industrial example: BASF's BASIL process uses an imidazole base to scavenge acid, forming an ionic liquid that separates as a liquid phase and is easily removed.

Common mistakes

• Saying ionic liquids are solutions of salts in water; they are pure molten salts.
• Claiming all ionic liquids are non-toxic and biodegradable.
• Defining them with "liquid at room temperature" only; the accepted cut-off is melting point below 100 °C.

How MU asks it

"What are ionic liquids? Explain their characteristics and applications as green solvents" (5 marks), or "Compare supercritical fluids and ionic liquids" (3–5 marks). For full marks: definition with the 100 °C criterion, examples of cations and anions with one full name, why melting point is low, 4–5 characteristics and 3–4 applications.