 Ionic liquid

An ionic liquid is a liquid composed of ions, conventionally a low-melting salt. Usage often restricts the term to salts with melting or glass-transition temperatures below $100{}^{\circ }\mathrm{C}$; a room-temperature ionic liquid must additionally be liquid at the stated room temperature. IUPAC notes the conventional restriction rather than making $100{}^{\circ }\mathrm{C}$ a physical discontinuity.^[ref]

A neat ionic liquid differs from a salt dissolved in water. The latter contains a molecular solvent as well as ions; a dry pure ionic liquid does not require that solvent to remain liquid. Its cation and anion need not be small monatomic species. Molecular shape, charge distribution and interactions affect the solid–liquid free-energy balance; the presence of electric charge alone does not determine melting temperature.

For an original number-density example, stipulate a pure 1:1 ionic liquid with molar mass $200\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and density $1.20\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. One litre contains $1200\mathrm{g}$, hence $6.00$ moles of formula units, or $6.00$ moles of each ionic species. The combined stoichiometric ion count is therefore $12.0$ moles per litre. That is a composition calculation, not an ideal-solution activity or a prediction of conductivity: ionic correlations and mobility matter.

Przemyslaw Kubisa's polymerization review gives a motivating use. An ionic medium can dissolve reactive components and affect polymerization kinetics or facilitate separation of polymer from a catalyst-containing phase.^[ref] “Ionic” does not by itself guarantee low viscosity, chemical inertness, environmental harmlessness or compatibility with every reaction. The selected ions, water content, temperature and intended measurement must be specified.