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Concept
Ionic liquid
Summary
An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.
Ionic liquids have many potential applications. They are powerful solvents and can be used as electrolytes. Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been considered as sealants due to their very low vapor pressure.
Any salt that melts without decomposing or vaporizing usually yields an ionic liquid. Sodium chloride (NaCl), for example, melts at into a liquid that consists largely of sodium cations (Na+) and chloride anions (Cl-). Conversely, when an ionic liquid is cooled, it often forms an ionic solid—which may be either crystalline or glassy.
The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids. Because of these strong interactions, salts tend to have high lattice energies, manifested in high melting points. Some salts, especially those with organic cations, have low lattice energies and thus are liquid at or below room temperature. Examples include compounds based on the 1-ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIMAc (acetate anion), EMIM dicyanamide, (C2H5)(CH3)C3H3N2+·N(CN)2-, that melts at ; and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below .
Low-temperature ionic liquids can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvents, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds. Certain mixtures of nitrate salts can have melting points below 100 °C.
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Molten salt
Molten salt is salt which is solid at standard temperature and pressure but has become liquid due to elevated temperature. Regular table salt has a melting point of 801 °C (1474 °F) and a heat of fusion of 520 J/g. A salt that is normally liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids. Molten salts have a variety of uses. Molten chloride salt mixtures are commonly used as quenching baths for various alloy heat treatments, such as annealing and martempering of steel.
Liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a nearly constant volume independent of pressure. It is one of the four fundamental states of matter (the others being solid, gas, and plasma), and is the only state with a definite volume but no fixed shape. The density of a liquid is usually close to that of a solid, and much higher than that of a gas. Therefore, liquid and solid are both termed condensed matter.
Ionic compound
In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl−) in sodium chloride, or polyatomic species such as the ammonium (NH4+) and carbonate (CO32−) ions in ammonium carbonate.