Ionic liquid

Ionic liquid
Name	Ionic liquid
Caption	A sample of a colorless ionic liquid in a vial.
OtherNames	Liquid salt, fused salt, ionic fluid

Ionic liquid. Ionic liquids are a class of salts that exist as liquids at relatively low temperatures, often below 100°C. They are composed entirely of ions, typically an organic cation paired with an inorganic or organic anion, which results in unique physical and chemical properties. Their low volatility, high thermal stability, and tunable nature make them distinct from conventional molecular solvents and molten salts.

Definition and basic properties

Ionic liquids are defined by their low melting points, which are arbitrarily set below 100°C, distinguishing them from traditional high-temperature molten salts like sodium chloride. A fundamental property is their negligible vapor pressure, which contributes to their reputation as "green solvents" by reducing atmospheric emissions. Their physical characteristics, such as viscosity, conductivity, and solvation ability, are highly dependent on the structures of the constituent ions. The interplay between the large, asymmetric cations and the anions often disrupts efficient crystal packing, depressing the melting point. Key properties like hydrophobicity and Lewis acidity can be finely adjusted by altering ion combinations, a concept known as "designer solvents."

History and development

The discovery of the first room-temperature ionic liquid, ethylammonium nitrate, is credited to Paul Walden in 1914, though its significance was not widely recognized for decades. Major development began in the mid-20th century with work at the United States Air Force and Oak Ridge National Laboratory on chloroaluminate salts for electroplating applications. The field expanded significantly in the 1990s with the introduction of air- and water-stable ionic liquids, such as those based on the 1-ethyl-3-methylimidazolium cation, pioneered by researchers like Kenneth Seddon and Robin Rogers. This period saw a surge in global research, with major contributions from institutions like the University of Alabama and the Queen's University Belfast.

Types and classification

Ionic liquids are primarily classified based on their cationic core. Common classes include imidazolium, pyridinium, pyrrolidinium, phosphonium, and ammonium-based salts. Anion variety is vast, encompassing halides like chloride, inorganic anions such as hexafluorophosphate and tetrafluoroborate, and organic anions like triflate and dicyanamide. A special category is protic ionic liquids, formed by proton transfer from a Brønsted acid to a Brønsted base. Other classifications consider functionality, leading to subclasses like task-specific ionic liquids, which have functional groups grafted onto the ion to perform specific chemical tasks, and chiral ionic liquids.

Synthesis and production

Synthesis typically involves two main steps: quaternization (Menshutkin reaction) to form the desired cation, often by alkylating a parent amine or phosphine, followed by anion exchange or metathesis to introduce the target anion. Common methods include reaction with silver salts or using ion-exchange resins. For large-scale production, companies like BASF and Merck KGaA have developed processes, including the BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process. Purification is critical and often involves washing, distillation, or column chromatography to remove impurities like water, halides, or unreacted starting materials.

Applications

Ionic liquids have found use as solvents and catalysts in organic synthesis and polymerization reactions, often improving selectivity and yield. In electrochemistry, they serve as electrolytes in batteries, supercapacitors, and for the electrodeposition of metals like aluminum and lithium. The NASA-developed ILFAM (Ionic Liquid as a Functional Material) project highlights their use in spacecraft lubrication. Other applications include gas capture, such as carbon dioxide sequestration, as stationary phases in gas chromatography, and in the processing of biomass like cellulose at institutions like the Joint BioEnergy Institute.

Environmental and safety considerations

While often labeled "green" due to low volatility, comprehensive assessments of their lifecycle and ecotoxicity are essential. Some ionic liquids, particularly those with fluorinated anions, can be persistent and toxic to aquatic organisms, as studied by agencies like the Environmental Protection Agency. Their synthesis can involve hazardous reagents and generate waste. Research into biodegradable ionic liquids, such as those derived from choline, is ongoing. Proper handling guidelines are issued by organizations like the Occupational Safety and Health Administration, focusing on potential dermal irritation and reactivity.

Category:Salts Category:Chemical compounds Category:Solvents