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Ionic liquids are salts that remain liquid at or near ambient conditions, typically defined by melting points below 100 °C. Unlike conventional molecular solvents such as water or hydrocarbons that consist of electrically neutral molecules, ionic liquids comprise discrete ions—cations and anions—that contribute to their unique physicochemical characteristics[1][2]. The ionic bonds within these substances are significantly stronger than the Van der Waals forces that dominate ordinary molecular liquids. This strength usually results in high lattice energies and correspondingly elevated melting points; however, specific organic cations with flexible alkyl substituents can reduce lattice energy sufficiently to maintain liquidity even below room temperature[1].
The melting behavior of ionic liquids is central to their chemistry and applications. For example, sodium chloride melts at a high temperature of 801 °C into a molten salt composed mainly of Na+ and Cl− ions[1]. In contrast, many ionic liquids melt well below this temperature due to the structural nature of their constituent ions. Organic cations such as the imidazolium family provide asymmetry and flexibility that disrupt crystalline packing. The compound EMIM dicyanamide \((0)\), melts at −21 °C, highlighting how ion design influences thermal properties[1]. Similarly, 1-butyl-3,5-dimethylpyridinium bromide forms a glassy phase below −24 °C instead of crystallizing conventionally[1].
The manipulation of both cationic and anionic components allows fine-tuning of melting points suitable for specific applications. Deep eutectic solvents—mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds—can achieve even lower melting points than pure salts by disrupting ion-ion interactions further[1].
The term "ionic liquid" has been in use since at least 1943[1]. Early examples include ethanolammonium nitrate with a melting range between 52 and 55 °C reported in 1888 by S. Gabriel and J. Weiner[1]. By the early twentieth century, ethylammonium nitrite \(\left(1\right)\), described in 1911 by Ray and Rakshit as a heavy yellow liquid resistant to solidification even when immersed in salt and ice mixtures, likely represents the first documented room-temperature ionic liquid[1]. Paul Walden’s work in 1914 introduced ethylammonium nitrate \(\left(2\right)\), stable at room temperature with a melting point of 12 °C[1].
Later developments in the 1970s and 1980s focused on alkyl-substituted imidazolium and pyridinium salts combined with halide or tetrahalogenoaluminate anions. These showed promise as battery electrolytes but were limited by moisture sensitivity and acidity issues until neutral weakly coordinating anions like hexafluorophosphate (\(PF_6^-\)) and tetrafluoroborate (\(BF_4^-\)) became more common after the work of Wilkes and Zawarotko in 1992[1].
The robust electrostatic interactions between ions confer several characteristic properties on ionic liquids. Their vapor pressures can be extraordinarily low—on the order of \(10^{-10}\) Pa—which reduces volatility dramatically compared to organic solvents[1]. This low vapor pressure contributes to negligible flammability risks and superior thermal stability.
Ionic liquids typically exhibit viscosities higher than conventional solvents due to strong coulombic attraction between ions. Electrical conductivity varies widely but is often lower than expected for fully ionic systems because ion pairing or clustering occurs to some extent.
Temperature-dependent phase behavior includes glass transitions detected well below ambient temperatures; for instance, N-methyl-N-alkylpyrrolidinium fluorosulfonyl-trifluoromethanesulfonylimide (FTFSI) demonstrates glass transition temperatures under −100 °C[1]. Certain ionic liquids remain liquid down to approximately −150 °C without crystallization[1]. These properties have spurred proposals for extreme environment applications such as lunar-based spinning liquid mirrors operating below cryogenic temperatures (~130 K)[1].
Room-temperature ionic liquids generally derive from organic cations with asymmetrical structures that prevent efficient crystal packing. The imidazolium family dominates this class—examples include:
- 1-Ethyl-3-methylimidazolium (EMIM)
- 1-butyl-3-methylimidazolium (BMIM)
- 1-octyl-3-methylimidazolium (OMIM)
- 1-decyl-3-methylimidazolium (DMIM)
- 1-dodecyl-3-methylimidazolium (dodecylMIM)
Longer alkyl chains increase hydrophobicity but also elevate viscosity. Other related imidazolium variants feature additional methyl groups or amino substituents such as BMMIM/DBMIM and DAMI respectively.
Pyridinium-derived cations like 4-methyl-N-butyl-pyridinium (MBPy) and N-octylpyridinium (C8Py) extend structural variety further.
Quaternary ammonium salts including tetraethylammonium (TEA) and tetrabutylammonium (TBA) form another important category with distinct solubility profiles[1].
Anion identity critically affects physical properties like viscosity, miscibility, thermal stability, and electrochemical window. Common anions encompass:
- Tetrafluoroborate (\(BF_4^-\))
- Hexafluorophosphate (\(PF_6^-\))
- Bis-trifluoromethanesulfonimide (\(NTf_2^-\))
Less common but functionally significant anions include trifluoromethanesulfonate (\(OTf^-\)), dicyanamide \(\left(N(CN)_2^-\right)\), hydrogensulfate (\(HSO^-_4\)), and ethyl sulfate (\(EtOSO_3^-\))[1]. Incorporation of paramagnetic anions such as tetrachloroferrate enables magnetic ionic liquids with novel functional behaviors.
Phosphonium-based cations like trihexyl(tetradecyl)phosphonium \((P_{6,6,6,14}^+)\) and tributyl(tetradecyl)phosphonium \((P_{4,4,4,14}^+)\), though less prevalent than ammoniums or imidazoliums, offer enhanced thermal stability and reduced toxicity profiles in select cases[1].
Protic ionic liquids arise from direct proton transfer between Brønsted acids and bases without requiring complex multi-step synthesis routes typical for aprotic ILs. This ease of formation allows rapid generation of ILs tailored for acid-base catalysis or proton conduction applications.
In contrast, aprotic ILs depend on carefully designed ion pairs where neither component acts as a proton donor; these exhibit broader electrochemical windows useful for battery electrolytes or solvent systems in catalysis.
Ionic liquids’ solubility characteristics reflect their charged nature combined with variable hydrophobic/hydrophilic balance depending on side chain length and ion identity. Saturated aliphatics generally dissolve poorly while alkenes display moderate solubility; aldehydes often mix completely with many ILs.
Gas solubility follows similar trends: carbon dioxide dissolves readily due to favorable interactions whereas carbon monoxide shows poorer solubility relative to conventional organic solvents. Hydrogen solubility remains low but consistent across common IL types.
These attributes underpin applications ranging from biphasic catalysis—where product separation benefits from solvent immiscibility—to gas capture technologies exploiting CO₂ affinity.
Despite being molten salts rather than molecular fluids per se, some ILs can be distilled under vacuum near ~300 °C; however vapor consists predominantly of ion pairs rather than free ions due to strong coulombic binding preventing dissociation in the gas phase[1]. Thermal decomposition pathways vary widely depending on ion selection but generally demonstrate superior resistance compared to volatile organic solvents.
Polymerized forms known as poly(ionic liquid)s retain partial ionicity since one ion is covalently fixed along the polymer backbone while counterions remain mobile. PILs combine traditional polymer mechanical properties with tunable ionic conductivity—opening avenues in advanced membranes or electrolytes where mechanical robustness is required alongside electrochemical function[4].
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Overall chemistry of ionic liquids hinges on intricate interplay between cation-anion combinations influencing melting behavior, solubility profiles, thermal stability, electrical conductivity, and chemical reactivity. Tailored design facilitates diverse roles including green solvents replacing volatile organics; electrolytes enabling safer batteries; catalysts modifiable through acid/base functionalization; and specialty materials adaptable through polymerization strategies[1][2][3][4].
These fundamental chemical principles continue guiding research toward optimized performance across industrial processes demanding sustainability alongside functionality.
[1] https://en.wikipedia.org/wiki/Ionic_liquid
[2] https://pubs.acs.org/doi/10.1021/acs.chemrev.7b00246
[3] https://royalsocietypublishing.org/rsta/article/384/2316/20240308/...
[4] https://link.springer.com/chapter/10.1007/978-3-032-02746-7_1
[5] https://solvionic.com/en/3-ionic-liquids
Italiano: Chimica dei liquidi ionici e le loro applicazioni
Español: Química de los líquidos iónicos y sus aplicaciones
Deutsch: Chemie der ionischen Flüssigkeiten und ihre Anwendungen
Français: Chimie des liquides ioniques et leurs applications modernes
Русский: Химия ионных жидкостей: основы и приложения
Română: Chimia lichidelor ionice: proprietăți și aplicații
Português: Explorando a Química dos Líquidos Iônicos na Ciência Atual
Hrvatski: Kemija ionskih tekućina i njihov značaj u kemiji
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