Physical and chemical properties



Ionic liquid.

11Ionic liquid: two solids make a liquid.

RTILs possess a unique array of physico-chemical properties that make them suitable in numerous task-specific applications in which conventional solvents are non-applicable or insufficiently effective.
Such properties include:

  • high thermal stability,
  • high electrical conductivity,
  • large electrochemical windowDictionary,
  • low nucleophilicity and capability of providing weekly coordinating or non-coordinating environment,
  • very good solvents properties for a wide variety of organic, inorganic and organometallic compounds: in some cases, the solubility of certain solutes in RTILs can be several orders of magnitude higher than that in traditional solvents.

Moreover, by fine-tuning the structure, these properties can be tailor-designed to satisfy the specific application requirements.5

As a result, ionic liquids are very popular materials and they enjoy a plethora of applications in various domains of physical sciences.Press here to see the chemical-physical properties Table.Table 1.

Stability5

In most of these applications, the stability of ionic liquids, at least at a certain extent, is crucial for optimum process performance. Several studies have indicated that, although not 100% inert, certain ionic liquids incorporating 1,3-dialkyl imidazolium cations are generally more resistant than traditional solvents under certain harsh process conditions, such as those occurring in oxidation, photolysis and radiation processes.

Color5

High quality ionic liquids incorporating [bmim]+ cation and a variety of anions, such as [PF6], [BF4], [CF3SO3], [CF3CO2] and [(CF3SO2)2N] have been reported to be colorless, even though they are not 100% pure. The color of less pure ionic liquids generally ranges from yellowish to orange.
The formation of the color has been attributed to the use of raw materials with color or excessive heating during the synthesis of imidazolium salt.
A number of precautions for synthesis of colorless ionic liquids have been described, and a procedure for removal of color from impure ionic liquids using acidic alumina and activated charcoal has also been proposed.

Hygroscopicity5

The water content has an influence on the viscosity of the ionic liquids. Viscosity measurement indicate that ionic liquids became less viscous with increasing water content. Hydrolysis problems can also occur.

Hydrophopicity

The degree of polarity can be varied by adapting the length of the 1-alkyl chain (in 1,3-substituted imidazolium cations), and the counterion. Long-chain IL salts have attracted some interest due to their liquid-crystalline (LC) properties.10
The anion chemistry has a large influence on the properties of IL. Though little variation in properties might be expected between same-cation salts of these species, the actual differences can be dramatic: for example, [bmim]PF6- is immiscible with water, whereas [bmim]BF4- is water-soluble.2

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