Liquid clathrates ionic liquids

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

The typical liquid clathrate is characterized by (a) a low viscosity relative to that of a neat ionic liquid, (b) immiscibility with excess aromatic solvents, and (c) non-stoichiometric compositions. The formation of air- and water-stable liquid clath-rates has been reported for the compositions consisting of aromatic hydrocarbons (e.g., benzene, toluene, and xylenes) and common salts of [AMIM] cation with the anions PF, [Tf2N] , BFJ, and Cl 91). 

Holbrey, J.D., Reichert, W.M., Nieuwenhuyzen, M., Sheppard, O., Hardacre, C., and Rogers, R.D., Liquid clathrate formation in ionic liquid-aromatic mixtures,... 

Ionic Liquids Liquid Clathrates Summary Study Problems References... 

The fact that liquid clathrates are thus liquids based on ionic compounds means that they are somewhat related in terms of their properties to ionic liquids (Section 13.4), but they are fundamentally a two or more component phase instead of a pure compound. A recent report has shown, however, that bmim... 

Ionic liquids are potentially Green solvents and offer interesting properties as designer solvents and can act as liquid clathrates, including neutral guest molecules. 

A potentially unique form of specific solute-solvent interaction has been proposed for ionic liquids. Blanchard and Brennecke [234] note that the solubilities of aromatic species are anomalously high in an imidazolium-based IL when compared to solutes of comparable molecular weight and dipole moment. This cannot be explained purely by ji-ji interactions, because while ji-ji interaction energies can be significant [235], the solubilization of a pure aromatic liquid must disrupt at least as many ji-ji contacts as it creates. However, work by Holbrey and co-workers [171] characterizes a cocrystalline clathrate form of an imidazolium-based IL with benzene, which shows distinctive ji-ji stacking. 

Holbrey JD, Reichert WM, Nieuwenhuyzen M (2(X)3) Liquid clathrate formation in ionic liquid—aromatic nrixtures. Chem Commun 476-477... 

Liquid clathrates, analogues of the gas clathrates but present in the liquid phase, are unusual two phase systems in which an upper layer of solvent lies above a denser layer of solvent saturated with ionic species. Some interesting examples are found in Atwood s work. Initially a salt, such as sodium chloride, is added to an aromatic solvent in which it is insoluble. Another reagent, an aluminium alkyl in one example, is added and the salt is solublized to form a dense phase [1] which may be interpreted as ... 

More recent research established that a much greater range of organic salts can form liquid clathrates when contacted with aromatic solvents,including liquid salts now known as ionic liquid.Formation of liquid clathrates is observed most often with halide and pseudohalide anions. Regardless of cation or anion, these materials can be expressed as nonstoichiometric liquid inclusion compounds forming complexes in conjunction with aromatic solvents ranging between 1.3-42.2 molecules of solvent. 

For a rationalization of the similar physical behavior of liquid clathrate formation with aromatics, for a chemically dissimilar set of materials. Zaworotko et al. suggested that liquid clathrate formation was chiefly dependent on the physical properties of the organic salts, rather than on their chemical natures. Thus, an approach to developing other systems capable of sustaining liquid clathrates would be to investigate organic salts with low melting points (or even salts that were liquid at room temperature). This is clearly a description of ionic liquids. 

In studies to determine the miscibility of ionic liquids with aromatic solvents, Holbrey et al. found that ionic liquid-aromatic mixtures form liquid clathrate systems. For example, ionic liquids of current interest, such as hydrophobic 1-alkyl-3-methylimidazolium salts ([Cn-mim]X) with hexafluorophosphate, / w(trifyl)imide, and tetrafluoroborate anions were observed to form liquid clathrates with benzene, toluene, and xylenes (Fig. 2). In... 

Fig. 2 Ratio of aromatic to ionic liquids in the lower phase of the liquid clathrate biphasic systems. The data shown for n-alkyl-methylimidazolium ionic liquids is from Ref [7]. Connecting lines are a visual guide to changes in the liquid clathrate phase composition.

The support for liquid clathrate formation under biphasic conditions in ionic liquid-aromatic systems is an important and underrecognized phenomena. As the use of ionic liquids as solvents and catalyst immobilization phases for transition metal catalyzed reactions becomes increasingly commonplace, the importance of understanding liquid clathrate formation as an activation and... 

The field of liquid clathrates can also aid in our understanding of new fields such as ionic liquids, where many ionic liquid compositions are being observed to behave as liquid clathrates. The ionic liquid researchers are searching for new and varied liquid salt compositions, and thus, one may predict that a plethora of new liquid clathrate hosts will soon appear in the literature. It will be incumbent on researchers in liquid clathrates and ionic liquids to make the connections between the two fields. 

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