Clathrates liquid

They are formed from a salt of a non-coordinating anion. 

There are special geometrical requirements for both cation and anion. 

Formation of small, closed units of association e.g. dimers, trimers etc.) is not permitted. Liquid clathrates are useful in a number of selective organic separations. 

Considering the process of liquid clathrate formation, solid-state liquid clathrate forming materials such as 13.21 are subject to a dissolution equilibrium or liquefying phase change upon exposure to aromatic solvents (Equation 13.1). 

The A/A values have been found to correlate extremely well with aromatic molecules van der Waals volumes, suggesting that the liquid cavity model in which the aromatic is a space filler is an accurate one. The constancy of the A/A value suggests that the cation-anion association produces fairly well-defined solution voids with a volume that is invariant with change of solvent. 

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Along the three-phase line liquid-clathrate-gas the variation of the composition with temperature is considerable (cf. CD in Fig. 3), because when applying Eq. 27 to this equilibrium, the relatively small quantity AH = 0.16 kcal/mole has to be replaced by the much larger difference/ —//ql between the partial molar heat functions of / -hydroquinone and the liquid phase, which amounts to about —6 kcal/mole. The argon content of the solid reaches a minimum at the quadruple point. 

In the P-T projection the difference in slopes of the three-phase lines -clathrate-gas and liquid-clathrate-gas at the quadruple point R is determined by the heat of fusion of the number of moles of hydroquinone associated with one mole of argon in the clathrate under the conditions prevailing at R. If we extrapolate the three-phase line liquid-clathrate-gas to lower pressures (where it is no longer stable), the value of yA decreases until it becomes zero when we are dealing with pure / -hydroquinone. Hence, the metastable part of this three-phase line ends in the triple point B of /1-hydro-... 

One of the more important recent developments in organometallic aluminum chemistry has been the formation and isolation of low-coordinate compounds, and, in particular, cations. These were first prepared in reactions of various aluminum reagents with crown ethers to form the inclusion compounds known as liquid clathrates. 71,72 Most of the evidence supports the presence of ion pairs as the basis of the solvent inclusion effect. Indeed, the compound [AlMe2-18-crown-6]+[AlMe2Cl2] was isolated from one such system (the cation is shown in Figure 6(a)).73 This was the first time the Me2Al+ unit had been structurally characterized. 

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). 

J. L. Atwood, Liquid Clathrates , in Inclusion Compounds (J. L. Atwood,... 

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... 

Jerry Atwood liquid clathrates from alkyl aluminium salts 1969 - Ron Breslow catalysis by cyclodextrins... 

The elegant match of H30+ for [18] crown-6 suggests that the crystallisation of crown ether and cryptand complexes from acidic liquid clathrate media might represent a straightforward... 

Liquid clathrates are useful in a number of selective organic separations. 

Figure 13.25 Favourable K+ interaction sites in [Al2Me6N3] (the anion in 13.21), [AlMe3N3] and N3 showing the difficulty experienced by the cations in approaching the anion as its bulk increases. This leads to the creation of regions of solvent-filled space (clathration) even in the liquid phase, which is the basis for liquid clathrate behaviour.

The model of the solution behaviour of liquid clathrates that appears to best fit the observations is depicted in Figure 13.26. Large regions of liquid order exist in which cations and anions interact strongly with one another. This prevents simple dissolution, but association into aggregates such as dimers and trimers (possibly leading to crystal nucleation) is also prevented by the steric properties of the anions and/or cations. The role of the aromatic solvent is to support the structure and stabilise... 

Figure 13.26 Two-dimensional model of liquid clathrate behaviour.

The majority of the interest in liquid clathrates has centred around their potential applications in separation science, particularly in the separation of closely related species such as benzene and toluene, or xylene isomers (cf. zeolite-based methods, Section 9.2). Separation occurs as a result of the greater ability of one organic solvent over another to stabilise the liquid clathrate phase. Hence, in a... 

Liquid clathrates offer a great advantage over solid-state separations (e.g. by formation of Hoffman-type inclusion compounds, Section 9.4) because of the extremely fast mixing kinetics, the avoidance of the need to wait for crystallisation to occur and the easy separation of the two liquid phases. It should also prove possible to run liquid clathrate separations in a continuous extraction manner. The avalues of a number of liquid clathrate-based separations have been reported and are summarised in Table 13.1. 

Clearly significant separations can be effected, especially between aromatics and aliphatics. The size basis for the effect is clear in the comparison of hexane and hexene results the presence of the 1-hexene double bond does not seem to enhance its inclusion into the liquid clathrate phase greatly. Also clear is the strong dependence on the dimensions of the anion, with the smaller, more compact materials giving better separations. 

A great deal of more recent research has concerned liquid clathrates formed from strong acids such as HC1 in aromatic solvents in which oxonium ions such as H30+ are stabilised by crown ethers and play the role of the cation. This has allowed the isolation of a range of interesting oxonium salts, as discussed in Section 3.11.1. 

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... 

Table 13.1 Liquid clathrate-based separations of 1 1 feed mixtures.

Figure 13.28 X-ray crystal structure of the liquid-clathrate-derived inclusion compound N,N -dimethylimidazolium+(PF6 )-0.5C6H6 showing the stacking of the benzene guest with the imidazolium...

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

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