Liquid clathrate behavior

Solid State Clues to Liquid Clathrate Behavior... 

Molar miscibility and saturation point measurements describing liquid clathrate behavior are commonly analyzed by the integration of NMR spectra. In most cases, the aromatic stoichiometries are the average of three preparations and integrations with an acceptable standard deviation result of 0.2 molecules. A common feature of all liquid clathrate NMR spectra is the shifting of the entire spectrum 0.2-0.5 ppm downfield relative to the pure aromatic substance. 

Since the initial discovery of liquid clathrate systems, hundreds of compounds having the structure M[Al2R6X] were found to exhibit liquid clathrate behavior.While these highly reactive air-sensitive salts are useful in such applications ranging from separations of aromatics from hydrocarbons to coal liquefaction, their air-sensitivity and reactivity posed drawbacks to further study. 

A model for liquid clathrate behavior is presented in Fig. 15. It is believed that the ions interact in a cooperative manner. One cation may be associated with two or more anions and vice versa. The cation-anion interaction must be strong, or the ions will separate and a normal solution will result. The aromatic molecules are necessary constituents of the layerlike structure. They are guests, but... 

FIGURE 15 Two-dimensional model of liquid clathrate behavior. 

The class of substances we refer to as liquid clathrates exhibits a new type of strong organometallic-aromatic involvement (9, 10, 11), However, before this behavior is described in detail, it is necessary to review the origin and characterization of the parent molecules. 

Liquid clathrate is a term coined by Professor Jeny L. Atwood, then at The University of Alabama.This term was used to describe the serendipitous discovery of the biphasic behavior of a wide selection of salts with aromatic solvents, such as toluene and benzene. These semiordered liquids containing complex salt hosts and aromatic hydrocarbons represent the most common examples of liquid clathrates. 

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. 

Of the several interesting applications of liquid clathrates is in the area of separations. Since the behavior is found for aromatic molecules but not for aliphatic ones, a separation is possible. It is possible to envision even difficult problems such as the separation of the xylene isomers being attacked by liquid clathrates. Liquid clathrates have also been reported to be useful as solvents for the liquefaction of coal. 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

If the gas phase activity of the host is controlled by the presence of a pure condensed phase, solid or liquid, the equilibrium between host and guest in a stoichiometric clathrate can be described in terms of the gas phase pressure of the guest. This is, in effect, a vapor pressure for the guest. At higher pressures the guest will condense to form clathrate, and at lower pressures the clathrate will decompose. Temperature variation of this pressure will follow the Clapeyron equation which, with the usual assumptions (ideal gas behavior of the vapor and negligible volume of the condensed phase), reduces to the Clausius-Clapeyron equation ... 

A new experimental apparatus was built to measure the clathrate dissociation pressures and equilibrium compositions of liquid phases which coexist with the clathrate phase. The pressure-temperature behavior of the binary phenol-carbon... 

Eutectics, LSE that occurs most often in metallurgy and also in water-mineral salt systems, involve one liquid and two solid phases at temperatures below the melting point of both of the solids. Hydrates (clathrates), GSL that occurs most often with hydrocarbons and water at temperatures above the melting points of the chemical species involved, involve a solid, sometimes a gas, and one or more liquid phases. For both the behavior follows the same thermodynamic rules as the other equilibria in this book, but with rntwe complexity. 

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