Guest components

Fig. 1. The principle of formation of an inclusion compound, (a) concave host (b) convex guest component (c) host—guest compound.

Fig. 5. Cyclophane-type inclusion compounds of different varieties. The guest component is shaded.

Crystallization of 5 in the open air from an initially aprotic solvent (N,N-dimethyl-acetamide) led to a non-layered structure which is characterized by a three-dimensional lattice of loosely-packed host species interspaced by channel-type zones accommodating the solvent guest components (Fig. 9). 

In the four isomorphous structures the different guest components are located in a similar crystalline environment. Since the species involved are of nearly identical... 

Beside a crystal cavity of suitable size, additional information is required, e.g. designed polarity gradients in the cavity or carefully located and specific binding sites, respectively. At best, the crystal inclusion will enjoy an ideal lock and key relation 23 between host and guest components from a chemical and spatial point of view. 

In polar solvents with high donor numbers, the complex is solvated poorly relative to its separate components. This difference in solvation energies nullifies the hydrogen bonding interactions present in the complex, which, as a result, dissociates into its host and guest components. 

Fig. 17 The host-guest composite structure of the tll9 phase of Na shown in projection down the c-axis. The host atoms are shown in light grey and guest atoms in dark grey. The monoclinic guest component unit cell is outlined with dashed lines, and a separate perspective view of the guest structure is shown...

If we systematize the growth conditions under which the three modes of growth mechanisms operate and express them in a simple manner in relation to the work required to cause coagulation of the guest component onto the surface of the host, Aa = 2a a, and the driving force A/r = we can formulate Table 7.1. This... 

Carson and Katz noted that their experimental hydrate composition changed at different temperatures and pressures in a manner indicative of a solid solution of mixtures, rather than segregated macroscopic quantities of pure hydrocarbons within the hydrate. The concept of a solid solution enabled the notion of the mole fraction of a guest component in the solid phase hydrate mixture, on a water-free basis. Carson and Katz defined a vapor-solid distribution coefficient (KVSi) for each component as... 

Note The 11-C4H10 conditions should only be evaluated for formation with at least one, smaller guest component. 

In hydrate equilibrium, it may seem slightly unusual to apply it to binary systems (water and one guest component) of three-phase (Lw-H-V or I-H-V) equilibrium to obtain the heats of dissociation. As van der Waals and Platteeuw (1959b) point out, however, the application of the Clapeyron equation is thermodynamically correct, as long as the system is univariant, as is the case for simple hydrates. 

M Total number of possible guest components in the mixture p Property of the empty hydrate crystal... 

For an ideal gas Equation 5.22a may be considered as elementary probability of cavity i occupation by molecule J. This is one of the most useful equations in the method of hydrate prediction, and it may also be recognized as the Langmuir isotherm. If the equation were written for one guest component J, it would contain the Langmuir constant Cjj as the only unknown for a given pressure and fraction of the cavities filled (or fraction of monolayer coverage). 

The value of fitting the Langmuir constants to simple hydrate formation data is in the prediction of mixture hydrate formation. When the formation data for the simple hydrates are adequately fitted, then mixtures of those guest components can be predicted with no adjustable parameters. Since there are only eight simple hydrate formers of natural gas which form si and sll, but an infinite variety of mixtures, such an advantage represents a substantial savings of time and effort. 

In sum, ab initio methods are beginning to fulfill their substantial promise for hydrates. For many hydrate guest components, ab initio methods have been shown to extend some of the most fundamental calculations from quantum mechanics to macroscopic properties, and to predict spectroscopic hydrate properties acceptably. 

Yet until these methods can be extended to all common natural gas guest components and their thermodynamic inhibitors, it will be difficult to use the programs pragmatically. To date the programs have proved the ab initio concept from an academic perspective. While extension of ab initio methods to all natural gas hydrate components can be done in principle, that task awaits the generation and maintenance of a complete program. 

To prevent water occlusion. Without agitation, Villard (1896) showed, for example, that nitrous oxide hydrate formation was continuous for a period longer than 15 days under a pressure of 2 MPa. Villard also determined that in previous research the ratio of water to guest molecules had been analyzed as greater than G 6H20 (Villard s Rule) due to either occlusion of water within the hydrate mass, or due to the loss of the guest component. 

Supermolecules generally comprise a host component with convergent binding sites and a guest component with divergent binding sites. 

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