Cavity occupation

Heats of dissociation for all three structures are largely determined by hydrogen bonding and the cavity occupation, as shown in Example 2.2. When similar cavities are occupied, the heats of dissociation are similar (regardless of the guest) within the restrictions listed. Detailed evidence for heats of dissociation effects is presented in Chapter 4.6.1.1. 

To a fair engineering approximation AH is not only a function of the hydrogen bonds in the crystal, but also a function of cavity occupation. Because the Clausius-Clapeyron equation determines the heat of hydrate formation by the slopes of plots of In P versus 1/T, one may easily determine relationships between heats of dissociation. 

Using the above equations, the Langmuir constant for the large cavity occupant can be determined explicitly from the chemical potential difference and the fugacity. However, for systems in which both cavities are occupied, a second method must be used to supplement Equation 5.27c and d. 

The ratio between the cavity and guest size in terms of their molecular volumes may be expressed in terms of the cavity occupancy factor (p). A p value of 1 represents a completely filled cavity. Occupancy factors for various cryptophanes and guests are shown in Table 6.9. 

The host molecules free energy contribution is independent of cavity occupation. This assumption implies that encaged molecules do not distort the cavity. 

Corrections to the last two assumptions have been the most beneficial in improving the theory. Together, the two assumptions (4. no interactions between guests, and 5. host molecules energy being independent of cavity occupation) comprise the ideal solid solution theory, which has been satisfactory for almost 40 years. However, a recent study showed that a 0.5% change in the lattice parameter can change the pressure prediction (at F-lOO bar) by 15%. 

Caldwell RL (1979) Cavity occupation and defensive behaviour in the mantis shrimp Gonodactylus festae evidence for chemically mediated individual recognition. Anim Behav 27 194-201 Caldwell RL (1982) Interspecific chemically mediated recognition in two competing stomatopods. 

Fig. 1 Space-filling representation of the common host framework adopted by 5-methoxysulfadiazine in three isostructural clathrates (space group P2]/c) and cavity occupation by the respective guests. Also shown are the unit cell data and PXRD patterns for the clathrates. (Adapted from Ref. [2] with permission from Gordon and Breach Publishing and Taylor Francis.)...

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