Ratio guest/cavity diameter

Table 2.4 shows the ratio of molecular diameter to cavity diameter for guests in structures I and II. The corresponding data for structure H is given in Table 2.7, and the data for cyclopropane and trimethylene oxide (which form both si and sll) are also provided. 

Hydrate nonstoichiometry appears to be related to the ratio of the guest molecule diameter to the free cavity diameter. Nonstoichiometry increases as that ratio approaches unity. 

Table 14.2 Ratios of molecular diameters (obtained from von Stackelberg and Muller 1954) to hydrate cavity diameters for various gases, including those commonly found in natural gas hydrate (from Sloan 1998). = indicates the cavity occupied by a single guest.

The ideal guest/water ratio is G 5%H20 for molecules that can occupy both cavities of structure I, and G-7(2/3 )H20 for occupants of only the 51262 of structure I. As indicated in Figure 2.13 molecules of transitional size (shaded region) such as cyclopropane (Majid et al., 1969) and trimethylene oxide (Hawkins and Davidson, 1966) with diameters of 5.8 and 6.1 A, respectively, may form either structure. 

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