Note the rounding off of the actual calculated compositions.
Source: Adapted from Schröder [1], de Forcrand [28].
It is interesting to examine a summary of the interpreted results on gas hydrates as reported by A. Bouzat in the 1920s [79], some one hundred years after Davy's description of the first gas hydrate.
1 Gas hydrates are stable dissociable bodies that form solids with low heats of formation from their constituents in the solid state.
2 They have the same composition and similar properties even though they are formed by vastly different substances, inorganic, organic, elements, or compounds.
3 They are able to combine to form hydrates of a more complex nature nM·n′M′·(n + n′) 6H2O.
4 The six water molecules are bonded to the molecule M through “subvalences” and can be assumed to take on the form of a regular octahedron around the guest molecule.
5 Since the two bodies in a hydrate combine with very small affinity, they retain in the compound a molecular state that is as similar as possible to their separated state.
We can take note of the hydration number – rounded off to a whole number, and the reluctance to recognize a set of hydrates with high hydration numbers.
2.5 Nikitin and von Stackelberg
In the 1930s, B.A. Nikitin [80] prepared mixed hydrates of the noble gases, Xe, Kr, Ar with SO2 by “isomorphous precipitation.” He showed that the noble gases could be separated by partitioning between the solid hydrate and the gas. Of some interest are the claims that he was able to trap neon and radon in SO2 hydrate, the latter to a high degree of enrichment. On the basis of data on mixed radon‐SO2 hydrates, he predicted that the decomposition temperature of radon hydrate at 1 atm would be above 0 °C. He took the noble gas hydrates to be prototypes of a large family of isomorphous hydrates formed by molecules of suitable size and van der Waals character ranging from argon to chloroform. He still held [81] to the general formula for hydrates as given in Bouzat's summary some 15 years earlier, i.e. M·6H2O, but where M could be a gas mixture. Implicit in Nikitin's observations of mixed guests in the hydrates was the solid solution nature of the hydrate, thus also confirming their non‐stoichiometry. However, Nikitin did not comment on any specific features of the hydrate structure [82].
The progression from an essentially molecular picture of hydrates based on Villard's rule, where a guest interacted with an octahedron of water molecules through “sub‐valences,” to an infinite lattice of four‐connected water molecules forming cages, was left to be proposed by Mark von Stackelberg (Figure 2.8). We can speculate that the development of the crystal structure of ice Ih may well have influenced the researchers working on hydrates to also look for ice‐like lattices. Although the first X‐ray structural