Clathrate Hydrates. Группа авторов

size (i.e. organic halides with boiling points greater than 110 °C appear to be too large to fit into the clathrate hydrate cages).⁴ The presence of hydrogen sulfide is not required for some of the compounds given in Table 2.1, but the stability of these hydrates is much less in the absence, than in the presence of H₂S.

      Three years before, in his first publication [60] on the subject of gas hydrates, de Forcrand had reported the formation of methyl iodide hydrate by the method said by Berthelot [33], to give a snow of CS₂ hydrate: bubbling moist air through the volatile liquid at a rate fast enough to cause substantial cooling. The new methyl iodide hydrate melted at approximately −4 °C and was found to have the composition CH₃I·H₂O. “Analogous hydrates” were said [59] to be formed by chloroform, ethyl bromide, and ethyl iodide. These results were not mentioned in his thesis, and by then, de Forcrand probably had doubts of their authenticity. In retrospect, this method of preparation of hydrates of volatile liquids is very likely to produce hydrates that contain varying amounts of air; thus, they were double hydrates in their own right. In turn, this would give considerable variability to the decomposition temperatures of the double hydrates and thus the difficulties in obtaining reproducible results. Since oxygen and nitrogen hydrates were not reported until 1960, these gases were assumed to be inert with respect to hydrate formation by the hydrate researchers of the day.

      Table 2.1 Molecules found by de Forcrand to form double hydrates with H₂S [1, 26].

Molecule	Boiling point (°C)	Molecule	Boiling point (°C)
CH3Cl	−23	C2HCl3	75
CH2Cl2^II	40	C2H5Br^III	38
CHCl3^III	61	CH3CHBr2^III	115
CCl4^III	78	C2H3Br^III	20
CH3Br	5	CH2CBr2	91
CH2Br2	80	C2H5I^III	71
CH3I^II	41	C2H3I	56
CBrCl3^II	104	n‐C3H7Cl	46
CCl3NO2^III	110	n‐C3H7Br^II	71
C2H5Cl^II	10	i‐C3H7Br	60
CH3CHCl2	64	CH2CHCH2Cl	46
CH3CCl3^III	75	CH2CHCH2Br	70
CH2ClCCl3^II,H	102	i‐C4H9Cl^H	67
CH2ClCH2Cl^III	83	i‐C4H9Br^H	90
C2H3Cl	18	CH3NO2	101
CH2CCl2	40	C2H5NO2	115
CS2	46

      The boiling points of the substance are those reported by de Forcrand. Other small halogenated alkanes which did not form clathrate hydrates were listed by de Forcrand.

      II Composition determined by de Forcrand by two component analysis.

      III Composition determined by de Forcrand by three component analysis.

      H Likely structure H hydrate formers synthesized by de Forcrand.

      Source: Adapted from Schröder [1], de Forcrand [26].

      As evidence for the close similarity of many of the double hydrates with hydrogen sulfide, de Forcrand found that chemical analysis for all three components of the nine hydrates marked “III” in Table 2.1 and for two components (the third being determined by difference) of the seven hydrates marked “II” all gave the same composition, namely M·2H₂S·23H₂O. In hindsight, the compounds listed in Table 2.1 probably represent two different hydrate structures, most of them belonging to the structure II (sII) hydrate family, the compounds flagged with “H” likely are hexagonal structure H (sH or HS‐III) hydrate formers, see Chapter 3 for further discussion.

      Another common feature of many of the double hydrates was the morphology of their crystals. The chloroform‐hydrogen sulfide hydrate was found to sublime as well‐defined crystals on the inner surface of the sealed tube. de Forcrand observed of this hydrate [26],

J'ai pu remarquer dans quelques cas des octaèdres presque parfaits. D'ailleurs l'examen au microscope polarisant ne peut laisser aucun doute sut la forme cubique de ces cristaux, qui n'agissent pas sur la lumière polarisée ⁵

Similar observations of several other of the most stable double hydrates showed cubic, cubo‐octahedral, and truncated octahedral forms (Figure 2.5a,b) which had no effect on polarized light when examined with the polarized light microscope. de Forcrand appears to have been the first to argue that the gas hydrate crystals examined belonged to the cubic system and thus were distinguishable from hexagonal ice crystals. The dissociation pressures of nine double hydrates measured by de Forcrand in the presence of liquid water and liquid hydrate former are reproduced in Figure 2.5c. These dissociation pressures and compositions of the gas phase were found to be dependent only on temperature and to be independent of the overall relative amounts of the three components. That these observations were a