Clathrate ices form from water and non-stoichiometric amounts of small non-polar molecules (hence usually gaseous) under moderate pressure (typically of a few MPa) and at cold temperatures (typically close to 0°C, but increased pressure raises the melting point). Every water molecule forms a vertex of four cages , which may, or may not, contain a small guest molecule. Their structures require a minimum amount of these small molecules to fit into and stabilize the cavities (usually one or none in each cavity) without forming any covalent or hydrogen bonds to the water molecules. Without these interstitial molecules the clathrate cavities, shown left, would collapse at positive pressures and they have been shown to dissipate, if surprisingly slowly, after the clathrate ice melts [897]. During formation and dissociation, the solid clathrates interact significantly with the structure of the neighboring aqueous solution [904].
CS-I and CS-II are the most stable structures and no other hydrate structure with a single guest component has been found at an ambient condition (except for bromine clathrate) [1732].
Type |
Lattice |
Space group |
Unit cell |
Unit cell formula a |
|---|---|---|---|---|
| Clathrate I, CS-I | Cubic | Pm3n |
a=1.203 nm | (S)2·(L)6·46H2O |
| Clathrate II, CS-II | Face-centered cubic | Fd3m |
a=1.731 nm | (S)16(L+)8·136H2O |
| Clathrate H, HS-III | Hexagonal | P6/mmm |
a=1.23 nm c=1.02 nm |
(S)5(L++)·34H2O |
| Clathrate TS-I, [1734] | Tetragonal | P42/mnm |
a=2.318 nm c=1.215 nm |
(L+)16-20·172H2O |
Cavity |
512 |
51262 |
51263 |
51264 |
51268 |
435663 |
|---|---|---|---|---|---|---|
H2O |
20 |
24 |
26 |
28 |
36 |
20 |
Mean cavity radius, Å |
3.95 |
4.33 |
4.53 |
4.73 |
5.71 |
4.06 |
free volume, Å3 |
51 |
77 |
98 |
120 |
213 |
44 |
| CS-I, /unit cell | 2 |
6 |
- |
- |
- |
- |
| CS-II, /unit cell | 16 |
- |
- |
8 |
- |
- |
| HS-III, /unit cell | 3 |
- |
- |
- |
1 |
2 |
| TS-I, /unit cell | 10 |
16 |
4 |
- |
- |
- |
| Guest molecules, for example; approximate radius, Å |
Ar, O2, N2,
CH4 |
CO2, C2H6 |
Br2 |
C3H8, (CH3)3CH |
(CH3)3CC2H5, Xe |
CH4 |
1.8-2.2 |
1.8-2.7 |
~2.4 |
2.8-3.1 |
3.5-4.3 |
1.8 |
a Not all cavities would normally be filled; S = small guest; L = large guest; L+ = larger guest; L++ = largest guest
The connectivity maps for the clathrate cages are shown right.
Some clathrate hydrates can form, at atmospheric pressure, at the interface between a liquid of suitable guest molecules and water (for example, CH3CCl2F in clathrate CS-II hydrate [408]). At low pressures (e.g. atmospheric) most clathrate hydrates decompose to release the guest molecules, except at low temperatures (for example, < 270 K) where they may remain in a metastable state, for several hours. At very high pressures, clathrate hydrates show complex phase behavior, often giving filled hexagonal ice [1144 ] with the smaller guest molecules/atoms, then at higher pressures they break down to give high density ice and a solid phase formed by the guest molecules (for example, see [898]. Gas hydrates have been recently reviewed [395]. Water itself cannot be contained in the cavities of solid clathrates [1114].
The relative content of the cavities can be determined by techniques such as Raman spectroscopy or NMR as the different cavities present differing environments. [Back to Top
]

For interactive Figures, see Chime
Shown opposite is the cubic clathrate CS-I network formed by small non-polar (gaseous) molecules, such as CH4 and CO2, in aqueous solution (for example, (CO2)8-y·46H2O) under pressure and at low but not necessarily (normally) freezing temperatures (only the oxygen atoms of water are shown.). The included molecules randomly occupy many of the cavities dependent on their size. Linear tetrakaidecahedral (51262) cavities form three orthogonal axes holding a dodecahedral cavity wherever they cross (ratio 6:2 respectively per unit cell); each dodecahedral cavity sitting (in a body-centered cubic arrangement) within a cube formed by six tetrakaidecahedral (51262) cavities. These (51262) cavities join at their hexagonal faces to form columns, going away from the viewer in the figure.
About 6.4 trillion (that is, 6.4x1012) tons
of methane lies at the bottom of the oceans in the form of
its clathrate hydrate [899].
Each kilogram of fully occupied hydrate (actually only about
96% occupancy is found) holds about 187 liters of methane
(at atmospheric pressure). [Back to Top
]
Opposite is shown the CS-II hydrate structure (cubic crystals containing sixteen 512 cavities, eight larger 51264 cavities and 136 H2O molecules per unit cell, and containing larger molecules such as 2-methylpropane in the larger cavities only). The tetrahedral 51264 cavities form an open tetrahedral network, with their centers arranged reminiscent to the cubic ice structure and separated by groups of three 512 cavities. The large proportion of 512 cavities is thought responsible for the similarities in the Raman spectra to gas saturated water [831].
Rather surprisingly the CS-II clathrate forms with molecular hydrogen (H2),
four molecules sitting in the large cages and one [1257b] or two [1257a] in
the small cages, that is, (2H2)16.(4H2)8.136H2O. [1257a]. [Back to Top
]

For interactive Figures, see Chime
Opposite is shown the HS-III hydrate structure. It has hexagonal crystals containing
three 512 cavities, two
small 435663 cavities, one large 51268 cavity
and 34 H2O molecules per unit cell, and containing
even larger molecules such as 2,2-dimethylbutane in
the larger cavities only). Each 51268 barrel -shaped
cavity is surrounded by six 435663 cavities around its central ring of 6 hexagons. These
(51268)
cavities join at their top and bottom hexagonal faces
to form columns, going away from the viewer in the figure. [Back to Top
]
Many other tiled three-dimensional structures are possible (see in JavaView) and other clathrate structures are being discovered; some related to the Frank-Kasper (FK) structures [1733].
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This page was last updated by Martin Chaplin on 26 July, 2011