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Gas Clathrates

V sI clathrate
V sII clathrate
V sH clathrate
Clathrate ice cavities

 

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). Their structures require a minimum amount of small molecules to fit into and stabilize the cavities 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].


Characteristic properties of the clathrates
Type
Lattice
Space group
Unit cell
 Unit cell formula a
Clathrate I  Cubic 
Pm3n
 a=1.20 nm
 (S)2·(L)6·46H2O
Clathrate II  Face-centered cubic 
Fd3m
 a=1.73 nm
 (S)16(L+)8·136H2O
Clathrate H  Hexagonal
P6/mmm
 a=1.23 nm
 c=1.02 nm
 (S)5(L++)·34H2O

 

Cavity
512
51262
51264
51268
435663
H2O
20
24
28
36
20
Mean cavity radius, Å
3.95
4.33
4.73
5.71
4.06
free volume, Å3
51
77
120
213
44
Clathrate I, /unit cell
2
6
-
-
-
Clathrate II, /unit cell
16
-
8
-
-
Clathrate H, /unit cell
3
-
-
1
2
Guest molecules,
for example,   approximate radius, Å
Ar, O2, N2, CH4
CO2, C2H6
C3H8, (CH3)3CH
(CH3)3CC2H5
CH4
1.8-2.2
1.8-2.7
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

 

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 sII 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 to top of page]

Clathrate-I crystal structure

For interactive Figures, see Chime

sI clathrate

Shown opposite is the cubic clathrate sI 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 to top of page]

sII clathrate

Clathrate-II crystal structure

For interactive Figures, see Chime

 

Opposite is shown the sII 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 sII 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 to top of page]

Clathrate-H crystal structure

For interactive Figures, see Chime

sH clathrate

Opposite is shown the sH 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 to top of page]

 

 

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This page was last updated by Martin Chaplin on 23 June, 2008


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