Chapter 3
Gas Hydrates
3.1 Physical Properties of Gas Hydrates
Understanding of nucleation of gas hydrates requires basic knowledge about the
physical properties of ice and clathrate hydrates. There are excellent textbooks on
the physical properties of clathrate hydrates, such as by Sloan and Koh [1] and by
Makogon [2, 3], and it is not our purpose here to reinvent their contents. Rather,
we will briefly review the selected contents here. The thermodynamic aspects of
clathrate hydrates will be discussed in Sect. 3.2.
3.1.1 Crystal Structures of Clathrate Hydrates
Common ice (Ih) floats on top of liquid water because it is less dense than liquid water.
That the crystalline form of a substance is less dense than its liquid counterpart is
rather unusual. In the case of ice, the tetrahedral orientations of the hydrogen bonding
render its structure hollow. The hollow structure of ice can accommodate small nonpolar molecules as “guests” in its cavities under the right temperature and pressure
conditions [1]. These crystalline solids in which guest molecules are accommodated
by water (“hosts”) are called clathrate hydrates, gas hydrates, or clathrates. Since
water molecules are polar and can form hydrogen bonds whereas the guest molecules
are non-polar, they have low affinity with each other and no chemical bonds form
between the guest and the host. Like ice that has 17 known phases [4, 5], several
lattice structures of clathrate hydrates form at different temperature and pressure
conditions and guest compositions [1]. Both pure (single-component) and mixed
(multi-component) gas hydrates can form [1].
There are three common crystal structures of clathrate hydrates called Structure I
(sI), Structure II (sII), and Structure H (sH) [1, 6–12], as shown in Fig. 3.1. There are
also less common structures of Structure III and Structure VI, but we will not discuss
them any further. The unit cell of Structure I consists of two 5
12 cages (“small cages”)
© Springer Nature Switzerland AG 2020
N. Maeda, Nucleation of Gas Hydrates,
https://doi.org/10.1007/978-3-030-51874-5_3
61
Gas Hydrates
3.1 Physical Properties of Gas Hydrates
Understanding of nucleation of gas hydrates requires basic knowledge about the
physical properties of ice and clathrate hydrates. There are excellent textbooks on
the physical properties of clathrate hydrates, such as by Sloan and Koh [1] and by
Makogon [2, 3], and it is not our purpose here to reinvent their contents. Rather,
we will briefly review the selected contents here. The thermodynamic aspects of
clathrate hydrates will be discussed in Sect. 3.2.
3.1.1 Crystal Structures of Clathrate Hydrates
Common ice (Ih) floats on top of liquid water because it is less dense than liquid water.
That the crystalline form of a substance is less dense than its liquid counterpart is
rather unusual. In the case of ice, the tetrahedral orientations of the hydrogen bonding
render its structure hollow. The hollow structure of ice can accommodate small nonpolar molecules as “guests” in its cavities under the right temperature and pressure
conditions [1]. These crystalline solids in which guest molecules are accommodated
by water (“hosts”) are called clathrate hydrates, gas hydrates, or clathrates. Since
water molecules are polar and can form hydrogen bonds whereas the guest molecules
are non-polar, they have low affinity with each other and no chemical bonds form
between the guest and the host. Like ice that has 17 known phases [4, 5], several
lattice structures of clathrate hydrates form at different temperature and pressure
conditions and guest compositions [1]. Both pure (single-component) and mixed
(multi-component) gas hydrates can form [1].
There are three common crystal structures of clathrate hydrates called Structure I
(sI), Structure II (sII), and Structure H (sH) [1, 6–12], as shown in Fig. 3.1. There are
also less common structures of Structure III and Structure VI, but we will not discuss
them any further. The unit cell of Structure I consists of two 5
12 cages (“small cages”)
© Springer Nature Switzerland AG 2020
N. Maeda, Nucleation of Gas Hydrates,
https://doi.org/10.1007/978-3-030-51874-5_3
61