Hexagonal ices

When water freezes the crystalline form adopted depends upon the detailed conditions employed. At least nine structurally distinct forms of ice are known and the phase relations between them are summarized in Fig. 14.9. Thus, when liquid or gaseous water crystallizes at atmospheric pressure normal hexagonal ice If, forms, but at very low temperatures (—120° to — 140°) the vapour condenses to the cubic form, ice Ic. The relation between these structures is the same as that between the tridymite and cristobalite forms of SiOa (p. 342), though in both forms of ice the protons are disordered. 

In the hexagonal ice crystal, each molecule is linked to four others. The molecules arrange themselves as stacks of hexagonal rings. 

In its solid state, however, the basic structural features of ordinary hexagonal ice (ice I) are well established. In this structure (Figure 1.2), each water molecule is hydrogen bonded to four others in nearly perfect tetrahedral coordination. This arrangement leads to an open lattice in which intermolecular cohesion is large. 

The combination of this knowledge and the results of quick-freezing processes provide a theoretical opportunity to freeze products into a solid, amorphous state. If the freezing velocity is smaller than required for vitrification, but large enough to avoid an equilibrium state, an amorphous mixture will result of hexagonal ice, concentrated solids and UFW. 

Figure 5.1 Hexagonal ice structure showing open, tetrahedrally coordinated structure...

A tunneling junction device was used to determine the water structure at the mercury electrode in an aqueous solution of 0.25MHg2 (N03)2 + 0.3M HNO3. It was found that the structure of water domains is the same as that of hexagonal ice. Hydrogen bonding is a dominant, structuredetermining factor in liquid water near the mercury electrode surface. ... 

Figure 7.6 Refraction by a hexagonal ice crystal showing the rays associated with the 22° and 46 haloes.

As an example of the usefulness of molecular visualization, Figure 1 shows a 128-molecule sample of ordinary hexagonal ice. The molecules are drawn in the Space Filling model style, commonly known to provide a reasonable representation of the effective size of most molecules. Figure la shows the ideal lattice structure (T = O K). It can be seen that the structure is exceptionally open, with channels that permeate the entire lattice. Essentially, the picture provides a hands-on molecular illustration of the uniqueness of water (the density of the solid is so low that it actually floats on the liquid). 

Figure 1. Molecular structure of hexagonal ice. Ideal lattice structure (a) vs. structure in the presence of thermal excitations prior to melting (b).

Here it is worth paying attention to the existence, now proved, of a cubic form of ice [2] with a diamond structure corresponding to the wurtzite-type structure of ordinary hexagonal ice. It is also of interest that this type of ice seems to be unstable at temperatures of above — 100°, and the enormous difference bel/veen its thermal stability and that of ordinary ice is a problem wbb-n requires explanation and whose solution may throw light on t.be fundamental nature of the hydrogen bond in ie < The existence of tetrahedral bonding suggests... 

Professor H. S. Frank has suggested that the difference is connected with the partially covalent character of the hydrogen bond, as is ako brought out by the differential thermal expansion in different crystalline directions of ordinary Hexagonal ice [3]. 

Figure 7.3 Qualitative phase diagram of H20 in the 0-10 kbar range, showing additional equilibrium solid phases (ice n, ice m, ice V, and ice VI) beyond normal ice I (hexagonal ice Ih) of the low-pressure limit. (The dotted line marks the approximate upper limit of pressures in Fig. 7.1.) Note that the phase boundaries are shown here as straight lines, although slight curvature is present in the actual coexistence curves.

The combined volume of all the billions of open rooms" in the hexagonal ice crystals of a piece of ice is equal to the volume of the part of the ice that extends above water when ice floats. When the ice melts, the open spaces are exaedy filled in by the amount of ice that extends above the water level. This is why the water level doesn t rise when ice in a glass of ice water melts—the melting ice caves in and exactly fills the open spaces. 

It gradually became clear that the clathrate hydrates distinguished themselves by being both nonstoichiometric and crystalline at the same time, they differed from normal hexagonal ice because they had no effect on polarized light. 

All common natural gas hydrates belong to the three crystal structures, cubic structure I (si), cubic structure II (sll), or hexagonal structure H (sH) shown in Figure 1.5. This chapter details the structures of these three types of hydrate and compares hydrates with the most common water solid, hexagonal ice Ih. The major contrast is that ice forms as a pure component, while hydrates will not form without guests of the proper size. 

The most common solid form of water is known as ice Ih (hexagonal ice), with the molecular structure as shown in Figure 2.1 from Durrant and Durrant (1962). In ice each water molecule (shown as a circle) is hydrogen bonded (solid lines) to four others in essentially tetrahedral angles (Lonsdale, 1958). A description of... 

Fig. 1.11. Phase diagram of water. L = liquid water Ih = hexagonal ice lc = cubic ice 111—IX crystal configurations of ice (Figure 1 from [1.7])...

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