Amorphous ice

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Image:Glassy water.jpg

Everyday ice is a crystal, which means its molecules are lined up in a repeating pattern. Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly oriented like the atoms of common glass. Amorphous ice is produced by cooling liquid water very quickly (around 1,000,000 K/s), so the molecules don't have enough time to form a crystal lattice.

Just as there are many different crystalline forms of ice (about thirteen), there are also different forms of amorphous ice, distinguished principally by their densities.

Contents 1 Formation techniques 2 Forms 2.1 Low-density amorphous ice 2.2 High-density amorphous ice 2.3 Very-high-density amorphous ice 2.4 Hyperquenched glassy water 3 Uses 4 References 5 External links

Formation techniques

The key to producing amorphous ice is the rate of cooling. The liquid water must be cooled to its glass transition temperature (about 165 K) in a matter of milliseconds to prevent the spontaneous formation of crystals. This is analogous to the production of ice cream, which must also be frozen quickly to prevent the growth of crystals and guarantee a smooth texture. The difference is that pure water forms crystals much more readily than the heterogeneous mixture of ingredients in ice cream, so amorphous water is more difficult to produce, requiring a physics lab rather than an ice cream churn.

Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another.

Forms

Low-density amorphous ice

Low-density amorphous ice (also called LDA, vapor-deposited amorphous water ice, amorphous solid water) is usually formed in the laboratory by a slow accumulation of water vapor molecules (physical vapor deposition) onto a very smooth metal crystal surface under 120 K. In outer space it is expected to be formed in a similar manner on a variety of cold substrates, such as dust particles. It is expected to be common in the subsurface of exterior planets, and comets ^[1].

Melting past its glass transition temperature (Tg) between 120 and 140 K, LDA is more viscous than normal water. Recent studies have shown the viscous liquid stays in this alternative form of liquid water up to somewhere between 140 and 210 K, a temperature range that is also inhabited by ice Ic, and ice IX ^[2]. LDA has a density of 0.94 g/cm³, less dense than the densest water (1.00 g/cm³ at 277 K), but denser than ordinary ice (ice I_h).

High-density amorphous ice

High-density amorphous ice (HDA) can be formed by subjecting lower pressure ices to very high pressures at very low temperatures. When cooled by liquid nitrogen to 77 K, HDA can form from ordinary natural ice at around 1.0 GPa and from LDA at around 0.5 GPa (atmospheric pressure at sea level is about 0.1 MPa), and has a density of 1.17 g/cm³. The resulting collapsed structure resists reverting to LDA at lower pressures and is stable for months at 77 K and 0.1 MPa.

Very-high-density amorphous ice

Very-high-density amorphous ice (VHDA), discovered in 2001, is usually made from high-density amorphous ice at 77 K by heating it up to about 160 K while under pressures of 1.15 GPa to create an annealing process. Once formed VHDA is more stable than either HDA or LDA and can retain its structure for months at normal atmospheric pressure. It has a density of 1.26 g/cm³.

Hyperquenched glassy water

Hyperquenched glassy water (HGW) is formed by spraying a fine mist of water droplets into a liquid such as propane around 80 K.

Uses

Amorphous ice is used in some scientific experiments, especially in electron cryomicroscopy of biomolecules ^[3]. The individual molecules can be preserved for imaging in a state close to what they are in liquid water.

References

• ^  Estimation of water-glass transition temperture based on hyperquenched glassy water experiments from Science (requires registration).
• ^  Liquid water in the domain of cubic crystalline ice I_c from AIP.
• ^  Dubochet, J., M. Adrian, J. J. Chang, J. C. Homo, J. Lepault, A. W. McDowell, and P. Schultz. Cryo-electron microscopy of vitrified specimens. Q. Rev. Biophys. 21:129-228. (1988).

External links

• Discussion of amorphous ice at LSBU's website.
• Journal of Physics article
• Glass transition in hyperquenched water from Nature (requires registration)
• Glassy Water from Science, on phase diagrams of water (requires registration)
• AIP accounting discovery of VHDA
• HDA in space
• Computerized illustrations of molecular structure of HDA
• Structure of amorphous ice