Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly arranged like the molecules of common glass. Everyday ice is a crystalline material where the molecules are regularly arranged in a lattice whereas amorphous ice is distinguished by a lack of long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water (so the molecules do not have enough time to form a crystal lattice) or by compressing ordinary ice at low temperatures.
Although almost all water ice on Earth is the familiar crystalline Ice Ih, amorphous ice dominates in the depths of interstellar medium, making this likely the most common structure for H2O in the universe at large.
The production of amorphous ice hinges on the fast rate of cooling. Liquid water must be cooled to its glass transition temperature (about 136 K or −137 °C) in milliseconds to prevent the spontaneous nucleation of crystals. This is analogous to the production of ice cream from heterogeneous ingredients, which must also be frozen quickly to prevent the growth of crystals in the mixture.
Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another.
Chemicals known as cryoprotectants can be added to water, to lower its freezing point (like an antifreeze) and increase viscosity, which inhibits formation of crystals. Vitrification without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation of cells and tissues.
Low-density amorphous ice
Low-density amorphous ice, also called LDA, vapor-deposited amorphous water ice, amorphous solid water (ASW) or hyperquenched glassy water (HGW), 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. It may also form in the coldest region of the Earth's atmosphere, the summer polar mesosphere, where noctilucent clouds exist.
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. 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 Ih).
Hyperquenched glassy water (HGW) is formed by spraying a fine mist of water droplets into a liquid such as propane around 80 K or by hyperquenching fine micrometer-sized droplets on a sample-holder kept at liquid nitrogen temperature, 77 K, in a vacuum. Cooling rates above 104 K/s are required to prevent crystallization of the droplets. At liquid nitrogen temperature, 77 K, HGW is kinetically stable and can be stored for many years.
High-density amorphous ice
High-density amorphous ice (HDA) can be formed by compressing ice Ih at temperatures below ~140 K. At 77 K, HDA forms from ordinary natural ice at around 1.6 GPa and from LDA at around 0.5 GPa (approximately 5,000 atm). At this temperature, it can be recovered back to ambient pressure and kept indefinitely. At these conditions (ambient pressure and 77 K), HDA has a density of 1.17 g/cm3.
Peter Jenniskens and David F. Blake demonstrated in 1994 that a form of high-density amorphous ice is also created during vapor deposition of water on low-temperature (< 30 K) surfaces such as interstellar grains. The water molecules do not fully align to create the open cage structure of low-density amorphous ice. Many water molecules end up at interstitial positions. When warmed above 30 K, the structure re-aligns and transforms into the low-density form.
Very-high-density amorphous ice
Very-high-density amorphous ice (VHDA) was discovered in 1996 by Mishima who observed that HDA became denser if warmed to 160 K at pressures between 1 and 2 GPa and has a density of 1.26 g/cm3 at ambient pressure and temperature of 77 K. More recently it was suggested that this denser amorphous ice was a third amorphous form of water, distinct from HDA, and called it VHDA.
Amorphous ice is used in some scientific experiments, especially in electron cryomicroscopy of biomolecules. The individual molecules can be preserved for imaging in a state close to what they are in liquid water.
- Debennetti, Pablo G; H. Eugene Stanley. "Supercooled and Glassy Water". Physics Today. Retrieved 19 September 2012.
- Velikov, V.; Borick, S; Angell, C. A. (2001). "Estimation of water-glass transition temperature based on hyperquenched glassy water experiments". Science 294 (5550): 2335–8. Bibcode:2001Sci...294.2335V. doi:10.1126/science.1061757. PMID 11743196.
- Murray, B. J.; Jensen, E. J. (2010). "Homogeneous nucleation of amorphous solid water particles in the upper mesosphere". J. Atm. Sol-Terr. Phys. 72: 51–61. Bibcode:2010JASTP..72...51M. doi:10.1016/j.jastp.2009.10.007.
- Jenniskens P., Blake D. F. (1994). "Structural transitions in amorphous water ice and astrophysical implications". Science 265 (5173): 753. Bibcode:1994Sci...265..753J. doi:10.1126/science.11539186. PMID 11539186.
- Jenniskens P., Blake D. F. (1996). "Crystallization of amorphous water ice in the solar system". Astrophysical Journal 473 (2): 1104. Bibcode:1996ApJ...473.1104J. doi:10.1086/178220. PMID 11539415.
- Jenniskens P., Banham S. F., Blake D. F., McCoustra M. R. (July 1997). "Liquid water in the domain of cubic crystalline ice Ic". Journal of Chemical Physics 107 (4): 1232–41. Bibcode:1997JChPh.107.1232J. doi:10.1063/1.474468. PMID 11542399.
- Mishima o., Calvert L. D., Whalley E. (1984). "‘Melting ice’ I at 77 K and 10 kbar: a new method of making amorphous solids". Nature 310 (5976): 393. Bibcode:1984Natur.310..393M. doi:10.1038/310393a0.
- Mishima, O.; Calvert, L. D.; Whalley, E. (1985). "An apparently 1st-order transition between two amorphous phases of ice induced by pressure". Nature 314 (6006): 76. Bibcode:1985Natur.314...76M. doi:10.1038/314076a0.
- Jenniskens P., Blake D. F., Wilson M. A., Pohorille A. (1995). "High-density amorphous ice, the frost on insterstellar grains". Astrophysical Journal 455: 389. Bibcode:1995ApJ...455..389J. doi:10.1086/176585.
- O.Mishima (1996). "Relationship between melting and amorphization of ice". Nature 384 (6609): 546–549. Bibcode:1996Natur.384..546M. doi:10.1038/384546a0.
- Loerting, Thomas; Salzmann, Christoph; Kohl, Ingrid; Mayer, Erwin; Hallbrucker, Andreas (2001). "A second distinct structural "state" of high-density amorphous ice at 77 K and 1 bar". Physical Chemistry Chemical Physics 3 (24): 5355. Bibcode:2001PCCP....3.5355L. doi:10.1039/b108676f.
- Dubochet, J.; Adrian, M.; Chang, J. .J; Homo, J. C.; Lepault, J-; McDowall, A. W.; Schultz, P. (1988). "Cryo-electron microscopy of vitrified specimens". Quarterly reviews of biophysics 21 (2): 129–228. doi:10.1017/S0033583500004297. PMID 3043536.
- Discussion of amorphous ice at LSBU's website.
- Journal of Physics article (requires registration)
- 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