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Amorphous ice

Amorphous ice

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Amorphous ice is an amorphous solid
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....

 form of water, meaning it consists of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 molecules that are randomly arranged like the atoms of common glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. Everyday ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 is a crystalline material where the atoms are regularly arranged in a lattice whereas amorphous ice is distinguished by a lack of long-range order in its atomic 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
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

) or by compressing ordinary ice at low temperatures.

Although almost all water ice on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 is the familiar crystalline Ice Ih
Ice Ih
thumb|Photograph showing details of an ice cube under magnification. Ice Ih is the form of ice commonly seen on earth.Ice Ih is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic which is...

, amorphous ice dominates in the depths of interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

, making this by far the most common structure for H2O in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 at large.

Just as there are many different crystalline forms of ice (currently fifteen known), there are also different forms of amorphous ice, distinguished principally by their densities
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

.

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 136 K or −137 °C) in a matter of milliseconds to prevent the spontaneous nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

 of crystals.

This is analogous to the production of ice cream
Ice cream
Ice cream is a frozen dessert usually made from dairy products, such as milk and cream, and often combined with fruits or other ingredients and flavours. Most varieties contain sugar, although some are made with other sweeteners...

, which must also be frozen quickly to prevent the growth of crystals in the mixture. The difference is that pure water forms crystals much more readily than the heterogeneous mixture of ingredients in ice cream, so amorphous ice is more difficult to produce, requiring a physics lab rather than an ice cream churn.

Pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 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 cryoprotectant
Cryoprotectant
A cryoprotectant is a substance that is used to protect biological tissue from freezing damage . Arctic and Antarctic insects, fish, amphibians and reptiles create cryoprotectants in their bodies to minimize freezing damage during cold winter periods. Insects most often use sugars or polyols as...

s can be added to water, to lower its freezing point (like an antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....

) and increase viscosity, which inhibits formation of crystals. Vitrification
Vitrification
Vitrification is the transformation of a substance into a glass. Usually, it is achieved by rapidly cooling a liquid through the glass transition. Certain chemical reactions also result in glasses...

 without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation
Cryopreservation
Cryopreservation is a process where cells or whole tissues are preserved by cooling to low sub-zero temperatures, such as 77 K or −196 °C . At these low temperatures, any biological activity, including the biochemical reactions that would lead to cell death, is effectively stopped...

 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
Physical vapor deposition
Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...

) onto a very smooth metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 crystal surface under 120 K. In outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

 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
Ice Ih
thumb|Photograph showing details of an ice cube under magnification. Ice Ih is the form of ice commonly seen on earth.Ice Ih is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic which is...

).

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
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

 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
Peter Jenniskens
Petrus Matheus Marie Jenniskens is a Dutch astronomer and a senior research scientist at the Carl Sagan Center of the SETI Institute and at NASA Ames Research Center. He is an expert on meteor showers. Jenniskens is the author of the 790 page book "Meteor Showers and their Parent Comets"...

 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.

Uses


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.

External links

  • Discussion of amorphous ice at LSBU
    London South Bank University
    London South Bank University is a university in south London. With over 25,000 students and 1,700 staff, it is based in the London Borough of Southwark, near the South Bank of the River Thames, from which it takes its name...

    's website.
  • Journal of Physics article (requires registration)
  • Glass transition in hyperquenched water from Nature
    Nature (journal)
    Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

     (requires registration)
  • Glassy Water from Science
    Science (journal)
    Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

    , on phase diagram
    Phase diagram
    A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...

    s of water (requires registration)
  • AIP accounting discovery of VHDA
  • HDA in space
  • Computerized illustrations of molecular structure of HDA