cryochemistry (rus. криохимия) — (from Greek kryos “cold”) - a branch of science and technology studying and using chemical conversion in liquid and solid phases at low (down to 70 K) and ultra-low (below 70 K) temperatures.

Description

The main objectives of cryochemistry include studying the mechanism of elementary chemical reactions at low temperatures; studying the effect of intermolecular interactions on the reactivity and its connection with physical and chemical properties of substances; obtaining chemical compounds and particles that are unstable or highly reactive at normal temperatures; identifying the low-temperature limit of chemical activity of substances.

Chemical reactions at low temperatures were observed for the first time by J. Dewar in the early 20th century (fluorination of hydrocarbons at 90 K; reaction of alkali metals, H2S and other compounds with liquid oxygen). Systematic studies in the field of cryochemistry have been conducted since 1950s; they were contributed by the emergence of new experimental techniques, especially methods of radio spectroscopy and matrix isolation (G. Porter , J. Pimentel, 1954). In liquid-phase reactions (for example, halogen with olefins) at low temperatures an important role is played by the relatively weak intermolecular interaction of reagents with each other and with molecules of the medium, which at ordinary temperatures is not significant due to thermal motion. Various features can be observed in cryochemical reactions. Thus, instead of the Arrhenius dependence of the rate constant on the temperature, the constant increases with the decreasing temperature, reaches its maximum and then decreases. In addition, the reaction mechanism, order and direction are often changed. Solid-state reactions at ultra-low temperatures usually require external initiating action (e.g. photolysis, UV radiation, mechanochemical effect) or the participation of high-activity reagents, such as atomic metals. Cryochemistry offers unique opportunities for obtaining and stabilizing chemically unstable particles and compounds. The particles are isolated in inert matrices (usually solid noble gases - Ar, Kr, Xe, Ne) at temperatures that exclude the possibility of thermal diffusion, usually below the boiling point of liquid nitrogen (method of matrix isolation). Matrix isolation can be used to obtain high-energy fuels, whose energy content exceeds the energy of the most performant modern fuels. New methods of cryochemical synthesis are based on low-temperature co-condensation of reagents (N.N. Semenov, A.I. Shalnikov, 1929). In the chemical industry low temperatures are used for ammonia synthesis, for catalytic conversion of methane and for cationic polymerisation of isobutylene, for obtaining amorphous and fine-grained metals, etc. Cryochemical processes based on physical and chemical conversions of aqueous solutions at low temperatures (cryocrystallisation, freeze-drying, extraction and dispersion), in combination with subsequent dehydration, thermal decomposition, sintering, etc., at temperatures above 70 K are promising for the production of ferrites, solid electrolytes, piezoceramics, catalysts, and adsorbents.

Author

  • Shlyakhtin Oleg A.

Sources

  1. Sergeev G.B., Batyuk V.A. Cryochemistry. (in Russian)— Moscow.: Khimija, 1978. — 296 pp.
  2. Goldanskii, V. I. Tunneling phenomena in chemical physics. — New York : Gordon and Breach Science Publishers, 334 p.
  3. Tretyakov Y. D. Cryochemical technology basics. (in Russian)— Moscow.: Vysshaja shkola, 1987. — 142 p.

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