freeze drying otherwise lyophilisation (rus. сублимационная сушка otherwise лиофилизация; лиофильная сушка) — a process of solvent removal from the frozen solutions, gelssuspensions and biological objects, which is based on the sublimation of the solidified solvent (ice) without forming macro-quantities of the liquid phase.

Description

The driving force behind freeze drying is the pressure difference between the solvent vapour above the object and the surrounding gaseous atmosphere. If the temperatures are quite low, freeze drying is possible even at atmospheric pressure, but the rate of this process is very slow. For practical purposes, vacuum freeze drying is used, with a much higher rate; however, even in this case, the drying rate is still lower than when using traditional drying methods at elevated temperatures, which is one of the major limitations for its commercial use.

Since the sublimation of ice is a strongly endothermic process, the intensity of its evaporation can be increased by an additional supply of heat, but its amount is strictly controllable (heating up a dehydrated product); overheating leads to the melting of the solvent and morphological changes in the freeze dried product including collapse. Another way to intensify the freeze drying process is to use cooled-down condenser traps, with the solvent vapour pressure above them being substantially less than the residual pressure in the freeze-drying chamber. In this case, along with the increasing driving force of the process, any solvent vapour is avoided in the vacuum system.

Along with the evaporation of ice, freeze-drying is accompanied by some physical and chemical processes: the softening and crystallisation of amorphous alloys; phase transitions, the formation and disruption of metastable solvates, which are particularly numerous in their variety when solutions of substances are being freeze-dried as they form stable chemical compounds with the solvent. One of the most unusual phenomena of this kind is the formation of intermediate states, with properties close to a viscous liquid at temperatures 40-70 degrees below the eutectic temperature for this system. The last stages of the freeze drying process often have a significant chemical component and are accompanied by the removal of the chemically bound solvent from the solvate.

The main solvent to be used in the freeze-drying process is water, then come tert-butanol, glacial acetic acid, dimethyl sulfoxide, dimethyl carbonate, and a number of aromatic compounds, including benzene and toluene. Freeze drying is substantially used in the pharmaceutical and food industries since it allows them to preserve the micromorphology and chemical homogeneity of the frozen objects to a large extent. Freeze drying is one of the most important techniques for the cryochemical synthesis of inorganic materials, and particularly one of the most effective methods to prevent agglomeration in the synthesis of isolated nanoparticles.

Illustrations

Laboratory freeze dryer ALPHA 1–4 (CHRIST).
Laboratory freeze dryer ALPHA 1–4 (CHRIST).

Author

  • Shlyakhtin Oleg A.

Sources

  1. How Freeze-Drying Works // TLC Cooking, 2009. Discovery Communications, LLC. — http://recipes.howstuffworks.com/freeze-drying.htm/printable (reference date: 01.09.2009).
  2. Wikipedia, the free Encyclopedia. Freeze drying. — http://en.wikipedia.org/wiki/Freeze_drying (reference date: 01.09.2009).
  3. Welcome to Freeze Drying // Lyophilization Info Online. —www.freezedryinginfo.com/index.html (reference date: 01.09.2009).
  4. Tretyakov Yu.D., Oleinikov N.N., Shlyakhtin O. A. Cryochemical Technology of Advanced Materials. — London: Chapman & Hall, 1997. P. 72–126.
  5. M. B. Generalov Cryochemical nanotechnology (in Russian). - Мoscow: Akademkniga, 2006. 143-169 pp.

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