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Adsorption

[ Note : Not to be confused with 'Absorption' whereby molecules, atoms or ions of a substance 'soak' into other materials. E.g. water / sponge ]

In adsorption, atoms, ions or molecules from a gas, liquid (or dissolved solid) adhere to a surface, and can dramatically alter the chemical and reactive properties of both components.

It's of fundamental importance in very many chemical. environmental and biological reaction scenarios, and is extensively used in industrial chemical processes. It's been very substantially investigated since it was first described in the 1880s.

Typically, molecules of a volatile substance 'stick' to surfaces due to a variety of physical forces - including Van Der Waals forces and co-valent bonding etc.. At the same time, the particles can 'desorp' away from the surface due to the heat-related (thermo-dynamic) movements of the particles concerned.

Depending on the materials involved, the adsorption process can involve multiple factors, many involving quantum mechanics, and can become extremely complex.

Over the years, many highly detailed theories about adsorption have been suggested, but to date, no overall mathematical or computational model can currently explain exactly what is happening in all cases. In any given example, it's likely that many of the processes described below are operating - and interacting - at the same time.

Technical details of the various theories - including :

  • the Freundlich equation (1906)
  • the Polanyi adsorption potential theory (1914)
  • the Langmuir equation theory (1918)
  • BET theory (1938)
  • the Single-molecule explanation (1946)
  • the Quantum mechanical model (1980)

can all be found at Wikipedia

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