Irving Langmuir investigated gas adsorption on solid surfaces. The Langmuir adsorption model treats a surface as a set of sites that can each be empty or occupied, balancing adsorption against desorption.
The finite number of sites produces saturation: increasing gas pressure cannot raise single-layer occupancy above one molecule per site in this model. Lateral interactions and multilayer formation require a different description.[ref]
The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum by Irving Langmuir. 1918, site occupancy and adsorption isotherms
Biographical archive: tile.loc.gov/storage-services/service/gdc/gdcfindingaidpdfs/ms012160/ms012160.pdf
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