Lynne Wong's PhD thesis

Langmuir (1916, 1917, 1918) proposed that adsorption on both liquid and solid surfaces
normally involves the formation of a monomolecular layer (the monolayer). To support
this concept, he collected evidence of isotherms in which the amount of gas adsorbed
increased with increasing pressure and then saturated to a plateau, i.e. a monolayer
coverage. This type of isotherm was later referred to as a type I isotherm (see Fig 5.1).
His work paved the way to subsequent progress in the interpretation of adsorption data.
Specific amount adsorbed a
Relative pressure
p '/p 0
Figure 5.1. The six main types of gas physisorption isotherms, as per
the IUPAC classification (Sing et al., 1985).
In his 1916 paper, Langmuir stated that with highly porous adsorbents such as charcoal, "it
is impossible to know definitely the area on which the adsorption takes place" and that
"there are some spaces in which a molecule would be closely surrounded by carbon atoms
on nearly all sides". He concluded that equations derived for plane surfaces were not
applicable to adsorption by charcoal. Dubinin (1960) later showed that the mechanism of
physisorption in very narrow pores is not the same as that in wider pores, or on open
surfaces. He identified three groups of pores of different width: micropores, transitional
pores (now termed mesopores), and macropores, of internal width less than 2 nm, between
2 and 50 nm, and greater than 50 nm respectively.
Emmett and Brunauer (1934) used low temperature adsorption of nitrogen to determine the
surface area of a synthetic iron catalyst for the production of ammonia. They noted that
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