Lynne Wong's PhD thesis

the adsorption isotherms of a number of gases, measured at temperatures at, or near, their
respective boiling points were all S-shaped with certain distinctive features. This
prompted the recognition that adsorption was not always restricted to monolayer coverage
and to the emergence of the Brunauer-Emmett-Teller (BET) theory (1938) of multilayer
adsorption. This type of isotherm shows the amount adsorbed increases with increasing
pressure up to a point B, then levels off and starts to increase again at higher pressures; it
was later referred to as a type II isotherm. Point B is usually considered to represent the
completion of the monolayer and the beginning of the formation of the multilayer.
In addition to the type I and II isotherms described above, there are type III isotherms,
where initially there is very little adsorption, then once a small droplet of adsorbate
nucleates on the surface, additional adsorption occurs more easily because of strong
adsorbate-adsorbate interactions. Type IV and type V isotherms occur when multilayers of
gas adsorb onto the surface of the pores in a porous solid. Initially, the adsorption
resembles that of type II or type III adsorption, then the adsorbed layer gets so thick that it
fills up the pores, no more gas can adsorb, and the isotherm saturates. Both type IV and V
isotherms exhibit a hyteresis loop, the lower curve of which represents measurements
obtained by progressive addition of gas to the adsorbent, and the upper curve by
progressive removal.
These five classifications of isotherms were proposed by S. Brunauer, L.S. Deming, W.S.
Deming and E. Teller and are termed the BDDT classification (1940), and are sometimes
also referred to as the Brunauer classification (1945).
The classification proposed by the International Union of Pure and Applied Chemistry
(IUPAC) in 1985 (Sing et al., 1985), shown in Fig 5.1, includes a type VI isotherm which
has been observed more recently. It appears as a stepped isotherm, associated with layerby-layer
adsorption on a highly uniform surface. This type of isotherm is relatively rare
(Rouquerol et al., 1999).
5.3 ADSORPTION OF MOISTURE
When a hygroscopic material is maintained in contact with air at constant temperature and
humidity until equilibrium is reached, the material will attain a definite moisture content.
This moisture is termed the equilibrium moisture content (EMC) under the specified
conditions. In order to characterise this water sorption mechanism, it is common practice
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