Based upon an extensive literature survey, performed by Brunauer, Demming, Demming and Teller (BDDT), the IUPAC published in 1985 a classification of six sorption isotherms, which reflects the situation discussed above in connection with below figure. Adsorption isotherms are classified into six types (Types I-VI) by the International Union of Pure and Applied Chemistry (IUPAC), based on the adsorbent's porosity and adsorbent-adsorbate interactions. Type I occurs in microporous solids, Type II and III in macroporous solids with strong and weak affinities, respectively, and Type IV and V in mesoporous solids, with hysteresis loops appearing in the latter. Type VI describes stepwise adsorption on uniform non-porous surfaces, which is rare. 

Schematic illustration of adsorption potential, E, on (a) planar, nonporous surface; (b) mesopore; (c) micropore.

The appropriate IUPAC classification is shown in below image. Each of these six isotherms and the conditions leading to its occurrence are now discussed according to Sing et al.

IUPAC classification of sorption isotherm

The reversible type I isotherm is concave to the P/Po axis and the adsorbed amount approaches a limiting value as P/P0 →1. Type I isotherms are obtained when adsorption is limited to, at most, only a few molecular layers. This condition is encountered in chemisorption, where the asymptotic approach to a limiting quantity indicates that all of the surface sites are occupied. In the case of physical adsorption, sorption isotherms btained on microporous materials are often of type I. Micropore filling and therefore high uptakes are observed at relatively low pressures, because of the narrow pore width and the high adsorption potential. The limiting uptake is being governed by the accessible micropore volume rather than by the internal surface area.

Type II sorption isotherms are typically obtained in case of nonporous or macroporous adsorbent, where unrestricted monolayer-multilayer adsorption can occur. The inflection point or knee of the isotherm is called point B. This point indicates the stage at which monolayer coverage is complete and multilayer adsorption begins to occur.

The reversible type III isotherm is convex to the P/Po axis over its entire range and therefore does not exhibit a point B. This indicates that the attractive adsorbate-adsorbent interactions are relatively weak and that the adsorbate-adsorbate interactions play an important role. Isotherms of this type are not common, but an example is nitrogen adsorption on polyethylene or the adsorption of water vapor on the clean basal plane of graphite.

Type IV isotherms are typical for mesoporous materials. The most characteristic feature of the type IV isotherm is the hysteresis loop, which is associated with the occurrence of pore condensation. The limiting uptake over a range of high P/Po results in a plateau of the isotherm, which indicates complete pore filling. The initial part of the type IV can be attributed to monolayer-multilayer adsorption as in case of the type II isotherm.

Type V isotherms show pore condensation and hysteresis. However, in contrast to type IV the initial part of this sorption isotherm is related to adsorption isotherms of type III, indicating relatively weak attractive interactions between the adsorbent and the adsorbate.

The type VI isotherm is a special case, which represents stepwise multilayer adsorption on a uniform, non-porous surface, particularly by spherically symmetrical, non-polar adsorptives. The sharpness of the steps depends on the homogeneity of the adsorbent surface, the adsorptive and the temperature. Type VI isotherms were for example obtained with argon and krypton  on graphitized carbons at liquid nitrogen temperature.

Below are easy-to-read and a comprehensive list of these six isotherm types:

Reference List

• Sing K.S.W., Everett D.H., Haul R.A.W., Moscou L., Pierotti R.A., Rouquerol J. ands Siemieniewska T. (1985) Pure Appl. Chem. 57,603.

• Brunauer S., Deming L.S., Deming W.S. and Teller E. (1940) JAm. Chem. Soc. 62, 1723.

• Hill. T.L. (1955)J Phys. Chem. 59,1065.

• Polley M.H., Schaeffer W.D. and Smith W.R. (1953) J Phys. Chem. 57,469.

• Greenhalgh E. (l967)J Phys. Chem. 71,1151.