There are six main types of adsorption isotherms (see Fig.). Type I is characteristic of microporous solids with a relatively small proportion of the outer surface. Type II refers to polymolecular adsorption in nonporous or macroporous adsorbents. Type III is characteristic of non-porous sorbents with low energy of adsorbent-adsorbate interaction. Types IV and V are similar to types II and III, but refer to porous adsorbents. Type VI isotherms are characteristic of non-porous adsorbents with homogeneous surface.
Adsorption isotherms are used to calculate the specific surface area of materials, mean pore size or mean size of deposited particles, as well as pore size or particle size distribution.
There are several methods of mathematically representing adsorption isotherms, with different models used to describe the adsorption process. At low surface coverage by the adsorbate the adsorption isotherm equation for a uniform surface is given by Henry’s equation: a = Kp, where a is the adsorption value, p is the gas pressure, K is a constant. At medium coverage Freundlich’s empirical equation can be applied: a = kpn, where k and n are constants.
A rigorous adsorption isotherm theory was proposed by I. Langmuir for the model of monolayer adsorption on a uniform surface, in which the attraction between adsorbate molecules and their mobility along the surface can be ignored. Langmuir’s isotherm equation has the form: a = ambp / (1 + bp), where b is the adsorption coefficient, which depends on the adsorption energy and temperature; am is the monolayer capacity.
Further development of the theory consisted in the exclusion of one of the assumptions used by Langmuir. Thus, S. Brunauer, P. Emmett and E. Teller proposed a theory of polymolecular adsorption (BET method); T. Hill and J. de Boer developed a theory that takes into account the interaction between the adsorbed molecules (Hill-de Boer isotherm), etc.
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