Multilayer Gas Adsorption on Composite Surfaces

The equation of Brunauer, Emmett, and Teller (BET) for the adsorption of a gas on a free plane homogeneous surface is extended to the case of a mixture of two such surfaces, in an attempt to explain the observed deviations below one monolayer. The resulting equation may be greatly simplified for the case that the adsorption energy parameter, c, for each component surface, is large with respect to unity, as is usually the case in low temperature adsorptions of nitrogen, argon, etc. This approximate form of the equation is applied to the nitrogen isotherm on an inorganic salt and gives substantially better agreement with experiment than does the simple BET equation. The region of agreement between the experimental and BET isotherms is discussed on the basis of a two-component surface, and conditions are given under which one might obtain two distinct regions of the isotherm which are fitted by the BET equation by the use of two sets of parameters. It is suggested that it is the lower of these two regions which gives rise to the occasionally observed ``saturation adsorption'' values corresponding to only small fractions of a monolayer.