Graphite adsorbed molecule interaction

The main difference between carbon nanotubes and high surface area graphite is the curvature of the graphene sheets and the cavity inside the tube. In microporous solids with capillaries which have a width not exceeding a few molecular diameters, the potential fields from opposite walls will overlap so that the attractive force which acts upon adsorbate molecules will be increased in comparison with that on a flat carbon surface [16]. This phenomenon is the main motivation for the investigation of the interaction of hydrogen with carbon nanotubes (Figure 5.14). 

After graphitization, the most notable feature is the sharp discontinuity at monolayer coverage. Beyond the monolayer the heat of adsorption is close to the heat of vaporization (as required by the BET theory) but does show some influence of the surface as well. At coverage below V/ Vm = 1 the heat of adsorption increases with increasing coverage, probably due to lateral interactions between the adsorbed molecules. 

The H-K method is based upon the model suggested by Everett and Powl100 which describes the interaction potential of a single adsorbate molecule between two parallel planes of the atoms of graphitized carbon. In the H-K expansion of the Everett and Powl s work, the space between the parallel carbon planes, i.e., the pore is assumed to be filled with adsorbed gas molecules. Thus, the contribution of adsorbate-adsorbate-adsorbent interaction to the total interaction potential is considered along with that of adsorbate-adsorbent interaction. 

The gas-solid interaction for an adsorbate molecule with a pore wall is taken as the well-known Steele potential for graphite [16] ... 

The classification of the low concentration region of the sample isotherm into linear, concave, and convex isotherms has been discussed (Section 2-4). For adsorption of a compound from the gas phase, Brunauer et al. (30) have described the five basic isotherm types shown in Fig. 3-5. Type I isotherms occur when monolayer adsorption is distinctly favored over multilayer adsorption. After an initial rapid uptake of adsorbate by the adsorbent, the surface monolayer is completed and further adsorption does not occur. If additional layers begin to adsorb prior to completion of the first monolayer, isotherms of type II result. These are typical of physical adsorption. Type III isotherms are less common and occur in adsorption systems where the attraction between adsorbed molecules are strong and adsorbent-adsorbate interactions are either relatively weak or are independent of surface coverage (e.g., uniform surface graphites). As a result the total attraction of an adsorbed molecule... 

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