First layer adsorption

The important advances in adsorption technology made possible by the BET theory (S, 4) clearly justify Model 1 in many applications. But one of the Models 2, 3, or 4 is apparently required for first-layer adsorption in a Type II isotherm. That an important condensation potential at the solid-gas interface is the Lennard-Jones 3-9 potential now seems well established (3, 7, 9, 10, 15). Equally clear is the fact that this potential is not solely responsible for physical adsorption the type of surface polarization predicted by the theory of structural adsorption (10) has been demonstrated by observed changes due to adsorption in linear... 

Substitution of Equation 6b in 4 for first-layer adsorption gives... 

The application of dispersion forces with surface polarization to account for a potential function in first-layer adsorption such as that described by Equation 7a, in which correlations with other adsorbate molecules result only in repulsion over the entire first-layer filling, seems more difficult to justify. Thus, b is apparently negative for the entire region below point B in most typical Type II isotherms, at least those in which point B appears at x < 0.05. If molecules adsorbed on the first layer of a Type II isotherm were to cause only repulsion, one would expect them to adsorb always in a pattern such as to remain as far apart as possible. But then when they are all a long distance from each other, as they would be near zero coverage, a repulsion term of sufficiently long range to account for a linear bO relationship is difficult to explain. Perhaps it may be possible to explain this situation in a Model 4 type adsorption process—i.e., in a model in which both the adsorbate and the adsorbent suffer polarization upon adsorption. 

This potential for first-layer adsorption agreed closely with the experimental one in the cases analyzed. The assumptions leading to Equation 6c were ... 

Muris et al. [31] used adsorption isotherms and calorimetric measurements, to study CH4 (and Kr) adsorbed on as-received carbon nanotubes produced by arc discharge, also from Montpellier. This study provided the first complete monolayer isotherm for any gas adsorbed on nanotube bundles. They found that for both CH4 and Kr there were two substeps present in the first-layer data. These two substeps indicate that in the first layer adsorption occurs on two... 

For thin film hematite e q>osed to a sodium pho hate solution, initially rapid uptake occurs during the first 10 minutes of solution exposure in ch phosphate conq)letes first layer adsorption. A short induction period ensues, lasting 10 min., which... 

Fig. 8.8 BET-based model for the uptake of a protic electrolyte by a basic polymer (e.g. a monoprotonic acid HA and a polymer with two protonable moieties/groups per repeating unit is depicted). The squares represent adsorption sites. The first layer adsorption sites, whose...

Only the modified BET (GAB) model, according to (8.22), is able to describe all adsorption isotherms on the whole concentration range satisfactorily. In Fig. 8.10 the corresponding fit curves are shown as solid lines. The three fit parameter, as the number of (first layer) adsorption sites z, the ratio a between the equilibrium constants and the equilibrium constant K for multilayer adsorption are reported in Table 8.2. 

Protonation of the z basic groups of the polymer chain. This corresponds to first layer adsorption of electrolyte molecules according to a BET model, described by the equilibrium constant K. 

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