Geometric heterogeneity

The preceding material has been couched in terms of site energy distributions—the implication being that an adsorbent may have chemically different kinds of sites. This is not necessarily the case—if micropores are present (see Section XVII-16) adsorption in such may show an increased Q because the adsorbate experiences interaction with surrounding walls of adsorbent. To a lesser extent this can also be true for a nonporous but very rough surface. 

The currently useful model for dealing with rough surfaces is that of the selfsimilar or fractal surface (see Sections VII-4C and XVI-2B). This approach has been very useful in dealing with the variation of apparent surface area with the size of adsorbate molecules used and with adsorbent particle size. All adsorbate molecules have access to a plane surface, that is, one of fractal dimension 2. For surfaces of Z 2, however, there will be regions accessible to small molecules 

In the case of mixtures of gases of different molecular size, an adsorbent of D 2 will effect some segregation by size. This segregation will also affect the probability of bimolecular reactions between molecules of different sizes [168]. 

The rate of physical adsorption may be determined by the gas kinetic surface collision frequency as modified by the variation of sticking probability with surface coverage—as in the kinetic derivation of the Langmuir equation (Section XVII-3A)—and should then be very large unless the gas pressure is small. Alternatively, the rate may be governed by boundary layer diffusion, a slower process in general. Such aspects are mentioned in Ref. 146. 

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Computer Simulations and Theory of Adsorption on Energetically and Geometrically Heterogeneous Surfaces... 

In this respect, this review provides a comprehensive survey of synthetic methods and physicochemical properties of the porous carbon materials. Furthermore, as electrochemical applications of the porous carbons to electrode materials for supercapacitor, the effects of geometric heterogeneity and surface inhomogeneity on ion penetration into the pores during double-layer charging/ discharging are discussed in detail by using ac-impedance spectroscopy, current transient technique, and cyclic voltammetry. 

Effect of Geometric Heterogeneity on Ion Penetration into the Pores during Double-Layer Charging/Discharging... 

In addition to structural (geometric) heterogeneity, characterized by different pore shapes and sizes, activated carbons like all porous solids may present surface energetic heterogeneity (SEH). This heterogeneity is mainly due to the presence of functional groups and strongly bound impurities on the surface of pores, whose characteristics depend on the type of row material used and the conditions of carbonization and activation processes,... 

The complex structure of activated carbon requires a different approach. In this case, the geometric heterogeneity of the micropore structure is best expressed primarily in terms of the micropore size distribution, although this simplest model is not entirely free from artifacts in the calculated distribution. Recognition of the inherent heterogeneity of the pore wall unit itself has, in early work, shown hope of providing a major advance in characterizing activated carbon structures. 

Sokolowska, Z. 1989. On the role of energetic and geometric heterogeneity in sorption of water vapor by soils Application of a fractal approach. Geoderma 45 251 -265. 

While the above models represent an improvement over classical capillary bundle models, they cannot reproduce the geometrical heterogeneity of natural porous media. As a result, several authors have tried to derive more sophisticated models based on random capillary networks. 

Another important conclusion concerns the geometrical heterogeneity of nanoporous carbons, which is characterized by the micropore and mesopore volume distributions. The current work demonstrates that in terms of the condensation approximation both these dishibutions are directly related to the adsorption potential distribution. As shown the pore volume distribution can be obtained by multiplication of the adsorption potential distribution... 

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