Adsorption Space Filling

The adsorption of vapors in complex porous systems takes place approximately as follows [1-3] at first, micropore filling occurs, where the adsorption behavior is dominated nearly completely by the interactions of the adsorbate and the pore wall after this, at higher pressures, external surface coverage occurs, consisting of monolayer and multilayer adsorption on the walls of mesopores and open macropores, and, at last, capillary condensation occurs in the mesopores. 

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If we assume that the adsorption interaction forces increases as the monolayer thickness, we can use the adsorption space filling model for the interfacial phase. 

Hence, the temperature coefficient of k1 having been measured, for an absolute calculation of k only kt) and bo must be known, and not the heat of adsorption, X. At the moment we are concerned with b0. A simple statistical estimate can be based on the assumption that in the absence of adsorption energy the adsorption space is filled at a proportion given by the ratio of the molecular adsorption volume (liquid volume Fm) to the molecular gas volume... 

As shown in Tables 1 and 2, iV-protectcd amino acids have been widely used as CDAs mainly because of their ready availability. However, it was noticed that chromatographic resolution of the diastereomeric amides increased when the space-filling protecting group (e.g., Boc) was cleaved, indicating that the bulky Boc group shields the diastereodifferentiation and the specific adsorption sites of the analyte with the stationary phase94. 

Micropore filling is the process in which molecules are adsorbed in the adsorption space within micropores. 

Equation 4.12 represents the work needed to compress the adsorptive from its equilibrium pressure in the gas phase to its saturation pressure P°. From this equation, since it is assumed that the liquefied adsorbate is incompressible, and knowing the normal density of the liquid at the given adsorption temperature, it is possible to obtain the volume of the filled adsorption space by... 

Dubinin and coworkers, during the course of their extensive studies on activated carbons, have developed the so-called theory of volume filling of micropores. Based on numerous experimental data, Dubinin and collaborators have added a second postulate to the Polanyi theory, which complements it. For an identical degree of filling of the volume of adsorption space, the ratio of adsorption potentials for any two vapors is constant ... 

These limits are to some extent arbitrary since the pore filling mechanisms are dependent on the pore shape and are influenced by the properties of the adsorptive and by the adsorbent-adsorbate interactions. The whole of the accessible volume present in micropores may be regarded as adsorption space and the process which then occurs is micropore filling, as distinct from surface coverage which takes place on the walls of open macropores or mesopores. Micropore filling may be regarded as a primary physisorption process (see Section 8) on the other hand, physisorption in mesopores takes place in two more or less distinct stages (monolayer-multilayer adsorption and capillary condensation). 

In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules so that not all adsorbed molecules are in direct contact with the surface layer of the adsorbent. In capillary condensation the residual pore space which remains after multilayer adsorption has occurred is filled with condensate separated from the gas phase by menisci. Capillary condensation is often accompanied by hysteresis. The term capillary condensation should not be used to describe micropore filling because this process does not involve the formation of liquid menisci. 

We now will consider the second case of adsorption on zeolites A and X of relatively small molecules, when, at the initial and final stages of the filling of the adsorption space of zeolites, adsorption is due to the predominant manifestation of various forms of adsorption interactions. Then for each variety of predominant interaction, for instance dispersion or electrostatic interaction. Equation 7 may be represented in the form... 

Note that the degree of filling of the limiting adsorption space is ... 

The principal feature of the micropore filling is the temperature invariance of the differential molar work of adsorption at a constant degree of filling of the adsorption space, that is ... 

Thus, the Dubinin and Radushkevich equation states the distribution of the adsorption space W according to the differential molar work of adsorption. A t)q)ical plot of the adsorption potential versus the reduced pressure is shown in Figure 4.2-1 for T = 77, 273 and 473 K. For low reduced pressure, the adsorption potential is high, while it is low for high reduced pressure. The latter means that less molar work is required for adsorption via micropore filling when the gas is approaching the vapour pressure. 

In equation (50), nj is the adsorption capacity for component 2 and 0 the fractional filling of the adsorption space. It is important to distinguish between these alternative forms. In the first three cases some model of the adsorption space must be employed. Thus if (e.g. in the case of microporous adsorbents)... 

FIGURE 3.8 (a) Side view of water adsorption on hydroxylated 1010) and (b) (1011) quartz surfaces. Crystal is shown as cage and water as space filled. Color key Si, gray O, black H, light gray. 

FIGURE 3.14 (a and b) Side view of water adsorption on wollastonite 100 and 001 surfaces, respectively where water is shown as space filled and the crystal as Si tetrahedral unit and oxygen and calcium as balls, (c) Top view of water adsorbed on wollastonite (shown as space filled) 102 surface. Color key Ca, light gray O, black Si, gray H, whitish gray. 

FIGURE 3.16 (a) Side view of methanoic acid adsorption on wollastonite 100 surface, (b) Top view of methanoic acid adsorption on wollastonite 001 surface, (c) Side view of methanoic acid adsorption on wollastonite 102 surfaee, shown as space filled. Color key Ca, light gray O, deep gray Si, gray C, black O (methanoic acid), deep gray H, white. 

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