Adsorbents, general properties

Doyen [158] was one who theoretically examined the reflection of metastable atoms from a solid surface within the framework of a quantum- mechanical model based on the general properties of the solid body symmetry. From the author s viewpoint the probability of metastable atom reflection should be negligibly small, regardless of the chemical nature of the surface involved. However, presence of defects and inhomogeneities of a surface formed by adsorbed layers should lead to an abrupt increase in the reflection coefficient, so that its value can approach the relevant gaseous phase parameter on a very inhomogeneous surface. 

The potential at an interface can be related through the abundant relationships of thermodynamics to the concentration and adsorbability of ions, but thermodynamics provides no information as to how the potential varies as we move through a small distance perpendicular to the surface. This observation reminds us of Figure 7.13, in which the profile of the variation in some general property in the immediate vicinity of a surface is shown. Figure 11.2a is essentially the same drawing with the property under discussion specified as the potential. 

The bulk properties of celluloses are generally influenced by adsorbed moisture. The effect of the change in bulk solid properties for microcrystalline cellulose has been demonstrated by a tableting operation in a very simplistic manner. Dry microcrystalline cellulose (%MC = 0.07) was compared with material with a moisture content above that associated with completion of the monolayer (%MC = 5.1). A thermodynamic picture of the character of water in these samples can be based on the adsorbate thermodynamic properties A// > 3.5kcal/mole (14.65 kJ/mole),... 

The origin of the coagulation behavior of Iler s silica sols is far from understood. The electrosteric barrier model herein proposed is designed to stimulate new experimental initiatives in the study of colloidal silica sol particles and their surface structure. The adsorbed steric layer, impregnated with bound (exchanged) cations, at the surface of 1-100-nm-diameter silica sol particles has the general properties needed to understand the anomalous coagulation behavior. The details await experimental and theoretical input. 

The results can be summarized as follows. A prewave (or prepeak) of the same shape and general properties as that described in Section 14.3.2 appears (Figure 14.3.5), representing the reduction of dissolved O to form a layer of adsorbed R. This response occurs at potentials more positive than the diffusion-controlled wave, because the free energy of adsorption of R makes reduction of O to adsorbed R easier than to R in solution. The prepeak is followed by the wave for reduction of dissolved O to dissolved R. While the latter resembles that observed in the absence of adsorption, it is perturbed by the depletion of species O at the foot of the diffusion wave during reduction of O to adsorbed R. The larger the value of )Sr, the more the prepeak precedes the diffusion peak (Figure 14.3.6). 

The behavior of protein molecules at solid surfaces is very complex. The interaction between the surface and the protein is determined both by the nature of the protein, the surface and the medium outside the surface. The situation is further complicated by the fact that exchange reactions between protein molecules of the same or different kinds take place on the surface. Except for these exchange reactions most protein molecules appear to be irreversibly adsorbed. Although the details of the interaction between protein molecules and surfaces are not known it is assumed that general properties of the surface and the protein such as hydrophobicity, charge density, ion binding, hydration etc. are involved. For reviews, see e.g (21.35-37). 

A third general approach to the correlation of adsorption equilibria for mi-croporous adsorbents was developed by Dubinin [15,16] from ideas originally suggested by Polanyi [17] and Berenyi [18,19]. The adsorbed phase within the micropores is assumed to behave as a liquid but, as a result of the force field of the adsorbent, the properties differ from those of the bulk liquid sorbate. The difference in free energy between the adsorbed fluid and the saturated liquid sorbate at the same temperature is referred to as the adsorption potential (s) which, assuming an ideal vapor phase, may be calculated from... 

The interplay of hydrogen-bonded dimers and substrate-adsorbate coordination is a general property of aromatic carboxyUc acids on low index gold single crystal electrodes. 

Though charcoal is the most generally useful adsorbent it is by no means the only possible one. Adsorption is a very general property of surfaces. 

Returning to multilayer adsorption, the potential model appears to be fundamentally correct. It accounts for the empirical fact that systems at the same value of / rin P/F ) are in essentially corresponding states, and that the multilayer approaches bulk liquid in properties as P approaches F. However, the specific treatments must be regarded as still somewhat primitive. The various proposed functions for U r) can only be rather approximate. Even the general-appearing Eq. XVn-79 cannot be correct, since it does not allow for structural perturbations that make the film different from bulk liquid. Such perturbations should in general be present and must be present in the case of liquids that do not spread on the adsorbent (Section X-7). The last term of Eq. XVII-80, while reasonable, represents at best a semiempirical attempt to take structural perturbation into account. 

In general there are two factors capable of bringing about the reduction in chemical potential of the adsorbate, which is responsible for capillary condensation the proximity of the solid surface on the one hand (adsorption effect) and the curvature of the liquid meniscus on the other (Kelvin effect). From considerations advanced in Chapter 1 the adsorption effect should be limited to a distance of a few molecular diameters from the surface of the solid. Only at distances in excess of this would the film acquire the completely liquid-like properties which would enable its angle of contact with the bulk liquid to become zero thinner films would differ in structure from the bulk liquid and should therefore display a finite angle of contact with it. 

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