Adsorption mechanism

The ideal carbon fitr pure physisorption can be derived directly fiom the Dubitiin-Radushkevich equation. The two carbon-related parameters are the structural constant B and the micropore volume Wo. Usually these values are obtained from N2-isotherms at 77K. A lower value of B will yield a higher value of We, i.e. a hi er adsorption capacity. In the original DR-equation B is related to the width of the Gaussian micropore distribution but it is also well known that B [K j is directly related to the adsorption energy E [kJ.moT ] by Eq.4, the latter being in turn related to the mean micropore half-width L [nm] by Eq.5 [37]. 

But even more important is the value of the micropore volume W . Whereas 5 is a measure of the adsorption force. Wo gives the available adsorption space. It is clear that a 

This solution contains approximately 8% Cu, 2% Cr and 0.05% Ag. Ammonia and ammonium carbonate are added to enhance solubillfy. The carbon is added to this solution (at 308K). After a sufficient equilibration time the wet carbon is removed and mixed with dry impregnated carbon, in order to absorb excess impregnation solution, Tlien the carbon is 

The exact reaction mechanisms, as far as they are known, are described in the next section. In some cases, e.g. the adsorption of CNCl, they are very complex, due to the fact that the carbon surface is not only an inert matrix but plays a significant role in the oxidation/reduction reactions. 

Finally one should note that a second disadvantage, apart from the loss of available micropore volume, is a noticeable reduction in self-ignition temperature. The presence of metal ions on the surface of die carbon will lower the temperature at which self combustion of the carbon can occur, bringing it down to around 500K. 

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Adsorption Mechanisms. The following mechanisms of adsorption are responsible for the formation of mineral—reagent bonds. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

The water removal mechanism is adsorption, which is the mechanism for ad Class 4 drying agents. The capacity of such materials is often shown in the form of adsorption isotherms as depicted in Figures 9a and 9b. The initial adsorption mechanism at low concentrations of water is beheved to occur by monolayer coverage of water on the adsorption sites. As more water is adsorbed, successive layers are added until condensation or capidary action takes place at water saturation levels greater than about 70% relative humidity. At saturation, ad the pores are fided and the total amount of water adsorbed, expressed as a Hquid, represents the pore volume of the adsorbent. 

The gases that have been used most often are hydrogen, carbon monoxide, and oxygen. Hydrogen is by far the most useful, and it has the best established adsorption mechanism. It dissociates at room temperature on most clean metal surfaces of... 

The above discussion has tacitly assumed that it is only molecular interactions which lead to adhesion, and these have been assumed to occur across relatively smooth interfaces between materials in intimate contact. As described in typical textbooks, however, there are a number of disparate mechanisms that may be responsible for adhesion [9-11,32]. The list includes (1) the adsorption mechanism (2) the diffusion mechanism (3) the mechanical interlocking mechanism and (4) the electrostatic mechanism. These are pictured schematically in Fig. 6 and described briefly below, because the various semi-empirical prediction schemes apply differently depending on which mechanisms are relevant in a given case. Any given real case often entails a combination of mechanisms. 

In the adsorption mechanism, adhesion is modeled as occurring across a well-defined interface by molecular interaction across that interface, and is often... 

Fig. 6. Four mechanisms of adhesion, (a) The adsorption mechanism (contact adhesion), (b) The diffusion mechanism (diffusion interphase adhesion), (c) The mechanical interlocking mechanism. (d) The electrostatic mechanism.

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

The standard methods of drying can be classified as deposition of the moisture as either water or ice decomposition of the water chemical precipitation absorption adsorption mechanical separation and vaporization. The completeness with which dryness can be accomplished by any process depends upon the factors controlling the equilibrium conditions achieved in the operation. A brief discussion of each method is first given. 

M. Tammaro, J. W. Evans. Monomer-dimer surface reaction models Influence of the dimer adsorption mechanism. Phys Rev E 52 2310-2317, 1995. 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

The presence of moisture on steel above the critical humidity but below the saturation point may be caused by an adsorption mechanism or by the presence of particles of deliquescent salts on the surface. Once rusting has begun, the composition of the rust already formed will influence the relative humidity at which further rusting will occur, because rusts formed in polluted atmospheres contain hygroscopic salts. The method by which moisture reaches the surface is probably less important, however, than the length... 

Hermosin MC, Martin P, Cornejo J (1993) Adsorption mechanisms of monobutyltin in clay minerals. Environmental Science and Technology, 27(12) 2606-2611. 

On the basis of this relation, the direct application of TLC adsorption in investigations of the adsorption mechanism from mobile phases is possible [61-65]. 

The adsorption mechanism in chromatography on alumina differs from that on silica gel because of the structural differences between these adsorbents. Relationships between the values of solutes and the adsorption data for the mobile phase components on sihca gel G and alumina G have been investigated by Rozylo [64,65]. The theoretical and experimental results obtained by the relation 2 = /( 1) show a good agreement for the two adsorbents. 

For an oxidized surface, the value of y is 10" - 1.7-10 and it decreases with increasing the experimental temperature. In this case the activation energy of a change in yis 2.1 kcal/mole. From these data it can be inferred that the heterogeneous de-excitation of singlet oxygen proceeds in terms of the physical adsorption mechanism similar to that described for glass. 

C. H. Giles, T. H. MacEwan, S. N. Natchwa and D. Smith, Studies in Adsorption, Part XI A System of Classification of Solution Adsorption Isotherms and its Use in Diagnosis of Adsorption Mechanism and its Measurement of Specific Surface Area of Solids, J. Chem. Soc., p. 3973,1960. 

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