Positive Adsorption Methods

In positive adsorption methods, the amount adsorbed of a given chemical species (either molecular or ionic) on an initially free surface is determined and, in principle, from the knowledge of the area covered by that species, the total solid area can be computed. These methods require three conditions (Sposito 1984)  

It should be a reaction between the surface and the adsorbate resulting in the accumulation of the adsorbate ( positive adsorption ). The adsorbate may be in gas phase or in solution. 

The amount of adsorbate corresponding to one monolayer must be determined. 

The packing area of the adsorbate (equivalent area occupied per molecule) must be known. 

Under such conditions, the adsorption specific surface area, can be calculated as follows  

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POSITIVE adsorptiqn METHODS. The measurement of specific surface area with a positive adsorption method requires the satisfaction of three basic experimental conditions ... 

Although the positive adsorption methods offer the advantage of convenient determination in heterogeneous samples, they suffer from the uncertainty involved in the calibration of the packing area (which usually must be done by comparison with results from a physical method using reference clay materials) and from the fact that the monolayer parameter is model-dependent through Eqs. 1.6 and 1.7. It must also be remembered that the specific surface area determined by positive adsorption is ultimately a function of the reaction between surface functional groups and some probe molecule. If the experimental conditions of the reaction are close to those under which the surface behavior of a sample is of interest, then this estimate of specific surface area has surface chemical relevance. 

S. J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity. Academic Press, London, 1982, TTie first two chapters of this well known monograph present a thorough discussion of the concept of the packing area and the measurement of specific surface area by positive adsorption methods. 

The determination of specific surface area of soils and soil coUoids is of great importance in characterizing the reactivity of a sample, among other factors. However, it is actually an operational concept, because the A5 value depends on the experimental method employed, as it will be shown in Section 7.6.4.I. The underlying fact is that the effective area available for a particular reaction or process is dependent on the reactants and/or external factors involved. The experimental methods can be broadly classified into three categories (Sposito 1984) physical methods, positive adsorption methods, and negative adsorption methods. 

The catalytic activity of hematin on the surface of alumina gel and graphitized carbon black has already been comprehensively studied by the adsorption method [93, 94], It is shown that at low filling degrees hematin molecules are placed flatly, and the iron atom contacts with the surface playing the role of the sixth ligand in this case. The deactivation process is associated with reorientation of the surface hematin molecule which induces iron bond break with the carrier and simultaneous formation of hematin dimer by sixth catalytically active coordinate position. 

Kofke et al. (168) determined the concentration of gaUium in the framework of MFI-type materials by adsorption methods. KUk et al. (76) investigated the gaUium incorporation by using high-field Ga MAS NMR spectroscopy, showing that gallium preferentiaUy occupies the framework positions in Ga-MFI materials that are prepared by direct synthesis from a gel. 

Thus, a plot of versus will be a straight line, and can be evaluated from the slope. The negative adsorption method has been applied to some phyllosilicates (Bolt and Warkentin 1958 Edwards, Posner, and Quirk 1965a, b) but has received less attention than its positive adsorption counterpart. 

Specific surface area determination is most commonly conducted by positive adsorption studies (Section 7.6.4.2), and here the election of the probe molecule is very important. The most traditional and widely used method is N2 adsorption at 77 K, using the BET isotherm to evaluate monolayer coverage (Section 7.6.4.2) sometimes other inert gases, such as argon, are employed in the same conditions (Sposito 1984). In soil characterization, other substances are also used in the BET... 

The use of adsorption methods to characterize porosity in carbons, and the (then considered) undisputable position of the N2 BET isotherm appeared to be permanent. However, the advent of immersion calorimetry and its application to carbon chemistry, together with the availability of commercial calorimeters, presented a significant challenge to the supremacy of the N2 (77 K) adsorption isotherm. The School of Adsorption, University of Alicante, made use of immersion calorimetry and reviewed their work over several years (Rodrfguez-Reinoso et al., 1997 Silvestre-Albero et al., 2001). Immersion calorimetry, as a method of characterization, is discussed at length in Section 4.7. 

Recent progress in the theory of adsorption on porous solids, in general, and in the adsorption methods of pore structure characterization, in particular, has been related, to a large extent, to the application of the density functional theory (DFT) of Inhomogeneous fluids [1]. DFT has helped qualitatively describe and classify the specifics of adsorption and capillary condensation in pores of different geometries [2-4]. Moreover, it has been shown that the non-local density functional theory (NLDFT) with suitably chosen parameters of fluid-fluid and fluid-solid interactions quantitatively predicts the positions of capillary condensation and desorption transitions of argon and nitrogen in cylindrical pores of ordered mesoporous molecular sieves of MCM-41 and SBA-15 types [5,6]. NLDFT methods have been already commercialized by the producers of adsorption equipment for the interpretation of experimental data and the calculation of pore size distributions from adsorption isotherms [7-9]. 

An additional method for increasing particle size deserves mention. When a precipitate s particles are electrically neutral, they tend to coagulate into larger particles. Surface adsorption of excess lattice ions, however, provides the precipitate s particles with a net positive or negative surface charge. Electrostatic repulsion between the particles prevents them from coagulating into larger particles. 

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

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