Features of adsorption

Of particular interest has been the study of the polymer configurations at the solid-liquid interface. Beginning with lattice theories, early models of polymer adsorption captured most of the features of adsorption such as the loop, train, and tail structures and the influence of the surface interaction parameter (see Refs. 57, 58, 62 for reviews of older theories). These lattice models have been expanded on in recent years using modem computational methods [63,64] and have allowed the calculation of equilibrium partitioning between a poly-... 

Hybrid Crystallization/Adsorption Process. In 1994, IFP and Chevron announced the development of a hybrid process that reportedly combines the best features of adsorption and crystallization (59,99). In this option of the Eluxyl process, the adsorbent bed is used to initially produce PX of 90—95% purity. The PX product from the adsorption section is then further purified in a small single-stage crystallizer and the filtrate is recycled back to the adsorption section. It is reported that ultrahigh (99.9+%) purity PX can be produced easily and economically with this scheme for both retrofits of existing crystallization units as well as grass-roots units. A demonstration plant was built at Chevron s Pascagoula refinery in 1994. 

A very important criterion for electron structure is the percent d-character, which characterizes the number of unpaired electrons in the rf-orbitals of the individual metal atom. Because of the vacancies existing in these orbitals, metals will interact with electron-donating species forming electron pairs. It is this interaction that determines the special features of adsorption of these species and, as a consequence, the catalytic activity of a given metal. 

Voltammetric features of adsorption coupled EC mechanisms (2.177) [128] and (2.178) [129] are rather unpredictable and deviate strongly from the EQ mechanism of a dissolved redox couple. Their voltammetric behaviour is mainly controlled by the adsorption parameter p, and the dimensionless chemical parameters k"s = j and... 

As an example, Fig. 9.7 shows adsorption isotherms of krypton on the (0001) face of graphite. The dashed lines were fitted using Eq. (9.35) with / = 4.5. In reality the coverage increases steeply and the two-phase region can be identified. Figure 9.7 shows another typical feature of adsorption The amount adsorbed decreases with increasing temperature. 

The second reaction used to illustrate various features of adsorptive reactors is the direct synthesis of hydrogen cyanide from ammonia and carbon monoxide ... 

A distinctive feature of adsorption from solution is that it always involves a competition between the solvent and solute which has be taken into account in any complete treatment of the data. 

The technique of gas adsorption manometry is now probably the most widely used it is simple and effective since the pressure transducer provides all the information required to determine the adsorption isotherm. Thus, the pressure and temperature of each dose of gas are measured and the gas is allowed to enter the adsorption bulb. After adsorption equilibrium has been established, the amount adsorbed is calculated from the change in pressure. The most critical features of adsorption manometry are summarized in the following checklist, with more detailed comments given in Section 3.4. 

To Judge by the number of papers published annually on adsorption from dilute solution, this subject is more important than adsorption from binary solutions. However, the basic issues can be better illustrated from the latter so we have emphasized them in the previous sections. Now we shall review some important features of adsorption from dilute solutions. The examples to be given are merely meant to illustrate certain points and do not claim to be a selection based on a "quality test" among the. say. 10 isotherms published in the literature. 

Another feature of adsorption from solution is the variety and complexity of molecules that may be involved in the processes. Indeed one can be interested either by a simple organic molecule, like benzene and its derivatives, or by much larger molecules like proteins, surfactants, or polymers, which bear many different chemical functions and may adopt a large number of conformations at the interface. For such molecules, a good knowledge of both the surface chemistry and the accessibility of porous materials are crucial to understand the adsorption phenomenon. 

Now, a particular feature of adsorption from solution is the variety of molecules which can be used. Playing on their polarity or charge, it is then possible to define applications where the interest is not to determine the total surface area of the sample but, rather, to define the percentage of the surface, which can be considered as polar or nonpolar, hydrophilic or hydrophobic, acid or basic, etc. 

Part 1 of the book is of an introductory nature. Thus, Chapter 1 provides an updated overview of adsorption by carbons, differentiates the features of adsorption on nonporous and porous carbons, and identifies the main obstacles still hindering the study of gas adsorption by porous carbons. Recent major developments and research needs in this field are also mentioned. Chapter 2 presents a survey of carbons in the context of adsorption and classifies them on the basis of structural criteria. This chapter might help nonspecialists in carbon to find materials that they can use for adsorption purposes. 

Incidentally, evidence of orientation is shown in a simple laboratory experiment in which a thin layer of melted beeswax is allowed to cool slowly on the surface of water when solidified, the beeswax will be wettable on the underside and non-wettable on the top side. Orientation constitutes an important feature of adsorption which helps us to understand the specificity so often found. 

A distinctive feature of adsorption from solution is that the surface is always completely covered and the sorption mechanism involves competition between the components of the liquid phase. Detailed theoretical interpretation of isotherms from liquid mixtures is most often based on binary liquid systems. The isotherms are measured by immersing the solid sample into the liquid phase. The concentration is monitored by UV, refractive index (RI), or other analytical methods. 

In previous chapters, the main modes of interaction between ions and soil mineral colloids have been discussed in Chapters 4 and 5, the principles of ion-surface interactions have been laid down, and in Chapters 8 and 9, the main features of adsorption onto silicate and oxide minerals have been reviewed. As introduced in Chapter 11, two main contributions should be considered, namely, the nonelectrostatic forces composed of physical (van der Waals forces) and/or chemical (specific bonding) interactions and the electrostatic forces arising from the charged nature of both the adsorbate and the surface this is reflected in two contributions (considered independent) to the Gibbs free energy, as in Equation 11.5 ... 

The theories of retention and efficiency (HETP) reflect the main features of adsorption capillary columns. 

Activated carbons are widely used in gas separation and purification processes because of their large adsorptive capacity and low cost. Adsorption processes occurring on the surface of activated carbons are connected with their surfiice area, porous structure and surface heterogeneity. To model adsorption processes using activated carbons, informatitm is need on the basic features of adsorption isotherms and pore difhision coefficients. 

Studying adsorption from solution of polymer mixtimes is of great interest for the theory of PCM because many binders for composites are two-and more-component systems. The presence of two components determines the specificity of the properties of the boundary layers formed by two different polymeric molecules. From another point of view, as the large majority of polymer pairs is thermodynamically immiscible,there may arise interphase layers between two components in the border layer at the interface. The selectivity of adsorption of various components, which is a typical feature of adsorption from mixture, leads to the change in composition of the border layer as compared with composition in the equilibrium solution. This fact, in turn, determines the non-homogeneity in distribution of components in the direction normal to the solid surface, i.e., creates some compositional profile. As compared with stud3ung adsorption from solution of individual polymers, adsorption from mixture is studied insufficiently. The first investigations in this field were done " for immiscible pair PS-PMMA on silica surface, in conditions remote from the phase separation. It... 

It is well known that in the hterature there are more than 100 isotherm equations derived based on various physical, mathematical, and experimental considerations. These variances are justified by the fact that the different types of adsorption, solid/gas (S/G), solid/liquid (S/L), and liquid/gas (L/G), have, apparently, various properties and, therefore, these different phenomena should be discussed and explained with different physical pictures and mathematical treatments. For example, the gas/solid adsorption on heterogeneous surfaces have been discussed with different surface topographies such are arbitrary, patchwise, and random ones. These models are very useful and important for the calculation of the energy distribution functions (Gaussian, multi-Gaussian, quasi-Gaussian, exponential) and so we are able to characterize the solid adsorbents. Evidently, for these calculations, one must apply different isotherm equations based on various theoretical and mathematical treatments. However, as far as we know, nobody had taken into account that aU of these different isotherm equations have a common thermodynamical base which makes possible a common mathematical treatment of physical adsorption. Thus, the main aim of the following parts of this chapter is to prove these common features of adsorption isotherms. 

An extension of the relative simple formulation used in SCMs for surfaces with permanent eharges (see Section II.B.l) has been published recently [84]. A fictitious surface species (X ) was defined and hypothetical complexation reactions on site X were written, and thus cation-exchange reactions of permanent negative layer charges were easily incorporated into such model. The model showed not only to fit satisfactorily all of the experimental data of transition metal adsorption on montmorillonite but also to explain specific features of adsorption on clays compared to oxides. 

For the SCLF model adsorption can be seen as purely partitioning behavior. Adsorption is quantified in this model by assigning a statistical mechanical partition function to each solute molecule. The partition function recognizes three major features of adsorption (1) entropy of adsorption, (2) energy of adsorption, and (3) solute-solvent interaction energy. 

An interesting feature of adsorption on MOFs is that some materials have framework flexibility and structural change may occur during the... 

Thus, one of the main features of adsorption phenomena on platinum metals is the development of chemisorption reactions between the hydrocarbons and the electrode, and this leads to considerable changes in the nature of the particles on adsorption. The presence of water makes it possible (particularly at elevated temperatures) for reactions forming oxygen-containing particles to occur. 

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