Adsorbed molecules interaction between

The determination of gas-solid virial coefficients can be a useful technique to explain the interaction between an adsorbed gas and a solid surface. The terms are defined so that the number of adsorbate molecules interacting can be readily ascertained. For example, the second order gas-solid interaction involves one adsorbate molecule and the solid surface the third order gas-solid interaction involves two adsorbate molecules and the surface, and so on. The number of adsorbed molecules under consideration is expanded in a power series with respect to the density of the adsorbed phase. 

At high coverages, adsorbate molecules interact not only with the surface, but also with neighbouring adsorbate molecules. In the most obvious case this involves the formation of bonds between neighbouring adsorbates. It has, for... 

It is likely that these small shifts will eventually be explained in terms of induced polarization interactions of the type described by the Kirk-wood-Bauer-Magat equation. At this time, however, it is not clear whether these interactions are between adjacent adsorbed molecules or between adsorbed molecules and surface OH groups. All studies of physical adsorption have involved surfaces which have OH groups. Further studies of physical adsorption on surfaces which are free of OH groups appears to be highly worth while. 

Heterogeneous photochemical processes depend strongly on the adsorbate and substrate characteristics, such as the electronic properties of the substrate, the nature of the adsorbate, the interaction between the substrate and the adsorbate and that between adsorbate molecules, the presence of surface impurities and the nature of the medium. Modern surface science provides powerful probes for the characterization of the structure and composition of surfaces and the dynamics of reactions occurring on them [3,7,8]. Several cases may be distinguished depending on the adsorbate and the substrate properties. 

Neither pure supercritical CO2 nor ethanol-modified CO2 eluted all the flavonoids tested in this experiment. But when phosphoric acid and ethanol modifiers were added to the mobile phase together, the separation on a silica-based column was significantly improved, and quercetin and risetin were eluted rapidly and efficiently. With an increase of phosphoric acid concentration, the peak shapes were also improved. Because the phosphoric acid molecules could be adsorbed onto the active sites of the stationary phase, which could prevent solute molecules from being strongly adsorbed, the interaction between solutes and stationary phase was eliminated, making the solutes easily elute from the chromatographic system. 

Because most solutions absorb infrared radiation in their bulk, the design of the electrochemical cell is an important consideration in interfacial reflection experiments. The optimum configuration involves a very thin solution layer which is a few microns thick and is sandwiched between the optical window and the reflective electrode (fig. 10.8). In order to achieve maximum sensitivity, the window has a hemispherical or triangular (prism) shape. Nevertheless, most radiation is absorbed in the bulk of the solution and the effect of interfacially adsorbed molecules cannot be seen unless special steps are taken. One procedure is to polarize the light in a cyclical fashion between s- and p-polarized light. If adsorbate molecules interact with the p-polarized light, the intensity of the... 

Stationary phase, which could prevent solute molecules from being strongly adsorbed, the interaction between solutes and stationary phase was eliminated, making the solutes easily elute from the chromatographic system. 

When the interfacial coverage increases, adsorbate molecules may occupy neighboring adsorption sites. As a rule, interaction between adsorbate and solvent molecules is different from those between adsorbate molecules and between solvent molecules. As a first approximation, only interactions between nearest neighbors are considered. 

Figure 7.11. Energetic scheme of a BET adsorbate. Adsorption sites can take not only one but several molecules. Interactions between admolecules are not taken into account [7.1-7.5].

The adsorption of gases on the surfaces of oxides differs greatly from that on metal surfaces. As oxides are ionic substances, interactions involve the acid-base properties of substrate and adsorbate, while interactions between uncompensated charges at the surface and dipole moments of the adsorbing molecules also play a role. In order to create a surface of an oxide MOx with cations and anions,... 

Various functional forms for / have been proposed either as a result of empirical observation or in terms of specific models. A particularly important example of the latter is that known as the Langmuir adsorption equation [2]. By analogy with the derivation for gas adsorption (see Section XVII-3), the Langmuir model assumes the surface to consist of adsorption sites, each having an area a. All adsorbed species interact only with a site and not with each other, and adsorption is thus limited to a monolayer. Related lattice models reduce to the Langmuir model under these assumptions [3,4]. In the case of adsorption from solution, however, it seems more plausible to consider an alternative phrasing of the model. Adsorption is still limited to a monolayer, but this layer is now regarded as an ideal two-dimensional solution of equal-size solute and solvent molecules of area a. Thus lateral interactions, absent in the site picture, cancel out in the ideal solution however, in the first version is a properly of the solid lattice, while in the second it is a properly of the adsorbed species. Both models attribute differences in adsorption behavior entirely to differences in adsorbate-solid interactions. Both present adsorption as a competition between solute and solvent. 

All gases below their critical temperature tend to adsorb as a result of general van der Waals interactions with the solid surface. In this case of physical adsorption, as it is called, interest centers on the size and nature of adsorbent-adsorbate interactions and on those between adsorbate molecules. There is concern about the degree of heterogeneity of the surface and with the extent to which adsorbed molecules possess translational and internal degrees of freedom. 

The following derivation is modified from that of Fowler and Guggenheim [10,11]. The adsorbed molecules are considered to differ from gaseous ones in that their potential energy and local partition function (see Section XVI-4A) have been modified and that, instead of possessing normal translational motion, they are confined to localized sites without any interactions between adjacent molecules but with an adsorption energy Q. 

The second general cause of a variable heat of adsorption is that of adsorbate-adsorbate interaction. In physical adsorption, the effect usually appears as a lateral attraction, ascribable to van der Waals forces acting between adsorbate molecules. A simple treatment led to Eq. XVII-53. 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

The state of the surface is now best considered in terms of distribution of site energies, each of the minima of the kind indicated in Fig. 1.7 being regarded as an adsorption site. The distribution function is defined as the number of sites for which the interaction potential lies between and (rpo + d o)> various forms of this function have been proposed from time to time. One might expect the form ofto fio derivable from measurements of the change in the heat of adsorption with the amount adsorbed. In practice the situation is complicated by the interaction of the adsorbed molecules with each other to an extent depending on their mean distance of separation, and also by the fact that the exact proportion of the different crystal faces exposed is usually unknown. It is rarely possible, therefore, to formulate the distribution function for a given solid except very approximately. 

A second criticism is that the model restricts attention to the forces between the adsorbent and the adsorbate molecules—the vertical interactions—and neglects the forces between an adsorbate molecule and its neighbours in the same layer—the horizontal interactions. From the nature of intermolecular forces (p. 5) it is certain that these adsorbate-adsorbate interactions must be far from negligible when a layer is approaching completion and the average separation of the molecules is therefore small in relation to their size. 

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

The total potential energy of adsorption interaction may be subdivided into parts representing contributions of the different types of interactions between adsorbed molecules and adsorbents. Adopting the terminology of Barrer (3), the total energy of interaction is the sum of contributions... 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

When two or more molecular species involved in a separation are both adsorbed, selectivity effects become important because of interaction between the 2eobte and the adsorbate molecule. These interaction energies include dispersion and short-range repulsion energies (( ) and ( )j ), polarization energy (( )p), and components attributed to electrostatic interactions. 

Adsorption (qv) is a phenomenon in which molecules in a fluid phase spontaneously concentrate on a sohd surface without any chemical change. The adsorbed molecules are bound to the surface by weak interactions between the sohd and gas, similar to condensation (van der Waals) forces. Because adsorption is a surface phenomenon, ah practical adsorbents possess large surface areas relative to their mass. 

The lecture deals with the advantages of IR spectroscopy at low or variable temperatures in the studies of molecule-surface interactions, lateral interactions between the adsorbed molecules and catalysis. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

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