Adsorption London dispersion forces

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. 

This difference in behavior between H-mordenite and 5A and 13X zeolites may be explained by a remark from Dubinin and Astarkhov (8). According to them, Equation 3 applies well in the absence of cations such as Na+ or Ca2+ in the microporous voids, when the London dispersion forces play the chief role during adsorption. On the contrary, the charged cations lead to interactions of a different nature, and Equation 3 is no longer valid. The behavior of the H-mordenite corresponds to the former... 

All adsorption processes result from the attraction between like and unlike molecules. For the ethanol-water example given above, the attraction between water molecules is greater than between molecules of water and ethanol As a consequence, there is a tendency for the ethanol molecules to be expelled from the bulk of the solution and to concentrate at die surface. This tendency increases with the hydrocarhon chain-length of the alcohol. Gas molecules adsorb on a solid surface because of die attraction between unlike molecules. The attraction between like and unlike molecules arises from a variety of intermolecular forces. London dispersion forces exist in all types of matter and always act as an attractive force between adjacent atoms and molecules, no matter how dissimilar they are. Many oilier attractive forces depend upon die specific chemical nature of the neighboring molecules. These include dipole interactions, the hydrogen bond and the metallic bond. 

The London dispersion forces are present and important in most adsorption processes and in adhesive interactions between dissimilar materials. The free energy of interaction per unit area between materials 1 and 2 in contact is where W 2 -s... 

Physical adsorption, or van der Waals adsorption, results from a relatively weak interaction between the solid and the gas. The forces responsible for adsorption are dispersion forces (characterized by London see 3.3.1) and/or electrostatic forces (Coulombic see 3.3.2) if either the gas or the solid is polar in nature. Physical adsorption is reversible hence all the gas adsorbed by physical adsorption can be desorbed by evacuation at the same temperature. Chemical adsorption is a result of a more energetic interaction between the solid and the gas than that of physical adsorption. Reversal of chemical adsorption using a vacuum requires elevated temperature, and even that may not be sufficient. Physical adsorption, being of more interest in gas-solid flows, is the focus of the following sections. 

London dispersion forces 0.1-40 van der Waals forces Physical adsorption... 

Physical adsorption is a universal phenomena, producing some, if not the major, contribution to almost every adhesive contact. It is dependent for its strength upon the van der Waals attraction between individual molecules of the adhesive and those of the substrate. Van der Waals attraction quantitatively expresses the London dispersion force between molecules that is brought about by the rapidly fluctuating dipole moment within an individual molecule polarizing, and thus attracting, other molecules. Grimley (1973) has treated the current quantum mechanical theories involved in simplified mathematical terms as they apply to adhesive interactions. 

Adsorption by dispersion forces, i.e., London-van der Waals dispersion forces acting between adsorbate and adsorbent... 

This chapter aims to give guidelines on how to use adsorption methods for the characterization of the surface area and pore size of heterogeneous catalysts. The information derived from these measurements can range from the total and available specific surface area to the pore sizes and the strength of sorption in micropores. Note that this spans information from a macroscopic description of the pore volume/specific surface area to a detailed microscopic assessment of the environment capable of sorbing molecules. In this chapter we will, however, be confined to the interaction between sorbed molecules and solid sorbents that are based on unspecific attractive and repulsive forces (van der Waals forces, London dispersion forces). 

Physical adsorption (physisorption) occurs when an adsorptive comes into contact with a solid surface (the sorbent) [1]. These interactions are unspecific and similar to the forces that lead to the non-ideal behavior of a gas (condensation, van der Waals interactions). They include all interactive and repulsive forces (e.g., London dispersion forces and short range intermolecular repulsion) that cannot be ascribed to localized bonding. In analogy to the attractive forces in real gases, physical adsorption may be understood as an increase of concentration at the gas-solid or gas-liquid interface imder the influence of integrated van der Waals forces. Various specific interactions (e.g., dipole-induced interactions) exist when either the sorbate or the sorbent are polar, but these interactions are usually also summarized under physisorption unless a directed chemical bond is formed. 

Adsorption by Dispersion Forces. Occurs via London-van der Waals dispersion forces acting between adsorbent and adsorbate molecules (Figure 2-9). Adsorption by this mechanism generally increases with an increase in the molecular weight of the adsorbate and is important not only as an independent mechanism, but also as a supplementary mechanism in all other types. For example, it accounts in part for the pronounced ability of surfactant ions... 

Here e is the static (zero frequency) relative dielectric constant hP is Planck s constant, i.e., 6.626 10 34 J s ve is the main UV adsorption frequency, which equals for most substances involved 2.9 — 3.0 1015 s 1 and n is the refractive index for visible light (generally taken at a wavelength of 589 nm). The first term in the equation is due to dipole-dipole and dipole-induced dipole interactions, and the second term is due to London dispersion forces (unretarded). The first term is always smaller than (3/4)kpT the second term can be much larger. 

In summary, mixed films of solute and solvent can be prepared as retracted films however, these are always transient or metastable systems, the probability of whose formation becomes greater the more fully does molecular adlineation of solute and solvent occur. If the concentration of polar solute is high enough, the film eventually becomes free of solvent as adsorption equilibrium is approached, and the retracted film is then free of solvent molecules. Where the solute and solvent molecules are so different in shape or size that the intermolec-ular cohesion between them through London dispersion forces becomes a minor factor, mixed films are never formed. Therefore, mixed films occur by the retraction process imder special conditions nonetheless, when they can be produced, a useful technique is available for studying the intermolecular interaction of solute and solvent molecules in the adsorbed state. 

The preferred technique for sampling organic vapors is collection on a porous sorbent by the van der Waals forces of adsorption (particularly the London dispersion force), with later desorption and instrumental analysis. The rate of adsorption at the sorbent surface is extremely fast and is not normally considered a rate-controlling step, and the adsorption equilibrium is normally shifted far enough in the direction of the adsorbed phase that the concentration at the sorbent surface can be regarded as insignificant. All molecules that arrive at the surface are therefore adsorbed. The adsorbed vapors are extracted from the sorbent by means of a solvent or heat, and analysis is normally carried out by gas chromatography (GC), or less often by LC. 

The adsorption theory of adhesion attributes adhesive strength to the action of London dispersion forces, combined in many instances with contributions from other forces (dipolar, polar or primary bonding) Calculations indicate that, in spite of their relatively low strength compared with the other types of bonding, these force can account for far greater strengths than are ever achieved experimentally. It is because all these forces are... 

Debye and Keesom forces together with London Dispersion forces are known coiiec-tively as van der Waals forces. See Lifshitz-van der Waals forces for a further discussion. They play a significant role in the Adsorption theory of adhesion and in surface phenomena such as Contact angles and interfacial tension. 

As it is known adsorption can be considered as a physical or chemical process. The forces responsible for physical adsorption are van der Waals or London dispersion forces. The hydrogen bonds are also responsible for this kind of adsorption. Physical adsorption is a rule, a reversible process, i.e., the physicosorbed layer is removed by reducing pressure or by increasing temperature. 

The second assumption (B), that is, is the size and orientation of the graphitic microcrys-tallite which controls the adsorption process, has to be addressed very seriously. Extents and enthalpies of adsorption are massively dependent on the London Dispersion Forces (van der Waals forces) which exist within the porosity of a carbon. Such porosity is unique among solid porous materials and exhibits considerable versatility. Calculations of areas of theoretical graphitic microcrystallites, as has been reported above, with the assumption that these areas are available for adsorption, have little relevance. 

The origins of these variable densities of adsorbates are with the adsorption potentials which exist with microporosity, as illustrated in Figure 4.2 and which will be discussed further under Section 4.6.2 (London dispersion forces). 

There are two classes of adsorption processes physical adsorption and chemical adsorption (chemisorption). In physical adsorption, the binding forces are London dispersion forces, dipole-dipole attractions, and so on. In chemical adsorption covalent chemical bonds are formed between the atoms or molecules of the surface and the atoms or molecules of the adsorbed substance. The Langmuir isotherm applies to both classes if only a monolayer of atoms or molecules can be adsorbed on the surface and if the adsorbed molecules do not dissociate. There are other isotherms that apply to the case of multiple layers. ... 

Thus, whenever there is contact between two materials at a molecular level, there will be adhesion. Physical adsorption or chemical adsorption (chemisorption) will occur. Specific types of bonding may be present, but there will always, at least, be London dispersion forces. That is the essential idea of the adsorption theory of adhesion. 

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