Waals-type dispersion force

Adsorption phenomena are divided into two main categories, physical (van der Waals or dispersion forces) and chemical (analogous to valence-bonding). The former type results in small... 

Such a model of surface modification has been used by Filippini, Gramaccioli, and Simonetta133 to investigate the naphthalene crystal (001) face. Their calculations, based on a Kitaigorodsky-type method,134 accounted for the reconstruction of the surface and the subsurface layers of the crystal. The principle of the calculation is to minimize the cohesion potential written as a sum of atom-atom potentials.134 The potentials include an attractive part in R"6 (of van der Waals or dispersion forces) which is of the type discussed in Section I.A.2, and a repulsive part which is generally given the form e cR to account for the impenetrability of the electronic clouds. The potentials are determined for each type of pair C , or by adjusting... 

Halogenated starch may incorporate halogen atoms bound either covalently ionically, or by other interactions, such as van der Waals and dispersion forces. Bonding of the latter type is responsible for the formation of various starch-halogen and halogenated compound-starch complexes. Since they were recently reviewed4 such compounds are omitted from this section. 

The forces controlling surfactant interactions with polymers are identical to those involved in other solution or interfacial properties, namely, van der Waals or dispersion forces, the hydrophobic effect, dipolar and acid-base interactions, and electrostatic interactions. The relative importance of each type of interaction will vary with the natures of the polymer and surfactant so that the exact characters of the complexes formed may be almost as varied as the types of material available for study. 

Interactions at polymer surfaces and interfaces arise from two types of force. One of these, comparatively weak van der Waals or dispersion forces, are universal. Stronger interfacial forces arise from ionic, chemical or covalent bonds. These non-dispersion forces, of course, are not universally present. Fowkes has proposed that non-dispersive interactions be represented quantitatively as (Lewis) acid/base, or electron acceptor/donor effects. Accordingly, the strength of an interface, as represented by the work of adhesion, W, can be written ... 

Forces Molecules are attracted to surfaces as the result of two types of forces dispersion-repulsion forces (also called London or van der Waals forces) such as described by the Lennard-Jones potential for molecule-molecule interactions and electrostatic forces, which exist as the result of a molecule or surface group having a permanent electric dipole or quadrupole moment or net electric charge. 

Boiling Point. When describing the effect of alkane structure on boiling point in Section 2.17, we pointed out that van der Waals attractive forces between neutral molecules are of three types. The first two involve induced dipoles and are often refened to as dispersion forces, or London forces. 

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

The physical adsorption is characterized by weak intermolecular forces of the van der Waals type. The adsorbed particle must get close to the solid surface, since the van der Waals energy is proportional to the sixth power of reciprocal distance. The main feature of this interaction is its non-specificity, a considerable velocity and reversibility. An example of the physical adsorption is the adsorption of apolar molecules on an apolar surface resulting form disperse forces. Beside these forces the dipol-dipol interactions occur when molecules of the adsorbent or adsorbate can form permanent or induced dipoles (adsorption of gases or dipol liquids on apolar surfaces). 

Chemisorption systems are sometimes used for removing trace concentrations of contaminants, but the difficulty of regeneration makes such systems unsuitable for most process applications so most adsorption processes depend on physical adsorption. The forces of physical adsorption are weaker than the forces of chemisorption so the heats of physical adsorption are lower and the adsorbent is more easily regenerated. Several different types of force are involved. For nonpolar systems the major contribution is generally from dispersion-repulsion (van der Waals) forces, which are a fundamental property of all matter. When the surface is polar, depending on the nature of the sorbate molecule, there may also be important contributions from polarization, dipole, and quadmpole interactions. Selective adsorption of a polar species such as water or a quadrupolar species such as CO2 from a mixture with other nonpolar species can therefore be accomplished by using a polar adsorbent. Indeed, adjustment of surface polarity is one of the main ways of tailoring adsorbent selectivity. 

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