Debye forces

It is clear from Table 1 that, for a few highly polar molecules such as water, the Keesom effect (i.e. freely rotating permanent dipoles) dominates over either the Debye or London effects. However, even for ammonia, dispersion forces account for almost 57% of the van der Waals interactions, compared to approximately 34% arising from dipole-dipole interactions. The contribution arising from dispersion forces increases to over 86% for hydrogen chloride and rapidly goes to over 90% as the polarity of the molecules decrease. Debye forces generally make up less than about 10% of the total van der Waals interaction. 

Induction forces, the so-called Debye forces ind> occur in the interaction between a permanent dipole of a solute or a polar solvent and an induced dipole in another compound. They are weak and appear during the analysis of the nonpolar polarized compounds, such as those with multiple... 

There are a number of different enthalpic interactions that can occur between polymer and packing, and in many cases multiple interactions can exist depending on the chemical structure of the polymer. Enthalpic interactions that are related to water-soluble polymers include ion exchange, ion inclusion, ion exclusion, hydrophobic interactions, and hydrogen bonding (12)- Other types of interactions commonly encountered in SEC, as well as in all other chromatographic separations, are dispersion (London) forces, dipole interactions (Keeson and Debye forces), and electron-donor-acceptor interactions (20). 

The net strength of a molecule s Debye interactions is a function of its polarizability and the number and magnitude of its local dipole moments. Since induced dipoles tend to be aligned for maximum attraction, the energy of interaction resulting from Debye forces is very nearly additive. 

A dipole-dipole interaction, or Keesom force, is analogous to the interaction between two magnets. For non-hydrogen bonding molecules with fixed dipoles, these interactions are likely to influence the orientation of the molecules in the crystal. This is because, unlike the Debye force which is always attractive, the interaction between two dipoles is only attractive if the dipoles are properly oriented with respect to one another, as is the case with magnets. 

An attractive interaction arises due to the van der Waals forces between molecules of colloidal particles. Depending on the nature of dispersed particles, the Keesom forces (or the dipole-dipole interaction), the Debye forces (or dipole-induced dipole interaction), and the London forces (or induced dipole-induced dipole interaction) may contribute to the van der Waals interaction. First, the van der Waals interaction was theoretically computed using a method of the pairwise summation of interactions between different pairs of molecules of the two macroscopic particles. This method called the microscopic approximation neglects collective effects, and, as a consequence, misrepresents the Hamaker constant. For many problems of a practical use, however, specific features of the total interaction are determined by a repulsive part, and such an effective, gross description of the van der Waals interaction may often be accepted [3]. The collective effects in the van der Waals interaction have been taken into account in the calculations of Lifshitz et al. [4], and their method is known in the literature as the macroscopic approach. 

The expression van der Waals attraction is widely used and is here defined as the sum of dispersion forces [9], Debye forces [17] and the Keesom forces [18]. Debye forces are Boltzmann-averaged dipole-induced dipole forces, while Keesom forces are Boltzmann-averaged dipole-dipole forces. The interaction for all three terms decays as 1 /r6, where r is the separation between the interacting particles, and they are combined into one term with the proportionality constant denoted the Hamaker constant. In order to determine the van der Waals force there are at least two approaches, either to calculate the force between two particles assuming that the interaction is additive, (this is usually called the Hamaker approach) or to use a variant of Lifshitz theory. 

It has to be emphasized that more refined approaches have been established, in particular by Van Oss and coworkers (1994). They introduced the so-called Lifschitz-Van der Waals (LW) interactions. These interactions include the dispersion or London forces ( / ), the induction or Debye forces (yD) and the dipolar or Keesom forces (, K), so that ... 

The very high coordination number in the gas hydrates makes a rather appreciable contribution from the Debye forces improbable, this is, however, favourable for the London interaction. In other compounds, such as the inert gases with hydrochloric acid and phenol, the first-mentioned interaction is perhaps of greater importance. 

Dipolar Interactions London, Keesom, and Debye Forces... 

The dd, di and ii interactions are known collectively as Van der Waals forces. The dd interactions are also called Keesom forces, the di interactions are also called Debye forces and the ii interactions are also known as London or dispereion forces. 

The solubility of the drug substance is attributable in large part to the polarity of the solvent, often expressed in terms of dipole moment, related to the dielectric constant. Solvents with high dielectric constants dissolve ionic compoimds (polar drugs) readily by virtue of ion-dipole interactions, whereas solvents with low dielectric constants dissolve hydrophobic substances (non-polar drugs) as a result of dipole or induced dipole interactions (Van der Waals, London, or Debye forces). This principle is illustrated in Fig. 1. The former is classified as polar solvents, with examples such as water and glycerin the latter are non-polar solvents, with example such as oils. Solvents with intermediate dielectric constants are classified as semipolar. The dielectric constants of some solvents are shown in Table 3. ... 

The van der Waals (VDW) attractive forces are the principal forces between dry, noncharged spherical aerosol particles [262] and may reduce stability and cause flocculation of suspended particles. The VDW forces arise from the attractive forces between permanent dipoles (Keesom forces), induced dipoles (London forces), and dipole-induced dipoles (Debye forces). For nonpolar or slightly polar compounds, the force of attraction between two particles with diameter d separated by a distance h (where h < d) is ... 

Nonpolar molecules can only interact by dispersion forces, while the interactions of polar molecules are often dominated by the Keesom forces. However, under certain circumstances it is still possible that dispersion forces might predominate over the other forces, even for polar molecules such as HCl. The Debye forces are often stronger than the London forces for highly polar molecules, and would predominate over Keesom forces for weakly polar molecules. Debye forces are selective, and important in explaining why certain nonpolar but polarizible molecules can still be soluble in polar solvents (Krishnan and Fredman, 1971). 

Polar forces between molecules in solution may arise from either permanent dipoles or Induced dipoles. When a molecule possessing a permanent dipole comes Into close association with a non-polar molecule, a relatively weak (0.0001 kcal/mole) permanent dipole-Induced dipole Interaction (Keesom force) arises. Close association of two permanent dipoles produces a stronger (1 to 2 kcal/mole) dipole-dipole Interaction or Debye force (12). In light of the relative magnitudes of these interactive forces, the Keesom forces and the extremely weak Induced dlpole-lnduced dipole Interactions will be omitted from further discussion. 

Dipole-induced dipole force The dipole-induced dipole force, also called the induction or Debye force, arises when a permanent dipole induces a redistribu-... 

Debye Forces Attractive forces between molecules due to dipole-induced dipole interaction. See also Dispersion Forces. 

Induction Forces Debye forces. See Dispersion Forces. 

As shown above, there have been identified several mechanisms involved in the interactions between atoms and molecules, denominated collectively as the van der Waals forces. In atomic and completely nonpolar molecular systems (hydrocarbons, fluorocarbons, etc.) the London dispersion forces provide the major contribution to the total interaction potential. However, in many molecular systems containing atoms of very different electronegativities and polarizabilities the dipole-dipole (Keesom) and dipole-induced dipole (Debye) forces may also make significant contributions to the total interaction. 

Polar forces K W ALLEN Nature of Keesom and Debye forces, attraction constants Lennard-Jones potential... 

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