Debye dispersion forces

There also exist dispersion, or London-van der Waals forces that molecules exert towards each other. These forces are usually attractive in nature and result from the orientation of dipoles, and may be dipole-dipole (Keesom dispersion forces), dipole-induced dipole (Debye dispersion forces), or induced dipole-induced dipole... 

Attractive and Repulsive Forces. The force that causes small particles to stick together after colliding is van der Waals attraction. There are three van der Waals forces (/) Keesom-van der Waals, due to dipole—dipole interactions that have higher probabiUty of attractive orientations than nonattractive (2) Debye-van der Waals, due to dipole-induced dipole interactions (ie, uneven charge distribution is induced in a nonpolar material) and (J) London dispersion forces, which occur between two nonpolar substances. 

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. 

Keesom, Debye, and London contributed much to our understanding of forces between molecules [111-113]. For this reason the three dipole interactions are named after them. The van der Waals4 force is the Keesom plus the Debye plus the London dispersion interaction, thus, all the terms which consider dipole-dipole interactions Ctotai = Corient+Cind- -Cdisp. All three terms contain the same distance dependency the potential energy decreases with l/D6. Usually the London dispersion term is dominating. Please note that polar molecules not only interact via the Debye and Keesom force, but dispersion forces are also present. In Table 6.1 the contributions of the individual terms for some gases are listed. 

Van der Waals postulated that neutral molecules exert forces of attraction on each other which are caused by electrical interactions between dipoles. The attraction results from the orientation of dipoles due to any of (1) Keesom forces between permanent dipoles, (2) Debye induction forces between dipoles and induced dipoles, or (3) London-van der Waals dispersion forces between fluctuating dipoles and induced dipoles. (The term dispersion forces arose because they are largely determined by outer electrons, which are also responsible for the dispersion of light [272].) Except for quite polar materials the London-van der Waals dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

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. 

Electroacoustics — Ultrasound passing through a colloidal dispersion forces the colloidal particles to move back and forth, which leads to a displacement of the double layer around the particles with respect to their centers, and thus induces small electric dipoles. The sum of these dipoles creates a macroscopic AC voltage with the frequency of the sound waves. The latter is called the Colloid Vibration Potential (CVP) [i]. The reverse effect is called Electrokinetic Sonic Amplitude (ESA) effect [ii]. See also Debye effect. 

The effect of the polarisability. and of the ionization potential IP may be directly related to their impact on the energy of the dispersive forces (London, Debye and Keesom) which govern physical adsorption onto activated carbon [4]. The lower positive effect of the molar mass M may be related to the influence of the molecular overcrowding which increases the surface contact with the solid, leading to more intensive interactions. 

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 dispersion forces. 

In the case of physical bonds (London dispersion, Keesom orientation, and Debye induction forces), the energy of interaction or reversible energy of adhesion can be directly calculated from the surface free energies of the solids in contact. 

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). 

The foundations for our present understanding of intermolecular forces were laid in the first decades of this century. First, Keesotn Debye and Falckenhagen elucidated the role played by permanent electric moments and polarizabilities. After the advent of quantum mechanics, Heitler and London S identified the exchange forces which keep molecules apart, and London discovered the dispersion forces which explained such puzzling phenomena as the condensation of noble gases. 

It is well known that atoms or molecules always attract each other at short distances of separation. The attractive forces are of three different types dipole-dipole interaction (Keesom) dipole-induced dipole interaction (Debye) and London dispersion force. Of these, the London dispersion force is the most importanL as it occurs for both polar and nonpolar molecules, and arises from fluctuations in electron density distribution. 

The van der Waals attractions between atoms or molecules are of three different types dipole-dipole (Keesom), dipole-induced dipole (Debye), and dispersion (London) interactions. The Keesom and Debye attraction forces are vectors, and... 

As we have seen, London dispersion interactions, Keesom dipole-dipole orientation interactions and Debye dipole-induced dipole interactions are collectively termed van der Waals interactions their attractive potentials vary with the inverse sixth power of the intermol-ecular distance which is a common property. To show the relative magnitudes of dispersion, polar and induction forces in polar molecules, similarly to Equation (78) for London Dispersion forces, we may say for Keesom dipole-orientation interactions for two dissimilar molecules using Equation (37) that... 

Dispersion Forces The dipolar interaction forces between any two bodies of finite mass, including the Keesom forces of orientation among dipoles, Debye induction forces, and London forces between two induced dipoles. Also referred to as Lifshitz—van der Waals forces. 

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

Induction Forces Debye forces. See Dispersion Forces. 

For certain types of gas-surface interactions, it may be useful to view the interaction as between the gas atom and a single surface atom. Weak attractive interaction between a pair of atoms can be due to dispersion forces (London [14, 15]) that represent the interaction of induced fluctuating charge distributions. In addition, molecules that possess permanent dipoles can further polarize each other (Debye [16, 17]) and can have dipole-dipole interactions (Keesom [18, 19]). All these pairwise interaction potentials fall off inversely as the sixth power of the distance. 

The van der Waals attraction between gas molecules may originate from three possible sources permanent dipole-permanent dipole (Keesom) forces permanent dipole-induced dipole (Debye) interactions and transitory dipole-transitory dipole (London) forces. This, of course, ignores the higher multipole interactions. Only the classical London dispersion forces contribute to the long-range attraction between colloidal particles. These London interactions are the self-same forces that are responsible for the liquefaction of the rare gases, such as helium and argon, at low temperatures. 

One can see from Table 4.4, the dispersion force contribution to the total van der Waals interaction can be quite significant. Except for very small and very polar molecules such as water, the dispersion force will exceed the Kee-som and Debye contributions and dominate the character of the interaction, even in the case of two dissimilar molecules. 

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. 

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