Keesom force

Dipole-dipole forces, the so-called Keesom forces 8ind8o> appear when both the solvent and solute have dipole moments. Strong interactions are produced as a result of dipole alignment. Dipole interactions are determined by the sum of all the dipoles within a molecule. 

Dipole-induced dipole forces. A molecule with a strong molecular or bond dipole can induce a dipole in a molecule nearby that is polarizable. These Keesom forces have the same inverse 6th power dependence with distance. An example could be the interaction of chlorobenzene with naphthalene. 

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. 

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. 

Keesom forces are a function of the number and magnitude of a molecule s local dipole moments, but since they are dependent upon the positioning of a molecule with respect to its neighbors, they may not always be strictly additive. Flowever, since the molecules in most crystals are aligned for maximum dipolar interaction, the group interactions are often roughly additive. 

The strengths of the forces described above all depend upon intermolecular distance. Keesom forces are a function of (distance)-3 and dispersion forces, (distance)-6. Since hydrogen bonds require overlap of the electron clouds of H and B, they are even more sensitive to distance. Hydrogen bonds are also sensitive to the A-H-B angle, 180° being optimal. 

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. 

Keesom forces, 43, 44 Kinetics of chromatography, 53 Kovats retention index, 78... 

Induction (or Debye) and Orientation (or Keesom) force 0°+K which are the specific (or polar) properties of the van der Waals attraction exist in the presence of the dipole moment and (total) polarizability, resulting in specific (or polar) intermolecular attraction. 

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 ... 

Keesom forces, 242 Kelley-Bueche equation, 611 Kerr effect, 299, 349 Kinetic(s)... 

In the first group of molecular compounds both partners possess a dipole moment and the compound can be considered as being produced thanks to the possibility of attaining a favourable mutual orientation of the dipoles (Keesom forces). Looked at in this way the numerous cases of association also belong to this group. These phenomena in which hydrogen dipoles, especially such as FH, OH, NH and (GH), play a part are discussed under the hydrogen bond . 

In this case it is not so much the whole moment of the molecule which plays a part but the moments of the separate groups. In view of the small radius of action of the Keesom forces also (00 1/r6), the interaction will only extend to the immediate neighbourhood. Therefore the interaction of the three isomeric dibromobenzene derivatives, as far as the dipole contributions also are concerned, will be practically equal. The action of one C-Br group is only influenced to a small... 

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 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 ... 

Note that temperature is a parameter of the equation. As the material temperature rises during processing, the value of orientation energy becomes negligible. In a typical system conflicting dipole fields are created which significantly reduce dipole-dipole net interaction. Keesom forces, unlike London forces, do not apply to nonpolar substances because both dipoles, which participate in the interaction, must be permanent dipoles (London forces do not require the presence of permanent dipoles). 

Van der Waals forces represent important intermolecular interactions between nonelectrolyte substances, and can be categorized into dipole-dipole, dipole-induced-dipole, and induced-dipole-induced-dipole forces. Polar molecules, by definition, will have a permanent dipole moment, and will interact with the oppositely charged portions or other molecules having permanent dipole moments. The dipole-dipole interaction is known as the orientation effect, or as the Keesom force. 

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-dipole force The dipole-dipole force, also called the Keesom force, arises from the interaction of the permanent dipoles of two interacting molecules. The interaction energy is inversely proportional to the sixth power of the distance between the two dipoles. Dipole-dipole interactions are temperature dependent as thermal motion of the molecules competes with the tendency toward favorable dipole orientations. The energy of two interacting dipoles of y, = 2 Debye at a distance of 5 A in vacuum is of the order of —0.25 kcal/ mol. 

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. 

Orientation Forces Keesom forces. See Dispersion Forces. 

Van der Waais forces opemie between molecules, and are classified as- (i) dii.wle dipole inleraclions or Keesom forces. 

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. 

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