Keesom orientation interactions

Keesom orientation interactions interactions between dipolar molecules (rotating)... 

These are often called Debye induced dipole interactions. It is interesting to note that the Keesom orientation interaction expression (Equation (37) in Section 2.4.3) may also be obtained from Equation (59) by replacing a with aorien = p2/3kT. This fact also indicates the presence of induction in orientation interactions. Thus, both Keesom and Debye interactions vary with the inverse sixth power of the separation distance and they both contribute to the van der Waals interactions, which we will see in Section 2.6. 

The origin of forces between neutral symmetrical molecules, such as hydrogen (H2) or the inert gases (e.g. A, Ne), is not obvious. Because of the symmetry of the electron configuration, there cannot be any permanent dipole so, there can be neither dipole-dipole interactions (Keesom orientation interactions) nor dipole-molecule interaction (Debye induction interactions) (see Polar Forces). Further, there appears to be no Coulombic electrostatic interaction since they are electronically neutral overall, nor can there be any covalent bonding. Yet, there must be forces of some type between these molecules as the existence of liquid and solid hydrogen and argon demonstrate. 

Besides the most basic and predominant nonpolar interactions (dispersion forces), there are polarization or polar interactions between molecules of counter bodies, such as dipole-dipole interactions (Keesom 1922) and dipole-induced dipole interactions (Debye 1921). The essential difference between dispersion and polarization forces is that, while the former involve simultaneous excitation of both molecules, those for the latter involve only a passive partner. The Keesom orientation interaction energy between two molecules with permanent dipoles is temperature dependent and proportional to the dipole moments as follows ... 

Keesom Orientation Forces. These forces result from the interaction of two permanent dipoles, with the hydrogen bond being the most important. Hydrogen bonds are stronger than dispersion or inductive forces. 

Keesom orientational force permanent dipole-permanent dipole interactions [45]. 

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. 

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... 

Thus, the Keesom dipolar orientation interaction coefficient, CP, can be written as... 

The values in Table 2.3 indicate that the most important contribution to van der Waals interactions results from the London dispersion interactions. Keesom dipolar orientation interactions are only operative for strongly polar and hydrogen-bonding substances such... 

Dispersion Forces Interaction forces between any two bodies of finite mass. Sometimes called van der Waals forces, they include the Keesom orientation forces between dipoles, Debye induction forces between dipoles and induced dipoles, and London (van der Waals) forces between two induced dipoles. Also referred to as Lifshitz—van der Waals forces. 

The van der Waals forces represent an averaged dipole-dipole interaction, which is a superposition of orientation interactions (between two permanent dipoles, Keesom 1913), induction interaction (between one permanent dipole and one induced dipole, Debye 1920) and dispersion interaction (between two induced dipoles, London 1930). The interaction between two macroscopic bodies depends on the geometry of the system (see Fig. 3). For a plane-parallel film with uniform thickness, h, from component 3 located between two semi-infinite... 

Keesom Orientation Forces. These forces result from the interaction of two permanent dipoles, of which the hydrogen bond is the most important. Hydrogen bonds are stronger than dispersion or inductive forces. If the two components have the same vapor pressure, separation can be achieved on the basis of several properties. These properties are (in the order of then-ease of separation) (1) difference in the functional groups, (2) isomers with polar functional groups, and (3) isomers with no functional groups. 

The form of interaction functiorrs such as those of Lennard-Jones, based on the model of Van der Waals forces irrvolving Keesom orientation effects, Debye induction and Lorrdon dispersion, which quickly decrease with distance beyorrd a certain distance between two molecules, the interaction can be negligible (for example, when the interaction is less than 5q/100)-This comes down to defming around each molecule a volume influence ... 

Weak, secondary forces, resulting from molecular dipoles, also act between materials. They are often classified according to the nature of the interacting dipoles. Keesom orientation forces act between permanent dipoles, London dispersion forces between transient dipoles, and Debye induction forces between a permanent and an induced dipole, see O Tables 2.1 and O 2.2. These are collectively known as van der Waals forces (but note alternative usage of this term, O Table 2.2), and occur widely between materials. They are much less dependent upon specific chemical structure than primary bonds. Indeed, dispersion forces are universal. They only require the presence of a nucleus and of extranuclear electrons, so they act between all atomic and molecular species. 

To reflect the contribution of the fundamental nature of the long-range interaction forces across the interface, it was suggested (Fowkes 1964) that surface free energies and work of adhesion may be expressed (O Eq. 3.11) by the sum of two terms the first one representative of London s dispersion interactions (superscript D) and the second representative of nondispersion forces (superscript ND), this latter include Debye induction forces, Keesom orientation forces, and acid—base interactions. 

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. 

The dipole-dipole interactions, frequently referred to as Keesom interactions, are historically included in the van der Waals interactions, even though they are purely electrostatic. For molecules that are free to orient themselves, the dipole-dipole interactions must be averaged over the molecular orientations, as the angular dependence of the interaction energy is comparable to the Boltzmann energy kBT (Israelachvili 1992, p. 62). With the averaging of the Keesom... 

This is the dipole-dipole interaction energy, often termed the orientation or Keesom interaction. Notice that it depends on the product of the squares of both dipole moments, but is inversely proportional to distance to the sixth power. This is a very short-range... 

Two freely rotating dipoles attract each other because they preferentially orient with their opposite charges facing each other. This randomly oriented dipole-dipole interaction is often referred to as the Keesom energy2 ... 

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. 

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 interaction occurs when a permanent molecular dipole creates an electric field, which orients other permanent dipoles in such a way that they will attract each other. 

Keesom interactions of permanent dipoles whose mutual angles are, on average, in attractive orientations ... 

The electrostatic energy Ei(es) is zero when averaged over the angles describing the relative orientation of the two interacting molecules. However, Keesom (1921) showed that if two dipolar molecules undergo thermal motions, they attract each other according to ... 

The Keesom formula (4.70) is easily derived (Magnasco, 2009a) by taking the Boltzmann average of the dipolar interaction over the angles of relative orientation of the two molecules for small values of the dimensionless parameter ... 

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. 

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