Debye induction interaction

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. 

Debye Induction Forces. These forces result from interaction between permanent and induced dipoles. 

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. 

Debye inductive force induced dipole-permanent dipole interaction [43,44]. 

Liquids containing permanent dipoles have additional attractive interactions called Keesnm forces, which are caused by the tendency of the permanent dipoles to align anti-parallel with each other. Finally, there are also Debye or induction interactions between permanent dipoles and fluctuating ones. The dispersion interactions are the most important of the three types, however, because they occur in all materials, and are usually stronger than the Keesom and Debye interactions, when the latter are present at all. 

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. 

The orientation and induction interactions are electrostatic effects, so they are not subjected to electromagnetic retardation. Instead, they are subject to Debye screening due to the presence of electrolyte ions in the liquid phases. Thus, for the interaction across an electrolyte solution the screened Hamaker constant is given by the expression " ... 

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. 

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... 

Table 59.3 is based primarily on the Zisman critical surface tension of wetting and Owens and Wendt approaches because most of the polymer data available is in these forms. The inadequacies of equations such as Eq. (59.7) have been known for a decade, and newer, more refined approaches are becoming established, notably these of van Oss and coworkers [24]. A more limited number of polymers have been examined in this way and the data (at 20 °C) are summarized in Table 59.4. is the component of surface free energy due to the Lifshitz-van der Waals (LW) interactions that includes the London (dispersion, y ), Debye (induction), and Keesom (dipolar) forces. These are the forces that can correctly be treated by a simple geometric mean relationship such as Eq. (59.6). y is the component of surface free energy due to Lewis acid-base (AB) polar interactions. As with y and yP the sum of y and y is the total solid surface free energy, y is obtained from... 

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 ... 

The molecular forces consist of three essentially different partSt of which two> the Keesom directional effect and the Debye induction effect, have been investigated earlier [muiaiia mutandis]. As the third part we have the interaction of the fast periodic mutual perturbation of the inner electronic motions in the molecule, which represents the mun portion of the molecular attraction for the most simple non-polar and weakly polar molecules. Especially, the assumption of the quadrup>ole structure of the noble gases, which was unavoidable up to now, is made superfluous. The purely theoretical determination of the molecular forces, which have to be treated as perturbational effects of second order, is hardly manageable. Instead the forces can be estimated ffom optical measurements through their theoretical relation with the /-values of the dispersion formula. The forces estimated in this way yield within the accuracy with which they have been established, the attraction part of the van der Waals equation of state. 

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. 

Debye argued that if the attraction energy was simply due to a Keesom effect, then the interaction energy should be drastically reduced at high temperatures. Since experimental results were contrary to the prediction, he concluded that an additional attractive effect should be involved. He showed that an additional polar interaction should he induced between a permanent dipole and an induced dipole. The Dehye induction interaction energy between two molecules with permanent dipoles is proportional to the square of the dipole moments and to the polarizabilities as follows ... 

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. 

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... 

Dipole-Dipole Interaction. The first of the four terms in the total electrostatic energy depends on the permanent dipole moment of the solute molecule of radius a (assuming a spherical shape) immersed in a liquid solvent of static dielectric constant D. The function f(D) = 2(D - l)/(2D + 1) is known as the Onsager polarity function. The function used here is [f(D) — f(n2)] so that it is restricted to the orientational polarity of the solvent molecules to the exclusion of the induction polarity which depends on the polarizability as of the solvent molecules, related to the slightly different Debye polarity function q>(n2) according to... 

Debye interactions in which a permanent dipole induces a dipole in another nonpolar molecule, with the induction necessarily in an attractive direction ... 

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 ... 

Debye derived a more general expression from Equation (59) for the interactions between dipolar molecules and induced dipolar molecules (rotating) in 1920. He found that when induction takes place, the pair potential energy between two different dipolar molecules each possessing permanent dipole moments of pi and p2 and polarizabilities a, and ah can be expressed as,... 

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. 

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... 

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