Debye interaction

When a charge approaches a molecule without a static dipole moment, all energies considered so far would be zero. Nevertheless, there is an attractive force, which arises from a charge shift in the nonpolar molecule induced by the charge. This induced dipole moment interacts with the charge. The Helmholtz free energy is 

a is the polarizability in m J The polarizability is defined by = aE, where E is the electric field strength. Often it is given as a/4jtEo in units of A .  

The polarizability of a water molecule in the gas phase has the value of 1.65 X 10 J. Which dipole moment is induced by a unit charge 

In analogy, a molecule with a static dipole moment will interact with a polarizable molecule by inducing a dipole moment in the polarizable molecule. If the dipoles can freely rotate, the Helmholtz free energy for interaction between a permanent dipole 

This interaction is called the Debye interaction [7]. It will also arise between two identical polarizable molecules that have a permanent dipole moment. In this case, a factor of 2 has to be inserted on the right-hand side of Eq. (2.20). 

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There are three types of interactions that contribute to van der Waals forces. These are interactions between freely rotating permanent dipoles (Keesom interactions), dipole-induced dipole interaction (Debye interactions), and instantaneous dip le-induced dipole (London dispersion interactions), with the total van der Waals force arising from the sum. The total van der Waals interaction between materials arise from the sum of all three of these contributions. 

Abbreviations are in parentheses. The dd interactions are also known as Keesom interactions di interactions are also known as Debye interactions ii interactions are also known as London or dispersion interactions. Collectively, dd, di and ii interactions are known as van der Waals interactions. Charge transfer interactions are also known as donor-acceptor interactions. 

Here, a0 and / are the polarizability and the ionization energy of the atom, respectively. The r6 dependence is also encountered in the so-called Debye interaction between a permanent dipole and an induced dipole, given by... 

Almost all interfacial phenomena are influenced to various extents by forces that have their origin in atomic- and molecular-level interactions due to the induced or permanent polarities created in molecules by the electric fields of neighboring molecules or due to the instantaneous dipoles caused by the positions of the electrons around the nuclei. These forces consist of three major categories known as Keesom interactions (permanent dipole/permanent dipole interactions), Debye interactions (permanent dipole/induced dipole interactions), and London interactions (induced dipole/induced dipole interactions). The three are known collectively as the van der Waals interactions and play a major role in determining material properties and behavior important in colloid and surface chemistry. The purpose of the present chapter is to outline the basic ideas and equations behind these forces and to illustrate how they affect some of the material properties of interest to us. 

The parameter (3l2 for heterogeneous (12) interactions plays a similar role as /3n (Equation (34)) does for homogeneous (11) interactions. Use entries from Table 10.1 to write an expression for (812. If Debye interaction makes a negligible contribution to /3I2 and vxv2/ + v2) — V2 yxv/)u2, show that... 

Since heteroatoms are generally more electronegative than carbon, a carbon-heteroatom bond will have an electron density that is localized towards the heteroatom. The result is a permanent dipole that is analogous in its electronic properties to a bar magnet. This permanent dipole can induce a dipole in an otherwise neutral portion of another molecule in the same manner that a magnet induces a dipole in a piece of steel. The resulting attraction is referred to as a dipole-induced dipole, or Debye interaction. 

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. 

The van der Waals force between atoms consists of three different dipole induced forces, the Keesom interaction, the Debye interaction and the London interaction. 

Debye interaction occurs when a permanent dipole induces a dipole in a polarisable atom or molecule. The induced dipole is oriented in such a way that attraction occurs. 

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

S.8.b. "Debye" interaction, permanent dipole and inducible dipole... 

The Debye interaction between a permanent dipole Mdipoie and an inducible dipole, built on the polarizability aind(0) in the limit of zero frequency ... 

The stability runs parallel to the polarizability but this can indicate both London and Debye interaction. For a very high polarizability the hydrate becomes unstable through the large cohesion energy of the organic liquid. 

The solubility will be but small in solvents in which the cohesion is based for a great part on dipole-dipole interaction (in particular through hydrogen bonds). The large specific cohesion energy of the solvent is only compensated to a small extent by the Debye interaction with the solute molecule. 

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. 

Instead of following conventional classifications, it would be reasonable to classify the intermolecular forces into three categories on the basis of their origin [2]. The first is the forces caused by the electronic polarization, i.e. van der Waals attraction, such as London dispersion (electronic polarization-electronic polarization) and Debye interaction (dipole-dipole-induced electronic polarization). The second is the forces caused by the electrostatic charges and/or the dipoles of the molecule these forces are based on the molecular structure and are independent of electronic polarizability. And the last category is the forces caused by exchange of elemental particles, such as an electron (covalent bond) and a proton (hydrogen bond). 

Debye interactions interactions between dipolar molecules and induced 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. 

As we have already discussed in Section 2.5.3 for excess polarizabilities of molecules dissolved in a solvent, and in Section 2.6.4 for van der Waals interactions in a medium, when two molecules 1 and 2 are dissolved in a medium 3, the van der Waals forces between them are reduced because of the dielectric screening of the medium. This reduction is particularly important for liquids with high dielectric constants. The attraction force is decreased by a factor of the medium s er for Keesom and Debye interactions and by a factor of e] for London dispersion interactions. This strong reduction in the attractive pair potential means that the contributions of molecules further apart tend to be relatively minor, and each interaction is dominated only by contributions from its nearest neighbors. 

As detailed in Chapter 2, van der Waals interactions consist mainly of three types of long-range interactions, namely Keesom (dipole-dipole angle-averaged orientation, Section 2.4.3), Debye (dipole-induced dipolar, angle-averaged, Section 2.5.7), and London dispersion interactions (Section 2.6.1). However, only orientation-independent London dispersion interactions are important for particle-particle or particle-surface attractions, because Keesom and Debye interactions cancel unless the particle itself has a permanent dipole moment, which can occur only very rarely. Thus, it is important to analyze the London dispersion interactions between macrobodies. Estimation of the value of dispersion attractions has been attempted by two different approaches one based on an extended molecular model by Hamaker (see Sections 7.3.1-7.3.5) and one based on a model of condensed media by Lifshitz (see Section 7.3.7). 

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. 

The LSER theory combined with IGC should be applied more in the future because it permits distinction between London, Keesom, and Debye interactions in addition to the acid-base scales. This is not done in the traditional IGC studies in relation to adhesion. 

Equation (15.8) that claims inverse six power dependence energy on separation is referred to as the Debye interaction between molecules. 

The interaction energy given by Equation (4.27), often referred to as the Debye interaction, represents the second of the three inverse 6th power contributions to the total van der Waals interaction between molecules. 

The basic derivations of the van der Waals forces is based on isolated atoms and molecules. However, in many particle calculations or in the condensed state major difficulties arise in calculating the net potential over all possible interactions. The Debye interaction, for example is non additive so that a simple integration of Equation (4.27) over all units will not provide the total dipole-induced dipole interaction. A similar problem is encountered with the dipole-dipole interactions which depend not only on the simple electrostatic interaction analysis, but must include the relative spatial orientation of each interacting pair of dipoles. Additionally, in the condensed state, the calculation must include an average of all rotational motion. In simple electrolyte solutions, the (approximately) symmetric point charge ionic interactions can be handled in terms of a dielectric. The problem of van der Waals forces can, in principle, be approached similarly, however, the mathematical complexity of a complete analysis makes the Keesom force, like the Debye interaction, effectively nonadditive. 

When a polar molecule and a nonpolar one are in proximity, the first will induce a transient dipole in the second, giving rise to a permanent dipole-induced dipole interaction. This is known as Debye interaction, given by... 

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