Debye induced dipole interactions

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. 

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. 

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. 

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. 

Permanent dipole/induced dipole interaction (Debye equation)... 

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 simplest, and least realistic, method for including induced dipole interactions is to average the potential for pair-polarizable point dipoles over all relative orientations, in which case the induced energy is the well-known Debye component of the van der Waals potential ... 

The solubility of the drug substance is attributable in large part to the polarity of the solvent, often expressed in terms of dipole moment, related to the dielectric constant. Solvents with high dielectric constants dissolve ionic compoimds (polar drugs) readily by virtue of ion-dipole interactions, whereas solvents with low dielectric constants dissolve hydrophobic substances (non-polar drugs) as a result of dipole or induced dipole interactions (Van der Waals, London, or Debye forces). This principle is illustrated in Fig. 1. The former is classified as polar solvents, with examples such as water and glycerin the latter are non-polar solvents, with example such as oils. Solvents with intermediate dielectric constants are classified as semipolar. The dielectric constants of some solvents are shown in Table 3. ... 

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. 

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. 

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... 

We may say for Debye dipole-induced dipole interactions for two dissimilar molecules using Equation (60) so that... 

In the Debye-Hilckel model the interactions contributing to the potential energy, 0, are long range coulombic interactions between the ions. However, because of the versatility of the computer simulation calculations, the statistical mechanical description of an electrolyte solution could include all conceivable electrostatic interactions such as the ion-ion, ion-dipole and dipole-dipole, dipole-quadrupole and quadrupole-quadrupole as weU as induced dipole interactions between the ions, and between the ions and the solvent and between solvent molecules. The total potential energy, 0, fed into the calculations which ultimately lead to g( (ri2) could be made up of contributions such as these and those given in Sections... 

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... 

The term van der Waals forces includes three types of intermolecular forces London (dispersion) forces, permanent dipole-dipole forces (sometimes referred to as Keesom forces) and permanent-induced dipole interactions (Debye forces). In 1910, van der Waals was awarded the Nohel Prize for his work on the equation of state for gases and liquids concerned with the reasons for non-ideal behaviour in real gases. His equation introduced compensatory terms to account for the non-zero size of the particles and the inter-particle forces between them. This broader definition of van der Waals forces runs contrary to the use of the term in many current textbooks, but is consistent with its use in the IB syllabus. 

The main forces responsible for adhesion are van der Waals, which for convenience are considered to be made of three main contributions Dipole-dipole interaction (Keesom force), dipole-induced-dipole interaction (Debye force) and London dispersion force. A hydrogen-bonding force can also be induded in the interaction. 

Orientation Forces. Besides the most basic non-polar interaction, dispersion forces, there are polar interactions between molecules of counterbodies, e.g. the dipole-dipole interaction (Keesom), the dipole-induced dipole interaction (Debye) and hydrogen bonding. The Keesom interaction (orientation) is temperature dependent and the energy is expressed as... 

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 ... 

Attractive forces between neutral molecules may include three contributions according to the nature of the molecule. These three interaction energies are dipole-dipole or Keesom interactions, dipole-induced dipole or Debye interactions, and induced dipole-induced dipole interactions or London dispersion forces (Table 3.1). Since all of them depend on the inverse of the sixth power of the intramolecular distance, they are generally combined in only one term, representing the total van der Waals attraction and this term is the sum of the three energies ... 

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... 

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... 

Note that AvapG encompasses dispersive (i.e., London), dipole-induced dipole (i.e., Debye), and dipole-dipole (i.e., Keesom) contributions (Section 3.2). However, in most organic liquids, dipole interactions are generally of secondary importance. Hence, as a first approximation, we consider only the dispersive interactions. Then we can use the approach described in Section 3.2 to quantify the vdW term (Eq. 3-10, Fig. 3.4). Since lnp, =-AvapG, / i r, we may express Eq. 4-23 as ... 

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

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. 

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