London-Keesom-Debye interactions

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. 

The total van der Waals interaction potential is obtained by simply adding the individual contributions arising from the Keesom, Debye, and London interactions. Because the radial power-law dependencies of all these interactions vary as 1 /r, the total van der Waals interaction can be expressed simply as... 

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. 

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. 

Compounds that undergo only vdW interactions (London plus Debye plus Keesom interactions) are commonly referred to as apolar. Examples include alkanes, chlorinated benzenes, and PCBs. 

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. 

Table 6.1 Contributions of the Keesom, Debye, and London potential energy to the total van der Waals interaction between similar molecules as calculated with Eqs. (6.6), (6.8), and (6.9) using Ctotal = Corient + Cind + Cdisp- They are given in units of 10-79 Jm6. For comparison, the van der Waals coefficient Cexp as derived from the van der Waals equation of state for a gas (P + a/V fj (Vm — b) = RT is tabulated. From the experimentally determined constants a and b the van der Waals coefficient can be calculated with Cexp = 9ab/ (47T21V ) [109] assuming that at very short range the molecules behave like hard core particles. Dipole moments /u, polarizabilities a, and the ionization energies ho of isolated molecules are also listed.

In certain circumstances, it may be necessary to distinguish between the different types of interactions. This can be performed in several ways (Barton, 1983 Van Krevelen, 1990). The most usual method is to make a distinction between dispersion (London), dipolar (Debye Keesom) and hydrogen-bonding components, each one being characterized by its contribution to CED and the corresponding solubility parameter, 8d, 8p, 8h, respectively, such that 8 = (8d + 8p + 8j )1/2. 

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

Hydrogen bonds" are not due to a separate potential they involve the attraction between an H atom that is covalently bonded to molecule 1 and electronegative atoms (O, N, etc.) in molecule 2 that are between 0.15 nm and 0.25 nm from the H atom. This hydrogen bond interaction is a combination of Keesom, Debye, and London interactions. 

Dipolar Interactions London, Keesom, and Debye Forces... 

All three types of polarization interactions—Keesom, Debye, and London—are included in the following formula for the total van der Waals interaction potential between two spherical molecules separated by a distance r ... 

Comparison of Keesom, Debye and London interactions in polar molecules... 

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. 

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... 

These three contributions (Keesom, Debye, and London) are collectively known as van der Waals interactions, and the general expression for the interaction between particles 1 and 2 is... 

A contains the Coulomb and Pauli repulsions, while B represents the attractive contributions from Keesom, Debye and London forces, as well as interactions due to higher order multipoles. Therefore Equation (1) represents general interactions commonly called Van der Waals forces. When the interacting partners possess some special... 

The two subsystems are not chemically bonded. For example, a solute molecule in a solution. The interactions between the fiagments are then weak interaction (Keesom, Debye, London, H-bond,. ..), and we will call them physical interactions. In such situations we will use the acroiym QM MM, where the colon symbohzes these non-bonded interactions (in the chemical sense). 

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. 

The surface free energy of a solid can be described as the sum of the dispersive and specific contributions. Dispersive (apolar) interactions, also known as Lifshitz-van der Waals interactions, consist of London interactions which originate from electron density changes but may include both Keesom and Debye interactions [6, 7]. Other forces influencing the magnitude of surface energy are Lewis acid-base interactions which are generated between an electron acceptor (acid) and an electron donor (base). Details of the widely accepted theoretical... 

Keesom, Debye, and London contributed much to our understanding of forces between molecules. For this reason, the three different types of dipole interactions are named after them. The van der Waals force is the Keesom plus the Debye plus the London dispersion interaction, thus all the terms that consider dipole-dipole interactions ... 

Table 2.1 Contributions of the Keesom, Debye, and London potential energy to the total van der Waals interaction between similar molecules as calculated with Eqs. (218),(220), and (2.21) using...

In Eqs. (2.50) and (2.51), the first term corresponds to the static (co = 0) dielectric response, which is due to the Keesom and Debye interaction and the second term corresponds to the London dispersion interaction. Except for materials with high values of the dielectric constant (e.g., water), the first term will be much smaller than the second one. In their original paper, Tabor and Winterton ignored this term since in their case medium 3 was vacuum n = 1). 

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