Dipolar interactions Keesom

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. 

Keesom forces are a function of the number and magnitude of a molecule s local dipole moments, but since they are dependent upon the positioning of a molecule with respect to its neighbors, they may not always be strictly additive. Flowever, since the molecules in most crystals are aligned for maximum dipolar interaction, the group interactions are often roughly additive. 

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 ... 

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

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 ... 

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 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 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 ... 

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

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

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). 

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 first type is the so-called Keesom interaction between molecules having a permanent dipolar moment such as water or formamide, as well as the Debye interaction between molecules with permanent dipolar moments and molecules with induced dipolar moments. These two kinds of interaction are directional and thus imply a specific orientation of the molecules, and they are strong interactions at about a few kilocalories per mole. For example, these may be the interactions on the polar-group side of the surfactant, such as between a sulfate group and water, or a carboxylate group and the next molecule in a similar molecular group. 

For some time it was thought that the Keesom dipole-dipole interactions should be treated separately from the Debye sind London interactions. Because of the dipolar nature of the Keesom phenomenon, the term polar was applied to these interactions, in contrast to the apolar Debye and London interactions. This distinction between all three apolar electrodynamic forces impeded progress in the search for the true polar surface interactions. After Chaudhury (1984) showed that the three, apolar, electrodynamic forces are simply additive, and should be treated as a single entity, the LW interactions, it became possible to examine the nature of the polar (Lewis) properties of surfaces as an entirely separate phenomenon from their electrod3mamic (LW) properties. 

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