Debye orientation forces

There also exist dispersion, or London-van der Waals forces that molecules exert towards each other. These forces are usually attractive in nature and result from the orientation of dipoles, and may be dipole-dipole (Keesom dispersion forces), dipole-induced dipole (Debye dispersion forces), or induced dipole-induced dipole... 

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. 

For a pair of identical molecules, it is noted in Eq. (13) that the first term determined with regard to the deformation polarizability is a so-called Debye inductive force , and the second term is generally called a Keesom orientational force between molecules when the dipole moment is considered in the intermolecular attractive system. 

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. 

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. 

Weaker secondary bonds act between molecules. Thus, below — 182°C, methane is a solid, the covalent molecules being held in a solid lattice be weak secondary bonds. These weak forces are associated with interactions between dipoles. Three different types of interaction have been described by London, Debye and Keesom, known respectively as dispersion, induction and orientation forces see Table 1 and Dispersion forces and Polar forces. The three types of interaction are often referred to collectively as van der Waals forces, as indicated in Table 1. However, it is necessary to note that some authors use the term van der Waals to refer exclusively to dispersion forces, the other two types being referred to as polar forces . Table 2. (The term dispersive is sometimes used by francophone authors writing in English where dispersion would be correct.)... 

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... 

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. 

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 first hint that there are non-covalent interactions between uncharged atoms and molecules came from the observations of van der Waals (1873, 1881). These interactions came to be known as van der Waals forces. The interactions responsible for these became clear with the work of Keesom (1915, 1920, 1921), Debye (1920, 1921) and London (1930) as, respectively, interactions between two permanent dipoles (orientation forces), a permanent dipole and an induced dipole (induction forces) and a fluctuating dip>ole and an induced dipole (dispersion forces). While these three kinds of interaction have different origins, the interaction energies for all three vary as the inverse of the distance raised to the sixth power ... 

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. 

The differences between ab initio and molecular mechanics generated dipole moments were discussed. The MM3(2000) force field is better able to reproduce experimental dipole moments for a set of forty-four molecules with a root mean squared deviation (rmsd) of 0.145 Debye compared with Hartree-Fock (rmsd 0.236 Debye), M0ller-Plesset 2 (rmsd 0.263 Debye) or MM3(96) force field (rmsd 0.164 Debye). The orientation of the dipole moment shows that all methods give comparable angle measurements with only small differences for the most part. This consistency within methods is important information and encouraging since the direction of the dipole moment cannot be measured experimentally. 

A dipole-dipole interaction, or Keesom force, is analogous to the interaction between two magnets. For non-hydrogen bonding molecules with fixed dipoles, these interactions are likely to influence the orientation of the molecules in the crystal. This is because, unlike the Debye force which is always attractive, the interaction between two dipoles is only attractive if the dipoles are properly oriented with respect to one another, as is the case with magnets. 

To be consistent with the experimental data for hydration forces measured between silica interfeces, Xm has to be of the order of 4 A, a value which is compatible with Eiq. (24) for a random orientation of the neighboring water molecules. Therefore, in what follows, we will employ only the value A, =4 A. For the van der Waals interactions, we will assume in all calculations A//-X.3 10 21 J and 2t= 15 A, as obtained previously form the fit with Eq. (43b). The magnitudes of the surface dipoles will be considered 4 Debyes (about twice that of a water molecule) and it will be assumed that each dipole occupies on the surface an area of 50 A" and that they are located at a distance A =1 A below the first water monolayer. 

Induction (or Debye) and Orientation (or Keesom) force 0°+K which are the specific (or polar) properties of the van der Waals attraction exist in the presence of the dipole moment and (total) polarizability, resulting in specific (or polar) intermolecular attraction. 

The equation 18.214 expresses the deformation energy of interaction of two dipoles and forces of this type do not depend to a first approximation on the temperature. For the calculation of the energy of orientation interaction it is possible to proceed in the same manner as in the deduction of the Debye equation (see above) but it is also possible to proceed directly from equation 18.214. 

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