London forces and

As the bulkiness of the substituents increases, the chains are prevented from coming into intimate contact in the crystal. The intermolecular forces which hold these crystals together are all London forces, and these become weaker as the crystals loosen up owing to substituent bulkiness. Accordingly, the value for the heat of fusion decreases moving down Table 4.2. 

The physicochemical forces between colloidal particles are described by the DLVO theory (DLVO refers to Deijaguin and Landau, and Verwey and Overbeek). This theory predicts the potential between spherical particles due to attractive London forces and repulsive forces due to electrical double layers. This potential can be attractive, or both repulsive and attractive. Two minima may be observed The primary minimum characterizes particles that are in close contact and are difficult to disperse, whereas the secondary minimum relates to looser dispersible particles. For more details, see Schowalter (1984). Undoubtedly, real cases may be far more complex Many particles may be present, particles are not always the same size, and particles are rarely spherical. However, the fundamental physics of the problem is similar. The incorporation of all these aspects into a simulation involving tens of thousands of aggregates is daunting and models have resorted to idealized descriptions. 

As well as these permanent dipole moments, random motion of electron density in a molecule leads to a tiny, instantaneous dipole, which can also induce an opposing dipole in neighbouring molecules. This leads to weak intermolecu-lar attractions which are known as dispersive forces or London forces, and are present in all molecules, ions and atoms - even those with no permanent dipole moment. Dispersive forces decrease rapidly with distance, and the attractions are in proportion to 1/r6, where r is the distance between attracting species. 

In this section we outline the molecular origins of the Debye, Keesom, and London forces and discuss the strengths of these forces relative to each other. In addition, we also outline how macroscopic properties and behavior (such as the heat of vaporization of materials, nonideality of equations of state, and condensation of gases) can be traced to the influence of the above van der Waals forces and illustrate these through specific examples. Another example of the van der Waals forces, namely, the relation between the surface tension (or surface energy) of materials and the London force, is discussed in Section 10.7. 

The two values agree remarkably well, showing the relation between the London force and the nonideality of the gas. 

SOLUTION Both substances can interact by London forces, and the strengths will be about the same in each case because they are composed of the same atoms. However, only the cis compound is polar, so only it can have dipole-dipole interactions. Therefore, we can expect cis-dibromoethene to have stronger intermolecular forces than trans-dibrornoethene. It follows that we should expect traws-dibromoethene to be the more volatile. 

Soft non-spherical potentials are one step towards a more realistic model. Luckhurst et al. [413] have used a potential having the prolate spheroidal symmetry discussed above but which is based on the well known Lennard-Jones or twelve-six potential. This involves an attractive 1/r6 potential based on London forces and a repulsive 1/r12 potential. Once again it is possible to predict the existence of a smectic phase. 

Because unbranched alkanes are neutral, nonpolar molecules, it is difficult to explain the existing intermolecular force between such alkanes that increases as the alkane molecules become larger. We will see that this attractive force is weak and tenuous. These molecules do not become overly friendly with each other. In theory, as atoms within one alkane molecule approach the atoms of another alkane molecule, the electrons around these atoms, for an instant, arrange themselves asymmetrically around the atoms so that instant dipoles are formed—the positive side of one atom attracts the negative side of another atom. This weak intermolecular attractive force is called a London Force. When there is a weak intermolecular attractive force between polar molecules, the force is called a dipole-dipole force. Together, London forces and dipole-dipole forces are called Van der Waals forces. 

In addition to hydrogen bonding, tertiary structure is maintained by London forces and, sometimes, disulfide bridges. London forces act between nonpolar... 

H. London Forces and Short-Range Electron Shell Repulsion.140... 

It has been traditional to define a van der Waals potential (which combines Coulomb s law and the Lennard-Jones 6-12 potential function) and thereby subsume electronic shell repulsion, London forces, and electrostatic interactions under the term van der Waals interaction. Unfortunately, the resulting expression is an oversimplified treatment of the electrostatic interactions, which are only calculated between close neighbors and are considered to be spatially isotropic. Both of these implicit assumptions are untrue and do not represent physically realistic approximations. We prefer to use the term van der Waals distance for the intemuclear separation at which the 6-12 potential function is a minimum (see Fig. 6), the van der Waals radius being one-half this value when the two interacting atoms are identical, and explicitly treat the Lennard-Jones and electrostatic terms separately. While the term van der Waals interaction may have some value as a shorthand in structure description, it should be avoided when energetics are treated quantitatively. 

The effectiveness of deep-bed filters in removing suspended particles is measured by die value of die filter coefficient which in turn is related to the capture efficiency of a single characteristic grain of the bed. Capture efficiencies are evaluated in the present paper for nil cases of practical importance in which London forces and convective-diffusion serve to transport particles to the surface of a spherical collector immersed in a creeping How field. Gravitational forces are considered in some cases, but the general results apply mainly to submicron or neutrally buoyant particles suspended in a viscous fluid such as water. Results obtained by linearly superimposing the in-... 

The objectives of the present paper are to (1) compute the rate of deposition of particles onto a spherical collector in a creeping flow field for all situations in which London forces and convective-diffusion act as transport mechanisms, (2) identify limiting behaviors according to the relative values of the characteristic parameters for each mechanism, (3) establish the physical conditions in which each of the limiting cases is valid, and (4) test the accuracy of the additivity rule. 

As a rule, it is then possible to note that intermolecular interaction can be considered as the sum of two components a dispersive (or nonpolar, superscript L) component, i.e., attributable to London force, and a specific (or polar, SP) component owing to all other types of interactions (Debye, Keesom, hydrogen bonding, and other weakly polar effects)... 

Dispersion forces -> London forces, and -> van der Waals forces... 

The three intermolecular forces are dipolar attractions, van der Waals forces (also called London forces), and hydrogen bonding. The effects of dipoles (Section 13.5) are considered first, followed by discussions of van der Waals forces and hydrogen bonding. 

Van der Waals forces comprise an attractive part, the induction or London forces, and repulsion between the atoms at short distance. For short contacts, these forces are usually treated according to the atom-atom method, which has recently been extensively reviewed by Pertsin and Kitaigorodsky (1987). This approach is based on empirical parameters. The interaction between two molecules is represented as the sum of all the interactions of all atoms of one molecule with all atoms of the other one. The atom-atom interactions are added as long as the summation converges, usually between 6 and 10 A. Two different analytical forms are usually assumed ... 

The calculation of lattice energy has been refined by including terms arising from the van der Waals (London) forces and from the zero-point energy. The former is important only if both ions are readily polarizable, as may be seen from the following figures ... 

London Component of the Surface Energy of Heated Treated Silicas. Surface energy is usually considered as the sum of two components the London component (y ), steming from London forces, and the specific component (y p), originating from all other types of forces (polar, H-bonding, metallic, etc). Two methods are commonly used for the measurement of surface energies wettability and adsorption techniques. 

The van der Waals (VDW) attractive forces are the principal forces between dry, noncharged spherical aerosol particles [262] and may reduce stability and cause flocculation of suspended particles. The VDW forces arise from the attractive forces between permanent dipoles (Keesom forces), induced dipoles (London forces), and dipole-induced dipoles (Debye forces). For nonpolar or slightly polar compounds, the force of attraction between two particles with diameter d separated by a distance h (where h < d) is ... 

Although the term van der Waals forces usually refers to all intermolecular attractions, it is also often used interchangeably with dispersion forces, as are the terms London forces and dipole-induced dipole forces. ... 

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