London forces 210 INDEX

Dispersion forces, the so-called London forces 5, are the highest when solute and solvent electrons are polarized. These forces are high when the refractive index values are high. Solvents with high refractive indexes will dissolve solutes with high refractive indexes. 

In most cases, the second (non-zero frequency) term in Equations 2.5 or 2.8 (including the refractive indexes), which is due to the London forces, dominates the Hamaker constant value and thus the forces between particles or surfaces, but for highly polar molecules, e.g. wato, the first term in Equations 2.5 or 2.8 (with the relative pamittivities) can be significant. This is investigated in the following example. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

Here e is the static (zero frequency) relative dielectric constant hP is Planck s constant, i.e., 6.626 10 34 J s ve is the main UV adsorption frequency, which equals for most substances involved 2.9 — 3.0 1015 s 1 and n is the refractive index for visible light (generally taken at a wavelength of 589 nm). The first term in the equation is due to dipole-dipole and dipole-induced dipole interactions, and the second term is due to London dispersion forces (unretarded). The first term is always smaller than (3/4)kpT the second term can be much larger. 

The influence of the medium through which the forces are t lansmitted may be roughly taken into account by dividing the constant A found in this way by the square of the refractive index, as the London-Van der Waals force is essentially of an electric nature. But as we are not fully informed as to the exact values of the London-VanderWaals constants, we shall leave this point out of the discussion for the present. 

If we consider the picture of the London-Van Der Waals forces as given above, viz., as an attraction between the temporary dipole of one atom and the dipole induced by it of the second atom, the finite velocity of propagation of electromagnetic actions causes the induced dipole to be retarded against the inducing one by a time equal to rn/c (if r is the distance between the two atoms and n the refractive index of the medium for the frequency coupled with the temporary dipole). The reaction of the induced dipole on the first one again is retarded by the same time, and if in this total time-lag of 2rn/c the direction of the first dipole is altered by 90 ", the force exerted is exactly nullified, and by a change of 180 " even reverted from an attraction into a repulsion. 

The interactiffli energy between nonpolar molecules should depend on the molar polarizability (London dispersion forces) and therefore the index of refraction. 

The title dispersion arises because, from the more theoretical development of this concept, an important parameter, characteristic of each molecule, is involved. This constant is proportional to the dispersion of the refractive index with frequency, and this in turn is approximately proportional to the ionization energy of the molecule. Hence the title. However, since the word Dispersion has other connotations and could be misleading, the name London , the name of the man who initially developed this theory, is frequently included in the title to give London Dispersion Forces . 

First Polymer Series. The first series of polymers was investigated, in order to limit the interaction between an AFM probe and polymer surfaces to only the dispersion (London) component of the Van der Waals force. Adhesional forces were measured between a S10,t probe and a set of nonpolar polymers that provided a range of refractive indexes (as measured) polystyrene (1.582), isotactic polypropylene (1.501), poly(vinylidene fluoride) (1.407), and poly(tetrafluoroethylene-co-hexafluoropropylene) (1.348). The histograms of the pull-off forces, measured with a SiOx probe, are shown in Figure 2 and tabulated with the calculated values for adhesion energy in Table 1. 

As the spectral shifts in hydrocarbons represent a susbstantial part as compared with the other solvents (excepting water and alcohols) we consider that the dispersion forces of the London [25] type have an important contribution to the solvation energies, and then to the red shiftj because the polarizability of solute molecule in the excited state increases [26], and an instantaneous redistribution of the electric charge will take place. From the McRae s [27] theory results a formula giving the spectral shift under the solvent influence (in terms of solute polarizability and dipole moment of the solute molecule and in terms of the refractive index and dielectric constant of the solvent), which, for nonpolar solvents, reduces to ... 

Because soiling with particulate soil is caused mainly by adhesion, resistance to soiling is anti adhesion. The latest generation of soil retardants for carpets are repellent compounds that can lower the adhesion of soil particles to fibers. The attraction between nonpolar soil and the fiber surface is probably caused by London dispersion forces arising from fluctuations of electron clouds. Hence, energy of the interaction between the soil and the fiber surface should depend on electron polarizability, which, in turn, is related to the refractive index by the Lorentz-Lorenz equation ... 

---