Text: "The origin of the van der Waals forces was once sought in gravitational attractions. These are now known to be much too small to account for the known intermolecular effects. The first conjecture (P. Debye) involved the assumption that one of the interacting molecules carries a permanent multi-pole, for example, a quadrapole, and induces by electric polarization a small multi-pole in the other. This action, known as the induction effect, was shown to lead to attractive forces. Although it does take place in many instances, it is too small to account for the full strength of the van der Waals forces in general. "If both molecules have permanent polarity, the multipoles tend to align themselves and thereby give rise to attractive forces. This alignment effect (W.H. Keesom) is an important part of the van der Waals forces between polar molecules, but does not explain the forces between nonpolar structures. "The missing factor is found upon application of quantum theory to molecular interactions. It may be described as follows: In general, the electrons in one molecule revolve rapidly and in a manner uncorrelated with the electronic motions in other molecules, but when two molecules are brought fairly close together, the electrostatic fields set up by the moving electrons constrain the motions in the two molecules to be more or less in phase, the phase agreement being closer the smaller the distance between interacting molecules. Attractive forces result, these forces being called dispersion forces (F. London). At large distances of separation, the potential energy associated with dispersion forces is proportional to -R-6. "A further development of the theory shows that the interaction involves not only the interplay between the rapidly fluctuating dipoles considered in the dispersion effect, but also the production of higher-order multipoles (H. Margenau). These give rise to asymptotic interactions proportional to R-8 (dipole-quadrapole interaction), R-10 (quadrapole-quadrapole and dipole-octupole interaction), and so forth. "Only for symmetric molecules are the intermolecular forces independent of the orientation of the molecular axis with respect to the line joining the molecules. In general, they display features which are not indicated in the figure. They are functions of angles and therefore noncentral forces. Moreover, when more than two molecules interact, the total potential energy of the system is not necessarily the sum of the potential energies of all pairs. Additivity in this sense holds only for the simpler kinds in dispersion forces. (See LAW OF SUPERPOSITION, BEATS, LONGITUDINAL WAVES) Also, van der Waals forces between molecules carrying permanent multipoles and between atoms or molecules in excited states can be repulsive as well as attractive. "The relative role played by the different constituents of the van der Waals forces is generally difficult to assess. There are only a few instances in which one type dominates all others. In the case of H2O, at a distance of separation equal to the diameter of the molecules as given by kinetic theory, the induction, alignment, and dispersion effects are all of comparable magnitude. For dispersion forces, the terms proportional to R-6, R-8, and R-10 are of comparable importance."

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