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Van der Waals force

In chemistry, the term Van der Waals force originally referred to all forms of intermolecular forces; however, in modern usage it tends to refer only to London forces: those forces which arise from induced rather than permanent dipoles. The forces are named after Johannes Diderik van der Waals.

Van der Waals interactions are observed in noble gases, which are very stable and tend not to interact. This is why it is difficult to condense them into liquids. However, the larger the atom of the noble gas (the more electrons it has), the easier it is to condense the gas into a liquid. This happens because when the electron cloud surrounding the gas atom gets large, it does not form a perfect sphere around the nucleus. Rather, it's only spherical if averaged over longer times and generally forms an oval, which has a slight negative charge on one side and a slight positive charge on the other. The atom becomes a temporary dipole. This induces the same shift in neighboring atoms and spreads from one atom to the next. Unlike charges attract and the induced dipoles are held together by dispersion force (or Van der Waals force).

The Van der Waals force is the force to which the gecko's unique ability to cling to smooth surfaces is attributed. A gecko can hang on a glass surface using only one toe. In 2003 a kind of synthetic adhesive tape was created using this principle.

Table of contents

London force

London forces, named after the German physicist Fritz London, are weak intermolecular forces that arise from the attractive force between transient dipoles (or better multipoles) in otherwise nonpolar molecules. London forces are also called London dispersion forces and sometimes Van der Waals forces.

London forces can be exhibited by nonpolar molecules because electron density moves about a molecule probabilistically. There is a high chance that the electron density will not be evenly distributed throughout a nonpolar molecule. When an uneven distribution occurs, a temporary multipole is created. This multipole may interact with other nearby multipoles.

Electron density in a molecule may be redistributed by proximity to another pole. Electrons will gather on the side of a molecule that faces a positive charge and retreat from a negative charge. Hence, a transient multipole can be produced by a nearby polar molecule, or even a transient multipole in another nonpolar molecule.

London forces are much weaker than other intermolecular forces such as ionic interactions, hydrogen bonding, or permanent dipole-dipole interactions.

This phenomenon is the only intermolecular force present between nonpolar species such as helium, nitrogen, or methane (to name a few). Without London forces, there would be no attractive force between these molecules and they could not then be obtained in a liquid form.

London forces become stronger as the atom (or molecule) in question becomes larger. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds. This trend is exemplified by the halogens (from smallest to largest: F2, Cl2, Br2, I2). Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid.

Relation to the Casimir effect

The London-Van der Waals force is related to the Casimir effect for dielectric media, the former the microscopic description of the latter bulk property. First detailed calculations of this were done 1955 by Lifshitz. This equivalence gained attention and some new papers, in the recent discussion of sonoluminescence.

See also

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References

  • E. M. Lifshitz, Zh. Eksp. Teor. Fiz. 29, 894 (1955)
    • English translation: Soviet Phys. JETP 2, 73 (1956)
  • I. D. Dzyaloshinskii, E. M. Lifshitz, and L. P. Pitaevskii, Usp. Fiz. Nauk 73, 381 (1961)
    • English translation: Soviet Phys. Usp. 4, 153 (1961)
  • L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, Pergamon, Oxford, 1960, pp. 368–376.







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