Van der Waals force
In
chemistry, the term van der Waals force refers to a particular class of intermolecular forces. The term originally referred to all such forces, and this usage is still sometimes observed, but it is now more commonly used to refer to those forces which arise from the
polarization of molecules into
dipoles. This includes forces that arise from fixed or angle-averaged dipoles and free or rotation dipoles as well as shifts in electron cloud distribution . The name refers to the Dutch physicist and chemist
Johannes Diderik van der Waals, who first documented these types of forces.
Encyclopedia
In
chemistry, the term
van der Waals force refers to a particular class of intermolecular forces. The term originally referred to all such forces, and this usage is still sometimes observed, but it is now more commonly used to refer to those forces which arise from the
polarization of molecules into
dipoles. This includes forces that arise from fixed or angle-averaged dipoles and free or rotation dipoles as well as shifts in electron cloud distribution . The name refers to the Dutch physicist and chemist
Johannes Diderik van der Waals, who first documented these types of forces. The
Lennard-Jones potential is often used as an approximate model for the Van der Waals force as a function of distance.
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 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 is 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 . Van der Waals forces are responsible for certain cases of pressure broadening of spectral lines.
London dispersion force
London dispersion forces, named after the German-American physicist Fritz London, are weak intermolecular forces that arise from the attractive force between transient
dipoles in
molecules without permanent multipole moments. London dispersion forces are also just called
dispersion forces or
London 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. London forces are also present in polar molecules, but they are usually only a small part of the total interaction force.
Electron density in a molecule may be redistributed by proximity to another multipole. 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 by a transient multipole in another nonpolar molecule.
London forces are weaker than other intermolecular forces such as ionic interactions,
hydrogen bonding, or permanent dipole-dipole interactions.
This phenomenon is the only attractive intermolecular force at large distances present between neutral atoms , and is the major attractive force between non-polar molecules, such as
nitrogen or
methane . Without London forces, there would be no attractive force between noble gas atoms, and they could not then be obtained in a liquid form.
London forces become stronger as the atom 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 .
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 E. M. Lifshitz.
Use by animals
The Van der Waals force is the force to which the
gecko's climbing ability is attributed. A gecko can hang on a glass surface using only one toe. Efforts continue to create a synthetic "gecko tape" that exploits this knowledge. So far, research has produced some promising results - early research yielded an
adhesive tape product, which only obtains a fraction of the forces measured from the natural material, and new research has yielded a discovery that purports 200 times the adhesive forces of the natural material.
Researchers at
Stanford University recently developed a gecko-like
robot which uses synthetic setae to climb walls .
See also
Sources
- Iver Brevik, V. N. Marachevsky, Kimball A. Milton, Identity of the Van der Waals Force and the Casimir Effect and the Irrelevance of these Phenomena to Sonoluminescence,
- I. D. Dzyaloshinskii, E. M. Lifshitz, and L. P. Pitaevskii, Usp. Fiz. Nauk 73, 381
- English translation: Soviet Phys. Usp. 4, 153
- R. H. French, University of Pennsylvania, Materials Science "".
- L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, Pergamon, Oxford, 1960, pp. 368–376.
- Mark Lefers, "". Holmgren Lab.
- E. M. Lifshitz, Zh. Eksp. Teor. Fiz. 29, 894
- English translation: Soviet Phys. JETP 2, 73
- Western Oregon University's "". .
