Van der Waals force

In physical chemistry, the van der Waals force (or van der Waals interaction), named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. The term includes:

• force between a permanent dipole and a corresponding induced dipole (Debye force)
• force between two instantaneously induced dipoles (London dispersion force)

It is also sometimes used loosely as a synonym for the totality of intermolecular forces. Van der Waals forces are relatively weak compared to normal chemical bonds, but play a fundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. Van der Waals forces define the chemical character of many organic compounds. They also define the solubility of organic substances in polar and non-polar media. In low molecular weight alcohols, the properties of the polar hydroxyl group dominate the weak intermolecular forces of van der Waals. In higher molecular weight alcohols, the properties of the nonpolar hydrocarbon chain(s) dominate and define the solubility. Van der Waals-London forces grow with the length of the nonpolar part of the substance.

Definition

Van der Waals forces include attractions between atoms, molecules, and surfaces. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations of nearby particles (a consequence of quantum dynamics).

Intermolecular forces have four major contributions:

1. A repulsive component resulting from the Pauli exclusion principle that prevents the collapse of molecules.
2. Induction (also known as polarization), which is the attractive interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is sometimes called Debye force after Peter J.W. Debye.
3. Dispersion (usually named after Fritz London), which is the attractive interaction between any pair of molecules, including non-polar atoms, arising from the interactions of instantaneous multipoles.

Returning to nomenclature, different texts refer to different things using the term "van der Waals force". Some texts mean by the van der Waals force the totality of forces (including repulsion); others mean all the attractive forces (and then sometimes distinguish van der Waals-Keesom, van der Waals-Debye, and van der Waals-London).

All intermolecular/van der Waals forces are anisotropic (except those between two noble gas atoms), which means that they depend on the relative orientation of the molecules. The induction and dispersion interactions are always attractive, irrespective of orientation, but the electrostatic interaction changes sign upon rotation of the molecules. That is, the electrostatic force can be attractive or repulsive, depending on the mutual orientation of the molecules. When molecules are in thermal motion, as they are in the gas and liquid phase, the electrostatic force is averaged out to a large extent, because the molecules thermally rotate and thus probe both repulsive and attractive parts of the electrostatic force. Sometimes this effect is expressed by the statement that "random thermal motion around room temperature can usually overcome or disrupt them" (which refers to the electrostatic component of the van der Waals force). Clearly, the thermal averaging effect is much less pronounced for the attractive induction and dispersion forces.

The Lennard-Jones potential is often used as an approximate model for the isotropic part of a total (repulsion plus attraction) van der Waals force as a function of distance.

Van der Waals forces are responsible for certain cases of pressure broadening (van der Waals broadening) of spectral lines and the formation of van der Waals molecules. The London-van der Waals forces are related to the Casimir effect for dielectric media, the former being the microscopic description of the latter bulk property. The first detailed calculations of this were done in 1955 by E. M. Lifshitz.

Calculation

$F(r)={\frac {\lambda }{r^{s}}}-{\frac {\mu }{r^{t}}}$

London dispersion force

Main article: London dispersion force

London dispersion forces, named after the German-American physicist Fritz London, are weak intermolecular forces that arise from the interactive forces between instantaneous multipoles in molecules without permanent multipole moments. These forces dominate the interaction of non-polar molecules, and also play a less significant role in Van der Waals forces than molecules containing permanent dipoles. or ionized molecules. London dispersion forces are also known as dispersion forces, London forces, or induced dipole–dipole forces. They increase with the molar mass, causing a higher boiling point especially for the halogen group.

van der Waals forces between macroscopic objects

One can use the equation above to obtain the total van der Waals attraction (or repusion) between macroscopic object. For this, one needs to integrate over the total volume of the object, which makes it therefore specific for the shape of the object. For example the van der Waals energy per unit area, E/a, between two infinite parallel surfaces is:

${\frac {E}{a}}=-{\frac {A}{12\pi D^{2}}},$

where A is the Hamaker constant which is a constant that depends on the material properties and can be positive or negative depending on the system, and D is the distance between the surfaces. Mathematical expressions for van der Waals interactions between other geometries have been published in the literature.

Use by animals

Gecko climbing glass using its natural setae

The ability of geckos – which can hang on a glass surface using only one toe – to climb on sheer surfaces has been attributed to Van Der Waals force, although a more recent study suggests that water molecules of roughly monolayer thickness (present on virtually all natural surfaces) also play a role. Efforts continue to create a dry glue that exploits the effect.

Further reading

• 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, hep-th/9901011
• 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.
• Dieter Langbein, "[Langbein, Dieter Theory of Van der Waals Attraction, ( Springer-Verlag New York Heidelberg 1974)]"
• Mark Lefers, "Van der Waals dispersion force". Holmgren Lab.
• E. M. Lifshitz, Zh. Eksp. Teor. Fiz. 29, 894 (1955)
  • English translation: Soviet Phys. JETP 2, 73 (1956)
• Western Oregon University's "London force". Intermolecular Forces. (animation)
• J. Lyklema, Fundamentals of Interface and Colloid Science, page 4.43

• Israelachvili J., Intermolecular and Surface Forces, Academic Press (1985–2004), ISBN 0-12-375181-0

External links

• Senese, Fred (1999). "What are van der Waals forces?". Frostburg State University. Retrieved March 2010. {{cite web}}: Check date values in: |accessdate= (help) An introductory description of the van der Waals force (as a sum of attractive components only)
• Robert Full: Learning from the gecko's tail. TED Talk on biomimicry, including applications of Van der Waals force.