Van der Waals Forces: Properties and Components

Van der Waals forces are the weak forces that contribute to intermolecular bonding between molecules. Molecules inherently possess energy and their electrons are always in motion, so transient concentrations of electrons in one region or another lead electrically positive regions of a molecule to be attracted to the electrons of another molecule. Similarly, negatively charged regions of one molecule are repulsed by negatively charged regions of another molecule.

Van der Waals forces are the sum of the attractive and repulsive electrical forces between atoms and molecules. These forces differ from covalent and ionic chemical bonding because they result from fluctuations in charge density of particles. Examples of van der Waals forces include hydrogen bonding, dispersion forces, and dipole-dipole interactions.

Properties of Van der Waals Forces

Certain characteristics are displayed by van der Waals forces:

• They are additive.
• They are weaker than either ionic or covalent chemical bonds.
• They are not directional.
• They act only over a very short range. The interaction is greater when molecules draw closer.
• They are independent of temperature, with the exception of dipole-dipole interactions.

Components of Van der Waals Forces

Van der Waals forces are the weakest intermolecular forces. Their strength typically ranges from 0.4 kilojoules per mole (kJ/mol) to 4 kJ/mol and acts over distances of less than 0.6 nanometers (nm). When the distance is less than 0.4 nm, the net effect of the forces is repulsive as electron clouds repel each other.

There are four major contributions to van der Waals forces:

1. A negative component prevents molecules from collapsing. This is due to the Pauli exclusion principle.
2. Either an attractive or a repulsive electrostatic interaction occurs between permanent charges, dipoles, quadrupoles, and multipoles. This interaction is called Keesom interaction or Keesom force, named for Willem Hendrik Keesom.
3. Induction or polarization occurs. This is an attractive force between a permanent polarity on one molecule and an induced polarity on another. This interaction is called the Debye force, for Peter J.W. Debye.
4. London dispersion force is the attraction between any pair of molecules due to instantaneous polarization. The force is named after Fritz London. Note that even nonpolar molecules experience London dispersion.

Van der Waals Forces, Geckos, and Arthropods

Geckos, insects, and some spiders have setae on the pads of their feet that allow them to climb extremely smooth surfaces such as glass. In fact, a gecko can even hang from a single toe! Scientists have offered several explanations for the phenomenon, but it turns out that the primary cause of the adhesion, more than van der Waals forces or capillary action, is electrostatic force.

Researchers have produced dry glue and adhesive tape based on analysis of gecko and spider feet. The stickiness results from tiny Velcro-like hairs and lipids found on gecko feet.

Real-Life Spider-Man

In 2014, the Defense Advanced Research Projects Agency (DARPA) tested its gecko-inspired Geckskin, a material based on the setae of gecko foot pads and intended to give military personnel Spider-Man-like abilities. A 220-pound researcher carrying an additional 45 pounds of gear successfully scaled a 26-foot glass wall using two climbing paddles.

Sources

• Kellar, Autumn, et al. "Evidence for Van der Waals Adhesion in Gecko Setae." Proceedings of the National Academy of Sciences, vol. 99, no. 19, 2002, 12252–6. doi:10.1073/pnas.192252799.
• Dzyaloshinskii, I. E., et al. "General Theory of Van der Waals' Forces." Soviet Physics Uspekhi, vol. 4, no. 2, 1961. doi:10.1070/PU1961v004n02ABEH003330.
• Israelachvili, J. Intermolecular and Surface Forces. Academic Press, 1985.
• Parsegian, V. A. Van der Waals Forces: A Handbook for Biologists, Chemists, Engineers, and Physicists. Cambridge University Press, 2005.
• Wolff, J. O., Gorb, S. N. "The Influence of Humidity on the Attachment Ability of the Spider Philodromus dispar (Araneae, Philodromidae)." Proceedings of the Royal Society B: Biological Sciences, vol. 279, no. 1726, 2011. doi: 10.1098/rspb.2011.0505.