Friday, November 8, 2019

Van Der Waals Forces Definition

Van Der Waals Forces Definition Van der Waals Forces are the weak forces which 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. Key Takeaways: Van der Waals Forces Van der Waals forces are distance-dependent forces between atoms and molecule not associated with covalent or ionic chemical bonds.Sometimes the term is used to encompass all intermolecular forces, although some scientists only include London dispersion force, Debye force, and Keeson force.Van der Waals forces are the weakest of the chemical forces, but they still play an important role in the properties of molecules and in surface science. Properties of Van der Waals Forces Certain characteristics are displayed by van der Waals forces: The forces are additive.The forces are weaker than either ionic or covalent chemical bonds.They are not directional.The forces act only over a very short range. The interaction is greater when molecules draw closer.Van der Waals forces 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 to 4 kJ/mol and act over distances less than 0.6 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: There is a negative component that prevents molecules from collapsing. This is due to the Pauli exclusion principle.Either an attractive or 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.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.London dispersion force is the attraction between any pair of molecules due to instantaneous polarization. The force is named after Fritz London. Note even nonpolar molecules experience London dispersion. Van der Waals Forces, Geckos, and Arthropods Geckos, insects, and some spiders have setae on their foot pads that allow them to climb extremely smooth surfaces, such as glass. In fact, a gecko can even hang from a single toe! Scientists have proposed numerous explanations for the phenomena, but it turns out the primary cause of the adhesion is the electrostatic force more than van der Waals forces or capillary action. Researchers have produced dry glue and adhesive tape based on analysis of gecko and spider feet. The stickiness results both from tiny velcro-like hair and also the addition of lipids, which are found on gecko feet. Gecko feet are sticky because of van der Waals forces, electrostatic forces, and lipids found on their skin. StephanHoerold / Getty Images Real-Life Spiderman In 2014, DARPA tested its Geckskin, a material based on the setae of gecko footpads. A 100-kg researcher, carrying an additional 20 kg of gear, scaled an 8 m (26 ft) glass wall using two climbing paddles. The goal is to give military personnel Spiderman-like abilities. Scientists have found a way to use van der Waals forces to help people cling to smooth surfaces, such as glass and walls. OrangeDukeProductions / Getty Images Sources Autumn, Kellar; Sitti, Metin; Liang, Yiching A.; Peattie, Anne M.; Hansen, Wendy R.; Sponberg, Simon; Kenny, Thomas W.; Fearing, Ronald; Israelachvili, Jacob N.; Full, Robert J. (2002). Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences. 99 (19): 12252–6. doi:10.1073/pnas.192252799Dzyaloshinskii, I. D.; Lifshitz, E. M.; Pitaevskii, L. P. (1961). General theory of van der Waals forces. Soviet Physics Uspekhi. 4 (2): 153. doi:10.1070/PU1961v004n02ABEH003330Israelachvili, J. (1985–2004). Intermolecular and Surface Forces. Academic Press. ISBN 978-0-12-375181-2.Parsegian, V. A. (2006). van der Waals Forces: A Handbook for Biologists, Chemists, Engineers, and Physicists. Cambridge University Press. ISBN 978-0-521-83906-8.Wolff, J. O.; Gorb, S. N. (18 May 2011). The influence of humidity on the attachment ability of the spider Philodromus dispar (Araneae, Philodromidae). Proceedings of the Royal Society B: Biological Sciences. 279 (1726): 139–143. doi:10.1098/rspb.2011.0505

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