Van der Waals force 1

Van der Waals force

Geckos can stick to walls and ceilings because of Van der Waalsforces; see the section below.

In physical chemistry, the van der Waals' force (orvan der Waals' interaction), named after Dutchscientist Johannes Diderik van der Waals, is the sum ofthe attractive or repulsive forces between molecules (orbetween parts of the same molecule) other than thosedue to covalent bonds, the hydrogen bonds, or theelectrostatic interaction of ions with one another orwith neutral molecules or charged molecules. The termincludes:

• force between two permanent dipoles (Keesomforce)

• force between a permanent dipole and acorresponding induced dipole (Debye force)

• force between two instantaneously induced dipoles(London dispersion force).

It is also sometimes used loosely as a synonym for thetotality of intermolecular forces. Van der Waals' forces are relatively weak compared to covalent bonds, but play afundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science,nanotechnology, surface science, and condensed matter physics. Van der Waals forces define many properties oforganic compounds, including their solubility in polar and non-polar media.

In low molecular weight alcohols, the hydrogen-bonding properties of the polar hydroxyl group dominate the weakervan der Waals' interactions. In higher molecular weight alcohols, the properties of the nonpolar hydrocarbon chain(s)dominate and define the solubility. Van der Waals forces quickly vanish at longer distances between interactingmolecules.In 2012, the first direct measurements of the strength of the van der Waals' force for a single organic molecule boundto a metal surface was made via atomic force microscopy and corroborated with density functional calculations.[1]

Definition

Attractive interactions resulting from dipole-dipoleinteraction of two hydrogen chloride molecules

Van der Waals forces include attractions and repulsions betweenatoms, molecules, and surfaces, as well as other intermolecularforces. They differ from covalent and ionic bonding in that theyare caused by correlations in the fluctuating polarizations ofnearby particles (a consequence of quantum dynamics[2]).

Intermolecular forces have four major contributions:1. A repulsive component resulting from the Pauli exclusion principle that prevents the collapse of molecules.2. Attractive or repulsive electrostatic interactions between permanent charges (in the case of molecular ions),

dipoles (in the case of molecules without inversion center), quadrupoles (all molecules with symmetry lower thancubic), and in general between permanent multipoles. The electrostatic interaction is sometimes called theKeesom interaction or Keesom force after Willem Hendrik Keesom.

3. Induction (also known as polarization), which is the attractive interaction between a permanent multipole on onemolecule with an induced multipole on another. This interaction is sometimes called Debye force after Peter J.W.Debye.

Van der Waals force 2

4. 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 textsmean 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 meansthat they depend on the relative orientation of the molecules. The induction and dispersion interactions are alwaysattractive, 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 outto a large extent, because the molecules thermally rotate and thus probe both repulsive and attractive parts of theelectrostatic force. Sometimes this effect is expressed by the statement that "random thermal motion around roomtemperature can usually overcome or disrupt them" (which refers to the electrostatic component of the van der Waalsforce). Clearly, the thermal averaging effect is much less pronounced for the attractive induction and dispersionforces.The Lennard-Jones potential is often used as an approximate model for the isotropic part of a total (repulsion plusattraction) 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 spectrallines and the formation of van der Waals molecules. The London-van der Waals forces are related to the Casimireffect for dielectric media, the former being the microscopic description of the latter bulk property. The first detailedcalculations of this were done in 1955 by E. M. Lifshitz.[3]

A general theory of Lifshitz van der Waals' forces was developed in 1961 by I. E. Dzyaloshinskii, et al. In 2011, Y.Zheng and A. Narayanaswamy provided a alternate formalism for calculating van der Waals or Casimir pressure.

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

Van der Waals' forces between macroscopic objectsFor macroscopic bodies with known volumes and numbers of atoms or molecules per unit volume, the total van derWaals force is often computed based on the "microscopic theory" as the sum over all interacting pairs. It is necessaryto integrate over the total volume of the object, which makes the calculation dependent on the objects' shapes. Forexample, the van der Waals' interaction energy between spherical bodies of radii R1 and R2 and with smooth surfaceswas approximated in 1937 by Hamaker[4] (using London's famous 1937 equation for the dispersion interactionenergy between atoms/molecules[5] as the starting point) by:

(1)

where A is the Hamaker coefficient, which is a constant (~10−19 − 10−20 J) that depends on the material properties (itcan be positive or negative in sign depending on the intervening medium), and z is the center-to-center distance; i.e.,the sum of R1, R2, and r (the distance between the surfaces): .

Van der Waals force 3

In the limit of close-approach, the spheres are sufficiently large compared to the distance between them; i.e.,, so that equation (1) for the potential energy function simplifies to:

(2)

The van der Waals' force between two spheres of constant radii (R1 and R2 are treated as parameters) is then afunction of separation since the force on an object is the negative of the derivative of the potential energy function,

. This yields:

(3)

The van der Waals forces between objects with other geometries using the Hamaker model have been published inthe literature.[6][7][8]

From the expression above, it is seen that the van der Waals force decreases with decreasing particle size (R).Nevertheless, the strength of inertial forces, such as gravity and drag/lift, decrease to a greater extent. Consequently,the van der Waals forces become dominant for collections of very small particles such as very fine-grained drypowders (where there are no capillary forces present) even though the force of attraction is smaller in magnitude thanit is for larger particles of the same substance. Such powders are said to be cohesive, meaning they are not as easilyfluidized or pneumatically conveyed as easily as their more coarse-grained counterparts. Generally, free-flow occurswith particles greater than about 250 μm.The van der Waals' force of adhesion is also dependent on the surface topography. If there are surface asperities, orprotuberances, that result in a greater total area of contact between two particles or between a particle and a wall, thisincreases the van der Waals force of attraction as well as the tendency for mechanical interlocking.The microscopic theory assumes pairwise additivity. It neglects many-body interactions and retardation. A morerigorous approach accounting for these effects, called the "macroscopic theory," was developed by Lifshitz in1956.[9] Langbein derived a much more cumbersome "exact" expression in 1970 for spherical bodies within theframework of the Lifshitz theory[10] while a simpler macroscopic model approximation had been made by Derjaguinas early as 1934.[11] Expressions for the van der Waals forces for many different geometries using the Lifshitz theoryhave likewise been published.

Van der Waals force 4

Use by geckos

Gecko climbing glass

The ability of geckos – which can hang on aglass surface using only one toe – to climbon sheer surfaces has been attributed to thevan der Waals forces between these surfacesand the spatulae (plural of spatula), ormicroscopic projections, which cover thehair-like setae found on their footpads.[12] Alater study suggested that capillary adhesionmight play a role, but that hypothesis hasbeen rejected by more recent studies. Therewere efforts in 2008 to create a dry glue thatexploits the effect,[13] and success wasachieved in 2011 to create an adhesive tapeon similar grounds.[14] In 2011, a paper waspublished relating the effect to bothvelcro-like hairs and the presence of lipidsin gecko footprints.[15]

References[1] http:/ / www. columbia. edu/ ~sva2107/ media/ Aradhya_NMat_2012. pdf[2][2] Chapter 6 Electromagnetic Radiation in an Absorbing Medium[3] For further investigation, one may consult the University of St. Andrews' levitation work in a popular article: Science Journal: New way to

levitate objects discovered (http:/ / www. sciencedaily. com/ releases/ 2007/ 08/ 070806091137. htm), and in a more scholarly version: NewJournal of Physics: Quantum levitation by left-handed metamaterials (http:/ / www. iop. org/ EJ/ article/ 1367-2630/ 9/ 8/ 254/ njp7_8_254.html), which relate the Casimir effect to the gecko and how the reversal of the Casimir effect can result in physical levitation of tiny objects.

[4][4] H. C. Hamaker, Physica, 4(10), 1058-1072 (1937)[5][5] F. London, Transactions of the Faraday Society, 33, 8-26 (1937)[6] R. Tadmor, JOURNAL OF PHYSICS: CONDENSED MATTER, 13 (2001) L195–L202[7] Israelachvili J., Intermolecular and Surface Forces, Academic Press (1985–2004), ISBN 0-12-375181-0[8][8] V. A. Parsegian, "Van der Waals Forces: A Handbook for Biologists, Chemists, Engineers, and Physicists," Cambridge University Press

(2006) ISBN 978-0-521-83906-8[9][9] E. M. Lifshitz, Soviet Phys. JETP, 2, 73 (1956)[10][10] D. Langbein, Phys. Rev. B, 2, 3371 (1970)[11][11] B. V. Derjaguin, Kolloid-Z., 69, 155-64 (1934)[12] Researchers discover how geckos know when to hold tight (http:/ / media-relations. www. clemson. edu/ article. php?article_id=2122).

Clemson.edu. Retrieved on 2011-01-08.[13] Gecko-like glue is said to be stickiest yet (http:/ / www. reuters. com/ article/ scienceNews/ idUSN0942431020081009?sp=true),

"reuters.com" 8 October 2008[14] Biologically inspired adhesive tape can be reused thousands of times Biologically inspired adhesive tape can be reused thousands of times

(http:/ / www. gizmag. com/ bioinspired-adhesive-tape-kiel/ 20406/ )[15] Scientists trace gecko footprint, find clue to glue (http:/ / www. physorg. com/ news/ 2011-08-scientists-gecko-footprint-clue. html)

Van der Waals force 5

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 (http:/ / arxiv. org/ abs/ 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 (http:/ / www. biochem. northwestern. edu/ holmgren/ Glossary/

Definitions/ Def-V/ Van_der_Waals_force. html)". 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 (http:/ / www. wou. edu/ las/ physci/ ch334/ lecture/ intermol/

london. htm)". Intermolecular Forces (http:/ / www. wou. edu/ las/ physci/ ch334/ lecture/ intermol/ ). (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?" (http:/ / antoine. frostburg. edu/ chem/ senese/ 101/

liquids/ faq/ h-bonding-vs-london-forces. shtml). Frostburg State University. Retrieved March 2010. Anintroductory description of the van der Waals force (as a sum of attractive components only)

• Robert Full: Learning from the gecko's tail. (http:/ / www. ted. com/ talks/ lang/ eng/robert_full_learning_from_the_gecko_s_tail. html) TED Talk on biomimicry, including applications of Van derWaals force.

Article Sources and Contributors 6

Article Sources and ContributorsVan der Waals force  Source: http://en.wikipedia.org/w/index.php?oldid=594579294  Contributors: *drew, 84user, 99of9, A13ean, A3RO, AWhiteC, Aboalbiss, Adabow, AdaronKentano,Adjwilley, Alansohn, Altenmann, Anna Frodesiak, Anomalocaris, Apeattie, Arbitrarily0, Ark2120, Arrektor, Arthur Rubin, Ashmoo, Atlantia, Aurum87, Bduke, Beetstra, Benplowman, Brewsohare, Brutaldeluxe, Bssquirrel, Bth, CaesiumX823, Can You Prove That You're Human, Canonymous, Cdbbench, Charles Matthews, ChemMater, Chris the speller, Christian75, Cless Alvein,Cwmhiraeth, Cybercobra, Cynorg, Dakrause, Danedori, Danfuzz, David Latapie, DavidSTaylor, Deglr6328, Diberri, Djfkrhd, Dllahr, Doodle77, Dr.Soft, EPadmirateur, Eequor, Effeietsanders,Eg-T2g, Elen of the Roads, Elkester, Eloc Jcg, Emperorbma, Emt14, Enviroboy, Eric Shalov, Everard Proudfoot, Falcon8765, Fama Clamosa, Fett0001, Flomar0, Ftc68, Gene Nygaard,Gentgeen, Gidonb, Giftlite, Giro720, Glane23, Harold f, Harry, Hjclark3, Hodja Nasreddin, Hoffmeier, Humanengr, Huwenting, Hyandrew, Imesswithwiki, Ita140188, Itub, JSarek, JSquish,Jamie C, Jason7825, Jawshoeaw, Jayrayspicer, Jcwf, Jeronimo, Jfpchem, Jgro, Jlove88, Jncraton, Jordgette, Jose Ramos, Jrockley, Jtgoesmoo, Keraunoscopia, Kutchkutch, LOL, La goutte depluie, Ledelste, Leifisme, Leszek Jańczuk, Lfh, Lightmouse, Linas, Lpm, MCB, Mani1, Marchije, Mareino, Materialscientist, Mattman723, MaximvsDecimvs, Meisterkoch, Meno25, Mets501,Michael Hardy, Mikemurphy, Mj sklar, Moreschi, Mtk180, My very best wishes, Nanobug, Nergaal, Netheril96, OlEnglish, Oneiros, Ortin19, P.wormer, PAR, Peterlin, Pgan002, Philwkpd,Physchim62, Piano non troppo, PierreAbbat, Piet Delport, Pinethicket, Pjacobi, Plumbago, Plutor, Poszwa, Psymier, Pure Oxygen, PureFire, Qmwne235, Rannyfash, Ray Eston Smith Jr, Reuqr,Rhetth, Richard Giuly, Richard W.M. Jones, Rjwilmsi, Rnash9, Rob Hooft, RuM, Ruakh, Ruislick0, S19991002, SHCarter, Sarregouset, SchreiberBike, Sergius-eu, Shehzadizeb, Shootbamboo,Shrommer, Silas S. Brown, Slicky, Snalwibma, Somoza, SpK, Spud Gun, Stan J Klimas, Steve Quinn, Stevertigo, Stismail, Sun Creator, Syp, Tangsl, Tatantyler, Tbhotch, TheLewisRepublic,Thecurran91, Thetimbo2000, Thingg, Thomaslau, Tobias Bergemann, TotoBaggins, Tr00st, Trefalcon, Trovatore, Twerbrou, Tynetrekker, Univirauniviracontenta, Urthen, V8rik, Van helsing,Vsmith, Wakebrdkid, Wavelength, Weierstrass, Widr, Wikeditor21, WikiBone, Willemhenskens, Williadb, Winderful1, Xhienne, Xoxclairexox, Yinchongding, Ylem, Ymblanter, Zac23, Zomno,340 anonymous edits

Image Sources, Licenses and ContributorsFile:Gecko on My Window 2 (17729540).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gecko_on_My_Window_2_(17729540).jpg  License: Creative Commons Attribution 2.0 Contributors: Steve Evans from Citizen of the WorldFile:Dipole-dipole-interaction-in-HCl-2D.png  Source: http://en.wikipedia.org/w/index.php?title=File:Dipole-dipole-interaction-in-HCl-2D.png  License: Public Domain  Contributors:Benjah-bmm27, Inductiveload, Odder, Wickey-nlImage:Gecko Leaftail 1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gecko_Leaftail_1.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Lpm,Quibik

LicenseCreative Commons Attribution-Share Alike 3.0//creativecommons.org/licenses/by-sa/3.0/