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Van der Waals force

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In molecular physics and chemistry, the van der Waals force (sometimes van de Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond;[2] they are comparatively weak and therefore more susceptible to disturbance. The van der Waals force quickly vanishes at longer distances between interacting molecules.

Rainwater flux from a canopy. Among the forces that govern drop formation: Van der Waals force, surface tension, cohesion, Plateau–Rayleigh instability.
Microfiber cloth makes use of van der Waals force to remove dirt without scratches.[1]

Named after Dutch physicist Johannes Diderik van der Waals, the van der Waals force plays a fundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. It also underlies many properties of organic compounds and molecular solids, including their solubility in polar and non-polar media.

If no other force is present, the distance between atoms at which the force becomes repulsive rather than attractive as the atoms approach one another is called the van der Waals contact distance; this phenomenon results from the mutual repulsion between the atoms' electron clouds.[3]

The van der Waals forces[4] are usually described as a combination of the London dispersion forces between "instantaneously induced dipoles",[5] Debye forces between permanent dipoles and induced dipoles, and the Keesom force between permanent molecular dipoles whose rotational orientations are dynamically averaged over time.

Definition

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Van der Waals forces include attraction and repulsions between atoms, molecules, as well as other intermolecular forces. 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[6]).

The force results from a transient shift in electron density. Specifically, the electron density may temporarily shift to be greater on one side of the nucleus. This shift generates a transient charge which a nearby atom can be attracted to or repelled by. The force is repulsive at very short distances, reaches zero at an equilibrium distance characteristic for each atom, or molecule, and becomes attractive for distances larger than the equilibrium distance. For individual atoms, the equilibrium distance is between 0.3 nm and 0.5 nm, depending on the atomic-specific diameter.[7] When the interatomic distance is greater than 1.0 nm the force is not strong enough to be easily observed as it decreases as a function of distance r approximately with the 7th power (~r−7).[8]

Van der Waals forces are often among the weakest chemical forces. For example, the pairwise attractive van der Waals interaction energy between H (hydrogen) atoms in different H2 molecules equals 0.06 kJ/mol (0.6 meV) and the pairwise attractive interaction energy between O (oxygen) atoms in different O2 molecules equals 0.44 kJ/mol (4.6 meV).[9] The corresponding vaporization energies of H2 and O2 molecular liquids, which result as a sum of all van der Waals interactions per molecule in the molecular liquids, amount to 0.90 kJ/mol (9.3 meV) and 6.82 kJ/mol (70.7 meV), respectively, and thus approximately 15 times the value of the individual pairwise interatomic interactions (excluding covalent bonds).

The strength of van der Waals bonds increases with higher polarizability of the participating atoms.[10] For example, the pairwise van der Waals interaction energy for more polarizable atoms such as S (sulfur) atoms in H2S and sulfides exceeds 1 kJ/mol (10 meV), and the pairwise interaction energy between even larger, more polarizable Xe (xenon) atoms is 2.35 kJ/mol (24.3 meV).[11] These van der Waals interactions are up to 40 times stronger than in H2, which has only one valence electron, and they are still not strong enough to achieve an aggregate state other than gas for Xe under standard conditions. The interactions between atoms in metals can also be effectively described as van der Waals interactions and account for the observed solid aggregate state with bonding strengths comparable to covalent and ionic interactions. The strength of pairwise van der Waals type interactions is on the order of 12 kJ/mol (120 meV) for low-melting Pb (lead) and on the order of 32 kJ/mol (330 meV) for high-melting Pt (platinum), which is about one order of magnitude stronger than in Xe due to the presence of a highly polarizable free electron gas.[12] Accordingly, van der Waals forces can range from weak to strong interactions, and support integral structural loads when multitudes of such interactions are present.

Force Contributions

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More broadly, intermolecular forces have several possible contributions. They are ordered from strongest to weakest:

  1. A repulsive component resulting from the Pauli exclusion principle that prevents close contact of atoms, or 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 centre), quadrupoles (all molecules with symmetry lower than cubic), and in general between permanent multipoles. These interactions also include hydrogen bonds, cation-pi, and pi-stacking interactions. Orientation-averaged contributions from electrostatic interactions are sometimes called the Keesom interaction or Keesom force after Willem Hendrik Keesom.
  3. 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. The interactions (2) and (3) are labelled polar Interactions.[13]
  4. Dispersion (usually named London dispersion interactions after Fritz London), which is the attractive interaction between any pair of molecules, including non-polar atoms, arising from the interactions of instantaneous multipoles.

When to apply the term "van der Waals" force depends on the text. The broadest definitions include all intermolecular forces which are electrostatic in origin, namely (2), (3) and (4).[14] Some authors, whether or not they consider other forces to be of van der Waals type, focus on (3) and (4) as these are the components which act over the longest range.[15]

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. Random thermal motion can disrupt or overcome the electrostatic component of the van der Waals force but the 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.[16][17] A more general theory of van der Waals forces has also been developed.[18][19]

The main characteristics of van der Waals forces are:[20]

  • They are weaker than normal covalent and ionic bonds.
  • The van der Waals forces are additive in nature, consisting of several individual interactions, and cannot be saturated.
  • They have no directional characteristic.
  • They are all short-range forces and hence only interactions between the nearest particles need to be considered (instead of all the particles). Van der Waals attraction is greater if the molecules are closer.
  • Van der Waals forces are independent of temperature except for dipole-dipole interactions.

In low molecular weight alcohols, the hydrogen-bonding properties of their polar hydroxyl group dominate other weaker van der Waals interactions. In higher molecular weight alcohols, the properties of the nonpolar hydrocarbon chain(s) dominate and determine their solubility.

Van der Waals forces are also responsible for the weak hydrogen bond interactions between unpolarized dipoles particularly in acid-base aqueous solution and between biological molecules.

London dispersion force

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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. In and between organic molecules the multitude of contacts can lead to larger contribution of dispersive attraction, particularly in the presence of heteroatoms. London dispersion forces are also known as 'dispersion forces', 'London forces', or 'instantaneous dipole–induced dipole forces'. The strength of London dispersion forces is proportional to the polarizability of the molecule, which in turn depends on the total number of electrons and the area over which they are spread. Hydrocarbons display small dispersive contributions, the presence of heteroatoms lead to increased LD forces as function of their polarizability, e.g. in the sequence RI>RBr>RCl>RF.[21] In absence of solvents weakly polarizable hydrocarbons form crystals due to dispersive forces; their sublimation heat is a measure of the dispersive interaction.

Van der Waals forces between macroscopic objects

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For macroscopic bodies with known volumes and numbers of atoms or molecules per unit volume, the total van der Waals force is often computed based on the "microscopic theory" as the sum over all interacting pairs. It is necessary to integrate over the total volume of the object, which makes the calculation dependent on the objects' shapes. For example, the van der Waals interaction energy between spherical bodies of radii R1 and R2 and with smooth surfaces was approximated in 1937 by Hamaker[22][full citation needed] (using London's famous 1937 equation for the dispersion interaction energy between atoms/molecules[23][full citation needed] 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 (it can 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):  .

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

  (2)

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

  (3)

with the force:

  (4)

The van der Waals forces between objects with other geometries using the Hamaker model have been published in the literature.[24][25][26]

From the expression above, it is seen that the van der Waals force decreases with decreasing size of bodies (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 dry powders (where there are no capillary forces present) even though the force of attraction is smaller in magnitude than it is for larger particles of the same substance. Such powders are said to be cohesive, meaning they are not as easily fluidized or pneumatically conveyed as their more coarse-grained counterparts. Generally, free-flow occurs with 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, or protuberances, that result in a greater total area of contact between two particles or between a particle and a wall, this increases 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 more rigorous approach accounting for these effects, called the "macroscopic theory", was developed by Lifshitz in 1956.[27][full citation needed] Langbein derived a much more cumbersome "exact" expression in 1970 for spherical bodies within the framework of the Lifshitz theory[28][full citation needed] while a simpler macroscopic model approximation had been made by Derjaguin as early as 1934.[29][full citation needed] Expressions for the van der Waals forces for many different geometries using the Lifshitz theory have likewise been published.

Use by geckos and arthropods

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Further information: Arthropod adhesion
 
Gecko climbing a glass surface

The ability of geckos – which can hang on a glass surface using only one toe – to climb on sheer surfaces has been for many years mainly attributed to the van der Waals forces between these surfaces and the spatulae, or microscopic projections, which cover the hair-like setae found on their footpads.[30][31]

There were efforts in 2008 to create a dry glue that exploits the effect,[32] and success was achieved in 2011 to create an adhesive tape on similar grounds[33] (i.e. based on van der Waals forces). In 2011, a paper was published relating the effect to both velcro-like hairs and the presence of lipids in gecko footprints.[34]

A later study suggested that capillary adhesion might play a role,[35] but that hypothesis has been rejected by more recent studies.[36][37][38]

A 2014 study has shown that gecko adhesion to smooth Teflon and polydimethylsiloxane surfaces is mainly determined by electrostatic interaction (caused by contact electrification), not van der Waals or capillary forces.[39]

Among the arthropods, some spiders have similar setae on their scopulae or scopula pads, enabling them to climb or hang upside-down from extremely smooth surfaces such as glass or porcelain.[40][41]

See also

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References

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  1. ^ Woodford, Chris (2 July 2008). "How do microfiber cloths work? | The science of cleaning". Explain that Stuff. Retrieved 11 February 2022.
  2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "van der Waals forces". doi:10.1351/goldbook.V06597
  3. ^ Garrett, Reginald H.; Grisham, Charles M. (2016). Biochemistry (6th ed.). University of Virginia. pp. 12–13.
  4. ^ Tschumper, Gregory S. (20 October 2008). "Reliable Electronic Structure Computations for Weak Noncovalent Interactions in Clusters". In Lipkowitz, Kenny B.; Cundari, Thomas R. (eds.). Reviews in Computational Chemistry. Vol. 26. John Wiley & Sons. pp. 39–90. doi:10.1002/9780470399545.ch2. ISBN 9780470399545.
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  7. ^ Batsanov, S. S. (2001). "Van der Waals Radii of Elements". Inorganic Materials. 37 (9): 871–885. doi:10.1023/A:1011625728803. S2CID 52088903.
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  9. ^ Wang, Shiyi; Hou, Kaiyi; Heinz, Hendrik (10 August 2021). "Accurate and Compatible Force Fields for Molecular Oxygen, Nitrogen, and Hydrogen to Simulate Gases, Electrolytes, and Heterogeneous Interfaces". Journal of Chemical Theory and Computation. 17 (8): 5198–5213. doi:10.1021/acs.jctc.0c01132. ISSN 1549-9618. PMID 34255965. S2CID 235823673.
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  12. ^ Heinz, Hendrik; Vaia, R. A.; Farmer, B. L.; Naik, R. R. (6 November 2008). "Accurate Simulation of Surfaces and Interfaces of Face-Centered Cubic Metals Using 12−6 and 9−6 Lennard-Jones Potentials". The Journal of Physical Chemistry C. 112 (44): 17281–17290. doi:10.1021/jp801931d. ISSN 1932-7447.
  13. ^ Leite, Fabio L.; Bueno, Carolina C.; Da Róz, Alessandra L.; Ziemath, Ervino C.; Oliveira, Osvaldo N. (8 October 2012). "Theoretical Models for Surface Forces and Adhesion and Their Measurement Using Atomic Force Microscopy". International Journal of Molecular Sciences. 13 (10): 12773–12856. doi:10.3390/ijms131012773. ISSN 1422-0067. PMC 3497299. PMID 23202925.
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  16. ^ "New way to levitate objects discovered". Science Daily. 6 August 2007.
  17. ^ Leonhardt, Ulf; Philbin, Thomas G. (2007). "Quantum levitation by left-handed metamaterials". New Journal of Physics. 9 (8): 254. arXiv:quant-ph/0608115. Bibcode:2007NJPh....9..254L. doi:10.1088/1367-2630/9/8/254. S2CID 463815.
  18. ^ Dzyaloshinskii, I. E.; Lifshitz, E. M.; Pitaevskii, Lev P. (1961). "General theory of van der Waals forces". Soviet Physics Uspekhi. 4 (2): 153. Bibcode:1961SvPhU...4..153D. doi:10.1070/PU1961v004n02ABEH003330.
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  22. ^ H. C. Hamaker, Physica, 4(10), 1058–1072 (1937)
  23. ^ London, F. Transactions of the Faraday Society 33, 8–26 (1937)
  24. ^ Tadmor, R. (March 2001). "The London–Van der Waals interaction energy between objects of various geometries". Journal of Physics: Condensed Matter. 13 (9): L195–L202. Bibcode:2001JPCM...13L.195T. doi:10.1088/0953-8984/13/9/101. S2CID 250790137.
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  29. ^ B. V. Derjaguin, Kolloid-Zeitschrift, 69, 155–164 (1934)
  30. ^ Russell, Anthony P.; Higham, Timothy. E. (2009). "A new angle on clinging in geckos: incline, not substrate, triggers the deployment of the adhesive system". Proceedings of the Royal Society B: Biological Sciences. 276 (1673): 3705–3709. doi:10.1098/rspb.2009.0946. ISSN 0962-8452. PMC 2817305. PMID 19656797.
  31. ^ 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. Bibcode:2002PNAS...9912252A. doi:10.1073/pnas.192252799. PMC 129431. PMID 12198184.
  32. ^ Steenhuysen, Julie (8 October 2008). "Gecko-like glue is said to be stickiest yet". Reuters. Retrieved 5 October 2016.
  33. ^ Quick, Darren (6 November 2011). "Biologically inspired adhesive tape can be reused thousands of times". New Atlas. Retrieved 5 October 2016.
  34. ^ Hsu, Ping Yuan; Ge, Liehui; Li, Xiaopeng; Stark, Alyssa Y.; Wesdemiotis, Chrys; Niewiarowski, Peter H.; Dhinojwala, Ali (24 August 2011). "Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy". Journal of the Royal Society Interface. 9 (69): 657–664. doi:10.1098/rsif.2011.0370. ISSN 1742-5689. PMC 3284128. PMID 21865250.
  35. ^ Huber, Gerrit; Mantz, Hubert; Spolenak, Ralph; Mecke, Klaus; Jacobs, Karin; Gorb, Stanislav N.; Arzt, Eduard (2005). "Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements". Proceedings of the National Academy of Sciences. 102 (45): 16293–16296. Bibcode:2005PNAS..10216293H. doi:10.1073/pnas.0506328102. PMC 1283435. PMID 16260737.
  36. ^ Chen, Bin; Gao, Huajian (2010). "An alternative explanation of the effect of humidity in gecko adhesion: stiffness reduction enhances adhesion on a rough surface". International Journal of Applied Mechanics. 2 (1): 1–9. Bibcode:2010IJAM....2....1C. doi:10.1142/s1758825110000433.
  37. ^ Puthoff, Jonathan B.; Prowse, Michael S.; Wilkinson, Matt; Autumn, Kellar (2010). "Changes in materials properties explain the effects of humidity on gecko adhesion". Journal of Experimental Biology. 213 (21): 3699–3704. doi:10.1242/jeb.047654. PMID 20952618.
  38. ^ Prowse, Michael S.; Wilkinson, Matt; Puthoff, Michael; Mayer, George; Autumn, Kellar (February 2011). "Effects of humidity on the mechanical properties of gecko setae". Acta Biomaterialia. 7 (2): 733–738. doi:10.1016/j.actbio.2010.09.036. PMID 20920615.
  39. ^ Izadi, H.; Stewart, K. M. E.; Penlidis, A. (9 July 2014). "Role of contact electrification and electrostatic interactions in gecko adhesion". Journal of the Royal Society Interface. 11 (98): 20140371. doi:10.1098/rsif.2014.0371. PMC 4233685. PMID 25008078. We have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.
  40. ^ Kesel, Antonia B.; Martin, Andrew; Seidl, Tobias (19 April 2004). "Getting a grip on spider attachment: an AFM approach to microstructure adhesion in arthropods". Smart Materials and Structures. 13 (3): 512–518. Bibcode:2004SMaS...13..512K. doi:10.1088/0964-1726/13/3/009. ISSN 0964-1726. S2CID 250841250.
  41. ^ Wolff, Jonas O.; Gorb, Stanislav N. (7 January 2012). "The influence of humidity on the attachment ability of the spider Philodromus dispar (Araneae, Philodromidae)". Proceedings of the Royal Society B. 279 (1726): 139–143. doi:10.1098/rspb.2011.0505. PMC 3223641. PMID 21593034.

Further reading

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