Editing Van der Waals force (section)

==Definition==
Van der Waals forces include attraction and repulsions between [[Atom|atoms]], [[Molecule|molecules]], as well as other [[Intermolecular force|intermolecular forces]]. They differ from [[covalent bond|covalent]] and [[ionic bond|ionic]] bonding in that they are caused by correlations in the fluctuating [[Polarizability|polarizations]] of nearby particles (a consequence of [[quantum dynamics]]<ref name=Abrikosov>
{{Cite book|first1=A. A. |last1=Abrikosov |first2=L. P. |last2=Gorkov |first3=I. E. |last3=Dzyaloshinsky |title=Methods of Quantum Field Theory in Statistical Physics |year=1963–1975 |publisher=[[Dover Publications]] |isbn=978-0-486-63228-5 |chapter=6: Electromagnetic Radiation in an Absorbing Medium}}</ref>).

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&nbsp;[[Nanometre|nm]] and 0.5&nbsp;nm, depending on the atomic-specific diameter.<ref>{{Cite journal |last=Batsanov |first=S. S. |date=2001 |title=Van der Waals Radii of Elements |url=http://link.springer.com/10.1023/A:1011625728803 |journal=Inorganic Materials |volume=37 |issue=9 |pages=871–885 |doi=10.1023/A:1011625728803|s2cid=52088903 }}</ref> When the interatomic distance is greater than 1.0&nbsp;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''<sup>−7</sup>).<ref>{{Cite book |last1=Hirschfelder |first1=Joseph O. |url=https://www.worldcat.org/oclc/534717 |title=Molecular theory of gases and liquids |date=1954 |publisher=Wiley |first2=Charles F. |last2=Curtiss |first3=R. Byron |last3=Bird |isbn=0-471-40065-3 |location=New York |oclc=534717}}</ref>

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 H<sub>2</sub> molecules equals 0.06 kJ/mol (0.6 meV) and the pairwise attractive interaction energy between O ([[oxygen]]) atoms in different O<sub>2</sub> molecules equals 0.44 kJ/mol (4.6 meV).<ref>{{Cite journal |last1=Wang |first1=Shiyi |last2=Hou |first2=Kaiyi |last3=Heinz |first3=Hendrik |date=2021-08-10 |title=Accurate and Compatible Force Fields for Molecular Oxygen, Nitrogen, and Hydrogen to Simulate Gases, Electrolytes, and Heterogeneous Interfaces |journal=Journal of Chemical Theory and Computation |language=en |volume=17 |issue=8 |pages=5198–5213 |doi=10.1021/acs.jctc.0c01132 |pmid=34255965 |s2cid=235823673 |issn=1549-9618|doi-access=free }}</ref> The corresponding vaporization energies of H<sub>2</sub> and O<sub>2</sub> 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 bond|covalent bonds]]).

The strength of van der Waals bonds increases with higher [[polarizability]] of the participating atoms.<ref>{{Cite journal |last1=Heinz |first1=Hendrik |last2=Lin |first2=Tzu-Jen |last3=Kishore Mishra |first3=Ratan |last4=Emami |first4=Fateme S. |date=2013-02-12 |title=Thermodynamically Consistent Force Fields for the Assembly of Inorganic, Organic, and Biological Nanostructures: The INTERFACE Force Field |url=https://pubs.acs.org/doi/10.1021/la3038846 |journal=Langmuir |language=en |volume=29 |issue=6 |pages=1754–1765 |doi=10.1021/la3038846 |pmid=23276161 |issn=0743-7463}}</ref> For example, the pairwise van der Waals interaction energy for more polarizable atoms such as S ([[sulfur]]) atoms in H<sub>2</sub>S and [[Sulfide|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).<ref>{{Cite journal |last=Halgren |first=Thomas A. |date=September 1992 |title=The representation of van der Waals (vdW) interactions in molecular mechanics force fields: potential form, combination rules, and vdW parameters |url=https://pubs.acs.org/doi/abs/10.1021/ja00046a032 |journal=Journal of the American Chemical Society |language=en |volume=114 |issue=20 |pages=7827–7843 |doi=10.1021/ja00046a032 |bibcode=1992JAChS.114.7827H |issn=0002-7863}}</ref> These van der Waals interactions are up to 40 times stronger than in H<sub>2</sub>, 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 [[Fermi gas|free electron gas]].<ref>{{Cite journal |last1=Heinz |first1=Hendrik |last2=Vaia |first2=R. A. |last3=Farmer |first3=B. L. |last4=Naik |first4=R. R. |date=2008-11-06 |title=Accurate Simulation of Surfaces and Interfaces of Face-Centered Cubic Metals Using 12−6 and 9−6 Lennard-Jones Potentials |url=https://pubs.acs.org/doi/10.1021/jp801931d |journal=The Journal of Physical Chemistry C |language=en |volume=112 |issue=44 |pages=17281–17290 |doi=10.1021/jp801931d |issn=1932-7447}}</ref> 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 ===
More broadly, [[intermolecular forces]] have several possible contributions. They are ordered from strongest to weakest:
# A repulsive component resulting from the [[Pauli exclusion principle]] that prevents close contact of atoms, or the collapse of molecules.
# Attractive or repulsive [[electrostatic]] interactions between permanent charges (in the case of molecular ions), dipoles (in the case of molecules without inversion centre), [[quadrupole]]s (all molecules with symmetry lower than cubic), and in general between permanent [[multipole]]s. These interactions also include [[hydrogen bond]]s, [[Cation–π interaction|cation-pi]], and [[Pi-Stacking (chemistry)|pi-stacking]] interactions. Orientation-averaged contributions from electrostatic interactions are sometimes called the [[Keesom force|Keesom interaction]] or Keesom force after [[Willem Hendrik Keesom]].
# Induction (also known as [[polarizability|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.
# Dispersion (usually named [[London dispersion force|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).<ref>{{cite web|url=https://www.ntmdt-si.com/resources/spm-theory/theoretical-background-of-spm/2-scanning-force-microscopy-(sfm)/22-cantilever-sample-force-interaction/224-the-van-der-waals-force/2241-intermolecular-van-der-waals-force|title=Intermolecular Van der Waals force|publisher=NT-MDT|accessdate=2024-05-30}}</ref> 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.<ref>{{cite journal|first1=Jianmin|last1=Tao|first2=John|last2=Perdew|first3=Adrienn|last3=Ruzsinszky|title=Long range Van der Waals interaction|url=https://www.sas.upenn.edu/~jianmint/Publications/Papers/413180.pdf|journal=International Journal of Modern Physics B|volume=27|year=2013|issue=18 |pages=1330011–1330032 |doi=10.1142/S0217979213300119 |bibcode=2013IJMPB..2730011T }}</ref>

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 molecule]]s.<!-- most use "Van der Waals molecules" ([http://pubs.acs.org/doi/abs/10.1021/cr00088a008], [http://www.wesleyan.edu/chem/faculty/novick/vdw.html], [http://doi.wiley.com/10.1002/anie.197204861], [https://books.google.com/books?id=PP6vtMJlWowC&pg=PA220]) few use "van" ([https://archive.today/20130202065320/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TFN-4CNJCXP-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1113973534&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=31e5934c994d877beafb7c335e513529], [http://www.infography.com/content/718688290900.html] (just a search aggregator)) --> The London–van der Waals forces<!-- see [http://www.clarkson.edu/projects/crcd/me437/downloads/5_vanderWaals.pdf] and [http://linkinghub.elsevier.com/retrieve/pii/S1672251507601560] --> 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 [[Evgeny Mikhailovich Lifshitz|E. M. Lifshitz]].<ref>{{cite web |url=https://www.sciencedaily.com/releases/2007/08/070806091137.htm |work=Science Daily |title=New way to levitate objects discovered |date=August 6, 2007}}</ref><ref>{{cite journal |doi=10.1088/1367-2630/9/8/254|title=Quantum levitation by left-handed metamaterials|year=2007|last1=Leonhardt|first1=Ulf|last2=Philbin|first2=Thomas G.|s2cid=463815|journal=New Journal of Physics|volume=9|issue=8|page=254|arxiv=quant-ph/0608115|bibcode=2007NJPh....9..254L}}</ref> A more general theory of van der Waals forces has also been developed.<ref>{{cite journal |last1=Dzyaloshinskii |first1=I. E. |last2=Lifshitz |first2=E. M. |last3=Pitaevskii |first3=Lev P. |title= General theory of van der Waals forces |journal= Soviet Physics Uspekhi|volume= 4|page= 153|year= 1961|doi= 10.1070/PU1961v004n02ABEH003330|bibcode=1961SvPhU...4..153D|issue= 2}}</ref><ref>{{cite journal |last1=Zheng |first1=Y. |last2=Narayanaswamy |first2=A. |s2cid=64619547 |title=Lifshitz Theory of van der Waals Pressure in Dissipative Media |journal=Physical Review A |volume=83 |issue=4 |page=042504 |year=2011|doi=10.1103/PhysRevA.83.042504 |arxiv=1011.5433 |bibcode=2011PhRvA..83d2504Z }}</ref>

The main characteristics of van der Waals forces are:<ref name="Sethi 1992">{{cite book | last1=Sethi | first1=M. S. | last2=Satake | first2=M. | title=Chemical bonding | publisher=Discovery Publishing House | publication-place=New Delhi | year=1992 | isbn=978-81-7141-163-4 | oclc=912437861 | page=}}</ref>

* 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 [[biomolecule|biological molecules]].