Editing Intermolecular force (section)

==Van der Waals forces==
{{Main|van der Waals force}}

The van der Waals forces arise from interaction between uncharged atoms or molecules, leading not only to such phenomena as the cohesion of condensed phases and physical absorption of gases, but also to a universal force of attraction between macroscopic bodies.<ref name=j1>{{cite journal | vauthors = Leite FL, Bueno CC, Da Róz AL, Ziemath EC, Oliveira ON | title = Theoretical models for surface forces and adhesion and their measurement using atomic force microscopy | journal = International Journal of Molecular Sciences | volume = 13 | issue = 10 | pages = 12773–12856 | date = October 2012 | pmid = 23202925 | pmc = 3497299 | doi = 10.3390/ijms131012773 | doi-access = free }}</ref>

===Keesom force (permanent dipole – permanent dipole) {{Anchor|Keesom force}}===
The first contribution to van der Waals forces is due to electrostatic interactions between rotating permanent dipoles, quadrupoles (all molecules with symmetry lower than cubic), and  multipoles.  It is termed the ''Keesom interaction'', named after [[Willem Hendrik Keesom]].<ref>{{cite journal| vauthors = Keesom WH |title=The second virial coefficient for rigid spherical molecules whose mutual attraction is equivalent to that of a quadruplet placed at its center |url=http://www.dwc.knaw.nl/DL/publications/PU00012540.pdf |journal= Proceedings of the Royal Netherlands Academy of Arts and Sciences |year=1915 |volume= 18 |pages= 636–646}}</ref> These forces originate from the attraction between permanent dipoles (dipolar molecules) and are temperature dependent.<ref name=j1/>

They consist of attractive interactions between dipoles that are [[canonical ensemble|ensemble]] averaged over different rotational orientations of the dipoles. It is assumed that the molecules are constantly rotating and never get locked into place. This is a good assumption, but at some point molecules do get locked into place. The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance. The Keesom interaction can only occur among molecules that possess permanent dipole moments, i.e., two polar molecules. Also Keesom interactions are very weak van der Waals interactions and do not occur in aqueous solutions that contain electrolytes. The angle averaged interaction is given by the following equation:

:<math>\frac{-d_1^2 d_2^2}{24\pi^2 \varepsilon_0^2 \varepsilon_r^2 k_\text{B} T r^6} = V,</math>

where ''d'' = electric dipole moment, <math>\varepsilon_0</math> = permittivity of free space, <math>\varepsilon_r</math> = dielectric constant of surrounding material, ''T'' = temperature, <math>k_\text{B}</math> = Boltzmann constant, and ''r'' = distance between molecules.

===Debye force (permanent dipoles–induced dipoles) {{Anchor|Debye force}}===
The second contribution is the induction (also termed polarization) or Debye force, arising from interactions between rotating permanent dipoles and from the polarizability of atoms and molecules (induced dipoles). These induced dipoles occur when one molecule with a permanent dipole repels another molecule's electrons. A molecule with permanent dipole can induce a dipole in a similar neighboring molecule and cause mutual attraction. Debye forces cannot occur between atoms. The forces between induced and permanent dipoles are not as temperature dependent as Keesom interactions because the induced dipole is free to shift and rotate around the polar molecule. The Debye induction effects and Keesom orientation effects are termed polar interactions.<ref name=j1/>

The induced dipole forces appear from the induction (also termed [[Dipolar polarization|polarization]]), which is the attractive interaction between a permanent multipole on one molecule with an induced (by the former di/multi-pole) 31 on another.<ref name=Blustin-1978>{{Cite journal |doi= 10.1007/BF00577166 |title= A Floating Gaussian Orbital calculation on argon hydrochloride (Ar·HCl) |journal= Theoretica Chimica Acta |volume= 47 |issue= 3 |pages= 249–257 |year= 1978 | vauthors = Blustin PH |s2cid= 93104668 }}</ref><ref name=Roberts-Orr-1938>{{Cite journal |doi= 10.1039/TF9383401346 |title= Induced dipoles and the heat of adsorption of argon on ionic crystals |journal= Transactions of the Faraday Society |volume= 34 |pages= 1346 |year= 1938 | vauthors = Roberts JK, Orr WJ }}</ref><ref name=Sapse-et-al-1979>{{Cite journal |doi= 10.1038/278332a0 |title= Ion-induced dipole H−n clusters |journal= Nature |volume= 278 |issue= 5702 |pages= 332–333 |year= 1979 | vauthors = Sapse AM, Rayez-Meaume MT, Rayez JC, Massa LJ |bibcode= 1979Natur.278..332S|s2cid= 4304250 }}</ref> This interaction is called the ''Debye force'', named after [[Peter J. W. Debye]].

One example of an induction interaction between permanent dipole and induced dipole is the interaction between HCl and Ar. In this system, Ar experiences a dipole as its electrons are attracted (to the H side of HCl) or repelled (from the Cl side) by HCl.<ref name=Blustin-1978/><ref name=Roberts-Orr-1938/> The angle averaged interaction is given by the following equation:

:<math>\frac{-d_1^2 \alpha_2}{16\pi^2 \varepsilon_0^2 \varepsilon_r^2 r^6} = V,</math>

where <math>\alpha_2</math> = polarizability.

This kind of interaction can be expected between any polar molecule and non-polar/symmetrical molecule. The induction-interaction force is far weaker than dipole–dipole interaction, but stronger than the [[London dispersion force]].

===London dispersion force (fluctuating dipole–induced dipole interaction)===
{{Main|London dispersion force}}

The third and dominant contribution is the dispersion or London force (fluctuating dipole–induced dipole), which arises due to the non-zero instantaneous dipole moments of all atoms and molecules. Such polarization can be induced either by a polar molecule or by the repulsion of negatively charged electron clouds in non-polar molecules. Thus, London interactions are caused by random fluctuations of electron density in an electron cloud.  An atom with a large number of electrons will have a greater associated London force than an atom with fewer electrons. The dispersion (London) force is the most important component because all materials are polarizable, whereas Keesom and Debye forces require permanent dipoles. The London interaction is universal and is present in atom-atom interactions as well. For various reasons, London interactions (dispersion) have been considered relevant for interactions between macroscopic bodies in condensed systems. [[Hamaker theory|Hamaker]] developed the theory of van der Waals between macroscopic bodies in 1937 and showed that the additivity of these interactions renders them considerably more long-range.<ref name=j1/>