There are several types of van der Waals attraction:
- Instantaneous-induced-dipole/Induced-dipole (the London force)
It is tempting to consider these forces to be of different strengths, but it is the distance range that is more important. Dipole/dipole attraction is relatively long range in action while the London spontaneous-dipole/Induced-dipole attraction requires contact between the van der Waals surfaces: the molecules need to touch.
Molecules with permanent dipole moments, polar molecules, such as iodine chloride, ICl, exhibit dipole-dipole attraction. The iodine end of iodine chloride is δ+ and the chlorine end is δ–. Molecules interact with each other so that the dipoles line up end-to-end:
All molecules with a permanent dipole exhibit permanent-dipole/permanent-dipole attraction. At temperatures below the material’s melting point, the structure will show long range order and crystallinity.
Permanent-Dipole/Induced-Dipole Attraction :
Molecular dipoles (polar molecules) are able to induce weak dipoles in adjacent non-polar species. The effect gives rise to a weaker attraction than dipole-dipole attraction.
London Dispersion Force (LDF) Attraction:
The very fact that it is possible to liquefy helium – and indeed all molecular materials – demonstrates that there must be some type of inter-molecular attraction taking place between the helium atoms. (Helium is a molecular material, where the helium molecule consists of just one atom.)
The attraction is known as the London dispersion force and is deemed to arise from short time scale fluctuations in the electronic structure of species which results in the formation of instantaneous dipoles.
The instantaneous-induced-dipole/induced-dipole London dispersion forces (LDF) are surprisingly strong but they only act at very short range. It is as if the surface of even neutral, non-polar molecules like methane, CH4, are 'sticky'.
Soccer Balls Covered in Velcro
Imagine a room filled with 50 soccer balls or so covered in Velcro and half a dozen four year old children.
The kids will kick the balls about, and the balls will fly around. But as the children become tired the balls will slow down and stick together.
So it is with a molecular gas. As the temperature is lowered the molecules will stick to each other via London dispersion forces, instantaneous-induced-dipole/Induced-dipole attractions, to give a condensed phase.
All molecules exhibit London dispersion forces and the strength increases with the size/surface area of the molecule. This logic can be used to explains the increasing boiling and sublimation temperatures of the halogens.
Going down the periodic table the atoms become larger, so the diatomic molecules become larger and their surface area becomes larger. Thus, the van der Waals forces increase and so do the boiling points:
F2 < Cl2 < Br2 < I2
Likewise, longer chain alkanes have higher boiling points than shorter chain alkanes. Branching, which decreases surface area, reduces boiling point.
Which is stronger: dipole/dipole or London forces?
Consider the molecular halogen bromine, Br2, and the interhalogen iodine chloride, ICl.
Both have a mass of close to 160, both are are 70 electron systems, but Br2 is non-polar and ICl is polar. Yet they have rather similar boiling points of 59 ° and 97° respectively:
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This implies that the dipole/dipole attraction makes only a minor contribution to the net attraction, and the most of the molecular stickiness is due to the the London dispersion force.
Gecko Toes, Setae and van der Waals Forces:
"The toes of the gecko have developed a special adaptation that allows them to adhere to most surfaces. Recent studies of the spatula tipped setae on gecko footpads demonstrate that the attractive forces that hold geckos to surfaces are van der Waals interactions between the finely divided setae and the surfaces themselves. Every square millimeter of a gecko's footpad contains about 14,000 hair-like setae." Wikipedia: