Types of Van der Waals Forces

1. Keesom Interactions

Keesom interactions can arise due to the following interactions (all of which are electrostatic in nature):

• The electrostatic interaction between the charges in ionic molecules.
• Interaction between dipoles in polar molecules.
• Quadrupole interactions in the molecules whose symmetry is lower than cubic.
• Interaction between permanent multipoles.

These forces are named after the Dutch physicist Willem Hendrik Keesom. It is important to note that Keesom interactions only originate from the interactions between two permanent dipoles and are temperature dependent.

2. Debye Forces

Debye forces are caused by the interactions between permanent dipoles and other atoms/molecules, which results in the formation of induced dipoles. For example, an induced dipole can be formed from the repulsive forces between electrons (belonging to a molecule) and a permanent dipole.

Unlike Keesom interactions, Debye forces are not dependent on temperature. These forces are named after the Dutch-American physical chemist Peter Debye.

3. London Dispersion Forces

London dispersion forces arise due to the interactions between an instantaneous dipole and an atom/molecule. These forces are named after the German physicist Fritz London and are also known as instantaneous dipole – induced dipole forces.

These forces are believed to be the weakest of all Van der Waals forces. The strength of the London dispersion force between two atoms/molecules depends entirely on the polarizability of the atom/molecule.

Factors Affecting Van der Waals Forces

1. Number of Electrons Held by the Atoms/Molecules

While traversing down a group in the modern periodic table, the atomic radii of the elements increase along with the number of electrons held by their respective nuclei. The presence of a relatively large number of electrons (along with the additional space for these electrons to disperse over) contributes to the formation of temporary dipoles. The greater the number of (instantaneous) dipoles formed, the greater the strength of the Van der Waals force.

An example of this relationship can be observed in the significantly different boiling points of xenon and neon – the boiling point of xenon is -108^oC whereas the boiling point of neon is -246^oC. The lower boiling point of xenon can be explained by the stronger dispersion forces experienced by its atoms.

2. Shape of the Molecule

Long, unbranched molecules tend to feature stronger dispersion forces than branched, short-chain molecules. For example, the structural isomers butane and isobutane (2-methyl propane) have different boiling points despite having the same chemical formulae. The boiling point of butane is -0.5^oC and that of isobutane is -11.7^oC.

The difference in the boiling points of these isomers can be accounted for by the stronger Van der Waals forces in the unbranched butane molecules (and the weaker Van der Waals forces in the short, branched isobutane molecules).

Applications of Van der Waals Forces

• It is widely believed that Geckos exploit Van der Waals forces hanging on to smooth surfaces with only their toes.
• The attractive forces that arise between the spatulae of the Gecko’s footpads and the smooth surface enable the lizard to effectively climb these surfaces. Similar biological designs can be observed in some spiders.