The London dispersal force is a form of Van Der Waals force. Van Der Waals forces are relatively weak electrical forces that allow neutral molecules to attract each other, whether they are in gaseous, liquid or solid form.
The London dispersion force is the weakest of the Van Der Waals forces. It is a temporary force of attraction that occurs between electrons that are placed in a specific way. The electrons must come from two adjacent atoms that form temporary dipoles. It is these forces of attraction that cause non-polar substances to condense into a liquid and solidify when the temperature is low enough.
Before going any further, it is important to remember that a molecule is a group of chemically bound atoms. For example, two hydrogen atoms covalently bonded would be a hydrogen molecule.
Understanding the Basics of the London dispersion force
Also known as the induced dipole force, this attractive force is caused by the constant motion of electrons. This motion can allow an atom or molecule to develop a temporary dipole when its electrons are asymmetrically distributed.
Prenons l’exemple ci-dessous. Voici deux atomes neutres qui dans des endroits séparés.
Lorsque ces deux mêmes atomes se rapprochent, les électrons d’un sont attirés par les protons de l’autre. Au même moment, les électrons des deux atomes se repoussent. L’illustration ci-dessous démontre de façon simplifiée ce principe.
This phenomenon represents the formation of a temporary dipole and the force of attraction that maintains this dipole is called the London dispersion force.
The London dispersion force is influenced by several aspects. In particular, the contact surface and the polarization capacity of the atoms
- Note that the polarization of atoms changes according to their size, the number of electrons and their valence.
The adsorption of carbon
In summary, carbon adsorbs different molecules thanks to its London dispersion force. Let us recall that the two aspects that have an impact on the strength of this force are the polarization capacity and the contact surface. Since carbon has 6 electrons, its polarization is high. Moreover, when the carbon is transformed into activated carbon, its contact surface is increased by almost 1000%. This is one of the reasons why activated carbon is such a good adsorbent medium.
Furthermore, it should be remembered that the dipoles created by the London dispersion force are temporary, which is why activated carbon is in theory reusable. On the other hand, the reuse of activated carbon is not always financially advantageous.