van der Waals forces

Van der Waals forces is the collective name given to the intermolecular interactions responsible for the weak attractions between neutral chemical species such as atoms and molecules. They are relatively weak forces with a very short range and that are constituted by the sum of three different types of forces that may or may not be present simultaneously. These three forces are the Keesom forces, the Debye forces, and the London dispersal forces.

Although they are much weaker interactions than the binding forces present in ionic, metallic, and covalent bonds, they can become considerable when the molecules involved are large enough.

Van der Waals forces are responsible for the ability of the gecko and arthropods to climb very smooth surfaces such as glass and ceramics.

They are also responsible for the adhesion forces between different surfaces and the tape, as well as other sticky substances. In fact, adhesive tape exists thanks to van der Waals forces. These forces are strong enough at close range to hold together the pieces we want to join (like the lids of a cardboard box, for example), but at the same time they are weak enough that we can easily detach them.

Characteristics of van der Waals forces

• Like all interactions between atoms and molecules, van der Waals forces are electrostatic in origin.
• They are very short-range forces, which means that they are only significant when the molecules are very close to each other and disappear rapidly as they get further apart.
• When two molecules get closer, below a certain minimum distance, the van der Waals forces become repulsive. This ensures that atoms and molecules do not collapse into each other.
• They are weak forces compared to ionic and covalent bonds. This is because attractive forces exist between small partial charges, some of which only exist for very short periods of time.
• Some of the components of the van der Waals forces do not have directionality. This means that two molecules that are close enough will always feel an attractive force towards each other regardless of their orientation relative to each other.
• They are additive, which, combined with their lack of directionality, means that they can become considerably intense if the contact surface between two molecules is large enough.
• All components of the van der Waals forces except the Keesom forces are independent of temperature.
• They can occur between any atom or molecule regardless of its structure or composition.

Components of Van der Waals forces

Van der Waals forces correspond to the sum of three different types of attractive forces. Some of these components are always present regardless of the atoms or molecules in question, while others only occur in the cases of polar molecules. These three components are:

Keesom forces or dipole-dipole interactions

Of the three components of the Van der Waals forces, the most intense interactions are those that come from the attraction between the opposite poles of polar molecules, that is, those that have a permanent dipole. These types of forces or interactions between two permanent dipoles are called Keesom forces, after the Dutch physicist Willem Hendrik Keesom who studied them at the beginning of the 20th century.

In these cases, the partial positive charge (δ+) of the dipole of a polar molecule is attracted (and vice versa) by the partial negative charge (δ-) of the dipole of a second, also polar molecule. These molecules can be the same as each other or not.

Keesom forces are primarily responsible for the solubility of polar substances in polar solvents. Also, for obvious reasons, they only occur between polar molecules.

Debye forces or induced dipole-dipole interactions

When a molecule that has a permanent dipole (a polar molecule) approaches a neutral molecule that is nonpolar, or approaches the nonpolar part of an amphipathic molecule (which has a polar head and a nonpolar tail), the partial charge of the dipole will attract electrons from the surface of the second molecule (if it is partially positive) or repel them (if it is partially negative). The effect is that the distribution of electrons on its surface will be distorted in the nonpolar molecule, inducing the formation of a small dipole. This induced dipole is then attracted to the dipole of the polar molecule.

These types of interactions between a permanent dipole and an induced dipole are called Debye forces and correspond to the second component in intensity to the van derWaals forces.

London dispersion forces or induced dipole-induced dipole interactions

In those cases in which a molecule does not have any permanent dipole moment or in the cases of neutral atoms which cannot have dipoles, there is still the possibility that an attractive force called the London dispersion force appears, named after to Fritz London who characterized it in 1930.

In this case, the attraction is between tiny instantaneous dipoles that appear and disappear on the surface of all atoms and molecules as a consequence of the fact that electrons are particles that cannot be everywhere at the same time. Because of its constant motion, there are times when there are more electrons on one side of an atom or molecule than on the other. This non-uniform distribution of electrical charges gives rise to a small dipole that disappears as soon as the electrons, which never stay still, return to the other side of the molecule.

Their short duration causes them to be called instantaneous dipoles, and they appear and disappear with surprising frequency on the surface of absolutely all chemical substances, be they molecules, atoms, or ions. Whenever two molecules approach each other, there will be attractive forces between the instantaneous dipoles of one molecule with those of the other. When one of these dipoles disappears, another appears on the other side, and there will always be a certain number of attracting dipoles in both molecules at any given moment.

London forces are the only intermolecular interactions present in nonpolar compounds and, furthermore, they are the weakest component of all van der Waals forces. However, the greater the contact surface between two molecules, the greater the number of instantaneous dipoles attracting them to each other, so the London forces can become considerable in the cases of apolar macromolecules such as the polymers that form the plastics.

Examples of van der Waals forces

• Dipole-dipole interactions between two water molecules.
• The adhesive strength of the packing tape.
• When condensing noble gases such as argon or krypton, the forces that hold the atoms together are London dispersion forces.
• Dipole-dipole interactions induced between a methanol molecule and the aliphatic tail of a triglyceride.
• The induced dipole-dipole forces that occur between water molecules (which is polar) and oxygen gas molecules (which are nonpolar) when this gas dissolves in water.
• In the case of plastics such as polyethylene , the London forces that occur between the long nonpolar chains of groups –CH ₂ -.
• The adhesion of the gecko’s pads to polished surfaces such as glass.
• The forces that hold together the molecules of bromine (Br ₂ ) in the liquid state and iodine (I ₂ ) in the solid state at room temperature.

References

Heltzel, Carl E. (October 2020). How Sticky Innovations Changed the World. ChemMatters. Retrieved from https://www.acs.org/content/dam/acsorg/education/resources/highschool/chemmatters/issues/2020-2021/october-2020/sticky-chemistry-pages.pdf

R. Moreno, E. Bannier (2015). 3- Feedstock suspensions and solutions. In Future Development of Thermal Spray Coatings, Editor(s): Nuria Espallargas. 51-80. Woodhead Publishing. Retrieved from https://www.sciencedirect.com/science/article/pii/B9780857097699000038

Adaira, JH, Suvacib, E., Sindela, J. (2001) Surface and Colloid Chemistry. In Encyclopedia of Materials: Science and Technology. 1-10. Elsevier. Retrieved from https://www.sciencedirect.com/science/article/pii/B0080431526016223

Van der Waals forces. (nd). Retrieved from https://e1.portalacademico.cch.unam.mx/alumno/quimica1/unidad2/tiposdeenlaces/vanderwaals

EcuRed. (nd). Van der Waals forces – EcuRed. Retrieved from https://www.ecured.cu/Fuerzas_de_Van_der_Waals