The van der Waals force was named after a Dutch scientist Johannes Diderik van der Waals (1837-1923). This is considered as the first type of intermolecular forces between atom and molecules. Like ionic or covalent bonds, these attraction forces do not result from a chemical bond as they are comparatively weak and hence can be easily disturbed. These van der Waals forces vanish very quickly as the distance between the interacting molecules increases.
The van der Waals forces play a fundamental role in different fields of sciences i.e. structural biology, supramolecular chemistry, nanotechnology, condensed matter physics, polymer science, and surface science. Many basic properties of organic and molecular solids underlie in van der Waals forces which also include solubility in polar and non-polar solutions.
Van der Waals forces are defined as:
“A short-range repulsive or attractive intermolecular forces between atoms, molecules, and surfaces that exist in both gas and liquid phase”.
As the atoms approach each other the distance between atoms at which the forces become repulsive other than attractive when no other force is present is called van der Waals contact distance.
This phenomenon is the result of mutual repulsion between the atoms. The origin point of van der Waals forces is the same as that of the Casimir effect that arises from quantum interactions with the field of zero point.
Types of van der Waals forces:
There are three types of van der Waals forces including:
- Dispersion (weak)
- Dipole-dipole (medium)
- Hydrogen (strong)
Dispersion which is also known as London dispersion forces is considered as the weakest type of intermolecular force. They were named as London dispersion forces after Fritz London (1900-1954) who was the first scientist considered that proposed the existence of these forces in 1930. London dispersion forces are the intermolecular forces which appear between all types of molecules either ionic or covalent – polar or non-polar when electrons start moving.
If the molecules are polar, it means that the electrons present are concentrated at one end and the molecules are partially negatively charged at the other end. This negative end makes the molecules present in the surrounding causing a diploe effect that attracts the surrounding molecules having positive ends. This process is called the London Dispersion Force of attraction.
Bromine, Br2 from a group of halogens, has more electrons than the chlorine, Cl2. So the bromine will have stronger London dispersion forces than chlorine which results in a high boiling point for bromine i.e. 59 ᵒC as compared to chlorine i.e. -35 ᵒC. The London dispersion forces does not require much energy for its break down which explains the reason why the nonpolar covalent compounds like methane, oxygen, and nitrogen that only have dispersion forces between molecules freezes at very low temperature.
Dipole-dipole forces are the type of attractive forces that occur between polar molecules. When the partially positive charge molecule interacts with the partially negative charged part of the neighboring molecule. The condition for this type of attraction to take place in the presence of partially charged ions.
In the case of polar covalent bonds e.g. Hydrogen Chloride, HCl, the molecules will align themselves in such a pattern that the oppositely charged molecules are close to each other.
This is an important kind of dipole-dipole interaction that takes place specifically with hydrogen atom which is bonded to nitrogen, fluorine or oxygen atom. The partially positive end of the hydrogen atom is attracted to the partially negative end of the nitrogen, oxygen or fluorine of another molecule. A strong force of attraction exists between the molecules of hydrogen bonding and a large amount of energy is required to break this hydrogen bond. This tells us the reason for very high boiling and melting points of compounds like water, H2O and hydrogen fluoride, HF. Hydrogen bonding also plays a very important role in biology as it holds nucleotide bases together in RNA and DNA.
The relative strength of intermolecular forces of attraction:
|Intermolecular force||Occurrence between||Relative strength|
|Dipole-dipole attraction||Partially oppositely charged ions||Medium|
|London dispersion forces||Temporary or induced dipoles||Weakest|
|Hydrogen bonding||H, O, N or F atom||strongest|
Van der Waals Equation:
When we need to calculate an actual value of non-ideal gases, van der Waals equation is required
The equation says:
V represents the volume of gas in moles, n.
a represents a specific value of a particular gas
P refers to measured pressure (expected to be low in most cases)
b refers to the eliminated volume per mole
R refers to a known constant having a value of 0.08206 L atm mol-1 K-1
T represents temperature
Properties of van der Waals forces
Van der Waals forces contain the following properties:
- These forces are additive.
- These forces are weaker as compare to ionic or covalent chemical bonds.
- These are non-directional forces
- These forces are present over a very short distance. There is a good interaction if molecules present are closer.
- This type of force is independent of temperature but with an exception of dipole-dipole interactions.
Components of van der Waals forces:
As these forces are the weakest form of forces, their strength generally ranges from 0.4 to 4kj/mol and within a distance range of less than 0.6nm. the electron clouds will repel each other if the distance is less than 0.4nm and the net effect of the forces will become repulsive.
Van der Waal forces contribute to four major points which are as follows:
- There is a negative component present that stops the molecules from collapsing. This is due to the Pauli exclusion principle.
- There is a repulsive or attractive electrostatic interaction that takes place between permanent charges, dipoles, multipoles, and quadrupoles. This interaction is known as Keesom interaction or Keesom force which is named for Willem Hendrik Keesom.
- Here induction or polarization occurs. There is an attractive force between a permanent polarity on one molecule or induced polarity on another. This interaction is known as the Debye force for Peter J.W. Debye.
- London dispersion force is an attraction between any two molecules due to the presence of instantaneous polarization. This force is named after a scientist named Fritz London. There also exist London dispersion forces even in nonpolar molecules.
Note: As the number of electrons increases, the size of the induced and oscillating dipoles, the size of the attractive forces between the molecules and the size of van der Waals forces also increases.
Melting and Boiling points of Halogens:
The elements present in the halogen group are nonpolar diatomic molecules and the boiling point comparison is given below:
Melting and Boiling points of Halogen Group
|Molecule||Total number of electrons||Melting Point (ᵒC)||Boiling Point (ᵒC)||Physical State at Room Temperature|
Iodine has the largest number of electrons, so the dispersion forces are strongest in it. Similarly, the relative stronger forces of the halogen group result in the highest melting and boiling points. The forces in the Iodine are strong enough that holds the iodine molecules close together in a solid-state at room temperature. The dispersion forces are permissively weaker for fluorine, bromine, and chlorine and this can be explained by their low melting and boiling points. Bromine is liquid at room temperature while chlorine and fluorine are gases because their molecules are far away from each other. Intermolecular forces do not exist in the gas state so the dispersion forces in chlorine and fluorine become measurable only when the temperature decreases and they condense into a liquid state.
Dependence of Van Der Waals Forces:
Van der Waals forces depend on the molecular surface area. For example, the boiling points of pentane and hexane are 36ᵒC and 69 ᵒC respectively. The van der Waal forces are strong in hexane than in pentane the surface area of hexane is larger than pentane to interact with the neighbouring molecules. The stronger intermolecular attraction results in holding the molecules together more tightly hence the vapour pressure decreases and results in giving a higher boiling point than pentane.
Van der Waals forces also depend on the molecular shape. For example, 2,2-dimethylpropane (neopentane) has a low boiling point than pentane. In shape, neopentane is more spherical than that of pentane. Hence it occupies less surface area than the more cylindrical pentane molecule. As a result, the van der Waals forces are smaller in neopentane and the boiling point is low.
Generally, the boiling points of isomeric alkanes depend on their shapes. Molecules which are tightly compact having small surface area show more good results. A small surface area shows less contact among adjacent molecules and smaller van der Waals forces. while considering any group of isomeric alkanes, the isomers having more branches has the lowest boiling point than the normal alkane which will have the highest boiling point.