Van der Waals Forces

When a species has a dipole, it has a partial positive (δ+) and a partial negative (δ-) end. These poles act like those of a magnet and will attract opposite charges while repelling like ones.

Van der Waals forces are essentially the pushing/pulling caused by these polar molecules coming close to each other.

Types of Van der Waals Forces

There are three types of Van der Waals forces that you need to be familiar with:

1. Dipole-dipole interactions
2. Dipole-induced dipole interactions
3. London dispersion forces (Instantaneous dipole-induced dipole) these forces are classified by whether the two molecules/atoms interacting have a permanent or temporary dipole, which we will discuss later.

Now, let's talk about each of these Van der Waals forces in detail.

Dipole-Dipole Interactions

The strongest of the Van der Waals forces are dipole-dipole interactions.

The way we determine whether a molecule is polar or not is based on electronegativity.

Electronegativity increases the further to the top right (fluorine) an element is on the periodic table, it decreases the further you get to the bottom right (francium). The difference in electronegativity is what determines bond polarity. If two atoms have a difference >0.5 in electronegativity, then the bond is polar. However, if a molecule is symmetrical, the molecule itself will be non-polar since the polarities will cancel themselves out.

Let's look at HF as an example:

Fig.1 HF is a polar molecule with a permanent dipole.

Fluorine is much more electronegative than hydrogen (3.98 > 2.2), so the electron density is pulled towards it. Because of this, the fluorine side of the molecule is partially negative (δ-). This also means that hydrogen's end is partially positive (δ+) since it is lacking electron density.

Molecules like HF participate in dipole-dipole interactions because of their dipole. Below is what these interactions look like:

Fig.2-Dipole-Dipole attraction and repulsion

The partial positive end is attracted to the partial negative end since it is lacking electron density, while the partial negative end has an excess of it. However, poles with the same charge will repel one another.

In addition, polar molecules can be at any orientation and still experience these forces. Molecules can be on top of each other or facing diagonally, and these forces will still occur as long as the molecules are in close enough proximity.

It's helpful to think of these molecules like magnets. If you hold a magnet above another magnet (opposite ends parallel), the magnets will be attracted to each other and possibly snap together.

Dipole-Induced Dipole Interactions

Now, let's talk about the second type of Van der Waals forces: dipole-induced dipole interactions.

Below is a diagram showing how an induced dipole is formed:

Fig. 3-A polar molecule induces a dipole in a neutral atom

In a non-polar molecule, electrons are spread evenly, however, as the electrons are moving around the electron cloud, they can randomly be spread out unevenly as shown below.

Fig.4 A neutral species forms an instantaneous dipole

Unlike induced dipoles, which only exist while a polar molecule is present, instantaneous dipoles both appear and disappear at random and on their own.While they are weak, an instantaneous dipole can induce dipoles. This is why non-polar molecules have some forces between them, despite being non-polar. London dispersion forces are the interaction between the instantaneous dipole and the induced dipole, as shown below.

Fig.5 The interaction between the instantaneous dipole and the induced dipole.

Since both species were originally non-polar, London dispersion forces are the weakest of the Van der Waals forces.

Factors Affecting Intermolecular Van der Waals forces

Van der Waals forces are affected by three things:

1. Number of electrons

   • The greater number of electrons, the more likely instantaneous dipoles will form.

     • Ex: Argon (18 electrons) has a lower boiling point than xenon (54 electrons), which means argon has weaker forces

2. Shape of the molecule

   • Long, unbranched molecules have stronger forces than short, branched ones.

     • Ex: Isobutane has a lower boiling point than butane, since isobutane is branched, while butane isn't

3. Distance

   • The farther apart molecules/atoms are, the weaker their interactions

     • Coulomb's law states that the force between two charges weakens the farther they are apart

       • $$F=k\frac{q_1q_2}{r^2} \text{where k is a constant, q₁ and q₂ are different charges and r is the distance between them}$$

While London dispersion forces are not affected by temperature, dipole-dipole interactions are.

Importance of Van der Waals forces

Van der Waals forces are important for several reasons. Here are just some examples:

• They impact the properties of various organic compounds and molecular solids.

• They are why non-polar molecules/atoms can become solids and liquids

• They are used in several fields such as structural biology and polymer science

• They stabilize protein structures

Just based on this small list alone, you can see why these forces are so important!

Van der Waals Forces Examples

As I just mentioned, Van der Waals forces are very important, and we can see evidence of that in our daily lives.

Geckos and spiders have small hair-like bristles on their feet which they use to stick to smooth surfaces and even be upside-down! This is because of the van der Waals forces of attraction between these bristles and the surfaces they are walking on. Scientists have tried to replicate this phenomenon by making “Geckskin” which also uses these forces. While tests are still ongoing, we may one day have a real-life Spider-Man!

As another example, Van der Waals forces are what keep the “rungs” of the DNA ladder together. Without these forces, DNA wouldn't have the necessary stability and would be prone to falling apart, which would be a big deal for us!