I recently got glasses. I really like them! It's nice to see long distances again. When you get glasses, they come with a cleaning cloth. The cloth is a fancy material called microfiber. You rub it on the lenses to pick up any dirt or smudges. But how does it not scratch the lens? You might assume you're pushing the crud around. But microfiber takes advantage of chemistry! The cloth's fibers attract the dirt. It lifts it from the lens! Let's learn more about this unique chemistry.
What are Van der Waal's Forces?
We've spent some time learning about bonding. Let's do a quick review. Both ionic and covalent bonds have a strong electrostatic attraction between atoms. Ionic bonds are the result of electron transfer creating ions. The ions have opposite charges. So, they attract. In covalent bonds, both nuclei attract the valence electrons. The octets are both filled by sharing electrons. Covalent molecules have more variety. Why? Because the elements have different electronegativities. So, the electron cloud shifts to one side of the molecule. What's the result? A polar molecule! One side is partially positive, while the other side is partly negative.
So, we understand chemical bonding. It will serve as our foundation for understanding a new set of forces. My glasses' cleaning cloth takes advantage of a remarkable collection of forces. Together we call them Van der Waal's forces. Van der Waal's forces is a catch-all name for intermolecular forces. Those are the electrostatic interactions that occur between molecules. It's different from bonding. Bonding is the electrostatic attraction that holds atoms together. They're the intramolecular forces or forces within molecules.
Have you ever wondered why water is liquid at room temperature? There are a lot of molecules that are gases. For example, oxygen is a diatomic molecule. But it's a gas. There are also a lot of materials that are solids. It depends on how the molecules interact. Intermolecular forces are much weaker than chemical bonds. They can't hold atoms together. But, they are responsible for a molecule's physical properties.
History of Van der Waal's Forces
Before we examine Van der Waal's forces, let's take a quick excursion through history. Johannes Diderik Van der Waal is a famous scientist. He did most of his scientific work in the late 1800s. During that time, chemists didn't know if molecules existed. In fact, many famous scientists argued that they did not. What was the consequence? They thought the various phases of matter were unique chemicals. For example, liquid water was a different chemical than water vapor. Van der Waal didn't accept this! He began to study the phases of matter. Eventually, he developed Van der Waal's equation. It's a variation of the ideal gas law that applies to real gases. His equation models that the gas and liquid phases merge into one another. His experiments provided massive support for the existence of molecules. In fact, he won the Nobel prize in 1910!
Van der Waal was never able to explain why gases condense into liquids. Why? Because we hadn't developed quantum mechanics. It wasn't until the 1930s that we modeled how electrons behave in atoms and molecules. Chemists realized there would be attractive forces between molecules. They began to classify the different intermolecular forces. In honor of Van der Waal's contribution, the whole collection got named after him. Now, let's look at the different types of intermolecular forces.
Dispersion Forces
I want you to imagine a noble gas. It's got a full octet. So, it's happy. But what are its electrons doing? Remember, because of the wave-particle duality, we can think of them as waves. That means their location is based on probability. We can't know its location with exact certainty. What's the likelihood that the electrons are evenly spaced around the atom? It's actually remarkably improbable! Let's imagine a neon atom. Let's say six of ten electrons are on one side. What happens? The protons in the nucleus are uniformly spaced. So, on one side, the protons outnumber the electrons. On the other side, the electrons outnumber the protons. We've created a dipole! We formed the dipole because of the electrons' dispersion. So, creatively we call them dispersion forces.
Let's imagine we line up two neon atoms. Could their dipoles align? Yes! If they do align, they attract one another. But remember, the electrons are constantly moving. So one nanosecond, the dipoles align. The next, they don't. That's why dispersion forces are the weakest intermolecular force.
What molecules experience dispersion forces? Technically, all molecules. But, it's most prominent for neutral atoms and nonpolar molecules. Why? Because the only dipole they experience is from dispersion forces. So, noble gases and diatomic elements are common examples. Dispersion forces have more impact at low temperatures. Why? A lower temperature equals less kinetic energy. So, the molecules aren't moving very fast. The attraction caused by the dispersion has more impact. That's why the boiling point of oxygen is −183oC. To be a liquid, it has to be unbelievably cold! That's the temperature where the dispersion forces overcome the kinetic energy.
Dipole Forces
We've studied polar molecules. So, we know some molecules have a dipole. What's the cause? One atom is more electronegative than the other. So, it pulls the electrons toward it. The electrons are more likely to be on that side of the molecule. So, it has a partial negative charge. That means the other side has a partial positive. The greater the electronegativity difference, the stronger the dipole.
What's an example? Water! The oxygen is far more electronegative than hydrogen. So, it hogs the electrons. In fact, the electronegativity difference is very prominent. So, it's an example of hydrogen bonding. Hydrogen bonding is a dipole created between hydrogen and fluorine, oxygen, or nitrogen. It's a little more substantial than a regular dipole. Why? The electronegativity differences are more pronounced.
Water has a bond between oxygen and hydrogen. So, water hydrogen bonds. What does that look like? The hydrogens of one molecule attract the oxygen of another. In water, it's particularly pronounced because of oxygen's lone pairs. The interaction is strong. That's why water has a much higher boiling point. You have to add a lot of heat to overcome the hydrogen bonds.
What are some non-hydrogen bonding polar molecules? You'll encounter a lot of examples in organic chemistry. For example, carbon bonded with nitrogen, oxygen, or halogens all form dipoles. Carbon gains a partial positive charge. In fact, it even elicits some chemical reactions!
Induction Forces
I want you to imagine placing a neutral atom next to an ion. Let's say we move neon toward a sodium ion. What happens to neon's electrons. They're going to be attracted to sodium's positive charge. They'll be pulled toward one side of the neon atom. What's the result? We've induced a dipole on neon. This can also happen when a polar molecule approaches a neutral atom. But, since polar molecules have partial charges, the effect is softer.
Microfiber Cloth
So, how do microfiber cloths work? They take advantage of dipole forces. Most of them are made of polyester. Polyester is a polymer. That's a unique material that has repeating units. So, it has repeating ester functional groups. If you take organic chemistry, you'll learn that an ester has a carbon-oxygen double bond. That's a polar bond! So, the cloth has lots of polar bonds that can attract grime. What if the dirt is nonpolar? It induces a dipole. So, the fibers attract anything on my glasses. It picked up without scratching the lens.
