Here is an exercise for you to try at home: Put some dirty dishes in your sink, and fill it with hot water, without adding any detergent. Now place a straw on top of the water (so that it floats). Position yourself in such a way that you can see light reflecting off the surface of the water. You should see a greasy film on the surface of the water. Now, while you are watching, get some dishwashing detergent and add a single drop – just one drop – and watch what happens. When the drop hits the surface of the water, rather than disappearing into the water, you will see it smear instantly across the surface of the water, pushing the oily film out of the way, and it will make the straw move as well.
What just happened? What are detergents and how do they work?
We have seen that water and oil are fundamentally different in their chemistries. Whereas water is polar, oil is nonpolar. Water molecules have an imbalance of charge, and oil molecules do not.
Well, if we know that detergents will allow oil to be dissolved in water, then clearly they are molecules that are able to interact both with water and oil. And the best way to do this, is where you simply have one part of the molecule as polar and the other part of the molecule nonpolar.
Let’s look at a detergent molecule:
So we can see that one in the molecule is hydrophobic (in other words, hates water) and the other end of the molecule is hydrophilic (in other words loves water).
So what happens when you wash the dishes, is that the oil loving part of the molecule looks for other bits of oil in the water, and wrap themselves around them, like this:
You can see that the non-polar part of the molecule is facing inwards, and is wrapped around the oil, and the polar part of the molecule is facing outwards where it can interact with the water. The resulting structure is called a micelle.
So the micelle can happily move through the water because the part that is facing the water is polar so it is perfectly happy, and the nonpolar part is locked away inside the micelle.
The more general term for detergents is surfactants. The word “surfactant” is simply an abbreviation of three words: surface active agent.
They are called surface active agents because they like interfaces. That is, in the case we just looked at, they look for the interface between the oil drop and the water. Because the two ends of the molecule are fundamentally different, it is happiest when one end of the molecule is facing a different environment from the other end of the molecule.
So is the surfactant molecule in your sink will go looking for, and attach itself to, interfaces such as the interface between the water and the oil in the sink, water and cutlery or crockery, water and the sides of the sink, or water and air.
So in a demonstration at the beginning of this post, the surfactant molecule will smear across the surface because they are more stable on the surface – the interface between the water and the air. The reason is that the water has a lot of tension because of the unbalanced charges.
In other words whereas water loves to stick to the water molecules to balance its charge, the water molecules right on the surface have drawn the short straw, because there’s only air above them and has nothing to balance the charge.
Surfactant molecules fulfil this need quite readily. The polar part of the molecule balances of charge and the nonpolar part sticks up into the air, where no balancing the charge is required.
Surfactants play a huge role in many household cleaning formulations and products, and will look at them in subsequent posts.