Have you ever wondered how soap works? No? Just me then…
Every now and then, an everyday phenomenon pops into my head and makes me think: Crivens. How does that actually work?
This happened with soap the other day. I was lying in bed, reading about water, which made me think of washing up, which led to soap cleaning greasy things. We all know that oil is insoluble in water, so how does the washing up get clean?
Mickey Mouse is integral here. No, really
First of all, you need to know a little something about water. (Water is fantastic stuff; really odd. I mean, really odd. But more about that another time.) Your common or garden H2O is a polar substance. That means it has an uneven distribution of electric charge.
In a nutshell, a water molecule comprises one oxygen atom (with six outer electrons) and two hydrogen atoms (each with one electron). Each hydrogen atom shares a pair of electrons with the oxygen atom, bonding them together in a shape strangely reminiscent of Mickey Mouse’s silhouette. (This is called covalent bonding.)
This molecular shape is important, because it gives water its polarity. The hydrogen atoms, on one side of the molecule, have a positive charge (the proton). While the oxygen atoms’ electrons cluster on the other side of the molecule, giving that side of the molecule a negative charge.
On why John Travolta’s hair is afraid of water
Next, you need to know why some substances dissolve in water, and others don’t. Substances that dissolve easily in water are generally polar substances. Like water, they have an uneven distribution of electric charge. So the positively-charged areas of the solute molecules are attracted to the negatively-charged areas of the water molecules. In this way, the solute molecules are distributed throughout water. These polar molecules are hydrophilic (water loving).
However, grease and oil are non-polar substances that do not dissolve in water. As anyone who has had greasy pans to deal with in a campsite will attest, water on its own will not get the cleaning job done. Water and oil do not mix.
The oil can be said to be hydrophobic (water fearing), and consequently clusters together into globules – or into a whole layer, as shown here. The oil molecules all want to be as far from the water as possible. I like to compare this with penguins during a snow storm: there is an outer layer of penguins around the main group, working a shift system. They are cold-phobic, so each spends a little time at the outer edge being cold, but the majority of their time is spent in the warm huddle.
Similarly with insoluble substances in water. To make them mix requires an emulsifier. Soap is an excellent emulsifier, capable of dispersing a liquid such as oil into another immiscible liquid, such as water. But how does it work?
Tadpole tails that despise water
A soap molecule has a comparable structure to that of a tadpole. It has a hydrophilic head (the polar end, consisting of – for example – oxygen and sodium) and a hydrophobic tail (a non-polar hydrocarbon chain).
The soap molecules work by embedding their hydrophobic tails into the (also hydrophobic) oil globules. Their hydrophilic heads, however, continue to interact with the water molecules, allowing the oil to be dispersed throughout the water.
But more than that, the soap molecules cluster together, presenting their polar heads to the water and hiding their non-polar tails together, forming a structure known as a micelle. Within these clusters are contained the oil and dirt that you want to clean off.
Just FYI, cell membranes work in a similar way. They are made primarily of phospholipids (more tadpole-shaped molecules), which have a polar head and a non-polar tail. Side-by-side they sit, with their heads and tails aligned, positioned back-to-back in double layers. So the hydrophobic tails are shielded from the water between the sheets – and these hydrophobic interactions hold the cell membranes together.
Making washing up more interesting (no need to thank me)
Next time you’re faced with a pile of washing up, instead of bemoaning the fact that it is a dull task, have a think about just how the mechanisms involved in cleaning your cutlery work. It’s fascinating stuff, even if you’re not a science nerd. Honest.