Why Don’t Water and Oil Mix? Part 2

We know then water and oil don’t mix. We are considering the reasons for this in terms of their various chemistries.

Water, as the have seen, is polar – that is, there is an uneven distribution of charge, and so water molecules like attaching themselves to other water molecules so that they can balance their charges.

Let’s now look at oil.  Oil is organic in nature.  By organic, we mean carbon-based.  Broadly speaking, all chemistry is split up into organic chemistry and inorganic chemistry.  Carbon atoms have the ability to form themselves into long chains, and these long chains are what make up all living organisms – this is called carbon chemistry, organic chemistry, or simply life chemistry.

It is quite funny really – one entire branch of chemistry is devoted to one element in the periodic table – carbon – whereas the other major branch of chemistry, inorganic chemistry, is devoted to the other hundred and seven elements.

So carbon has this incredible ability to link itself together in long chains, rather like Lego.  In just the same way that we can take a box of Lego bricks, and put them together into all sorts of shapes and configurations, carbon atoms can be linked together to form an infinite number of molecules, these molecules are the amino acids, the carbohydrates, the proteins that make up our bodies and make up life.  They also the chemistry of oils and fats.  So it’s look at them now.

Let’s consider the simplest organic molecule that there is: methane.

Methane consists of a single carbon atom, with four hydrogens attached to it in the shape of a tetrahedron:

because the molecule contains no electron withdrawing atoms, like oxygen, there is no uneven distribution of charge.

But of course methane is a gas.  Let’s consider a simple organic molecule such as octane:

We can see immediately that there is no imbalance of charge on this molecule.  There is therefore no requirement for it to mix with anything to balance a charge.

Now consider what happens if we mix the octane with the water. If you are a water molecule and you bump into an octane molecule, you’re not going to get along.  The reason is that as a water molecule, you are going to be looking for other water molecules so you can balance your charges.

So if you mix water and octane together all the water molecules will run around until they bump into each other and link up with each other so that the positive and negative charges can be balanced out.  That’s why we get to separate layers – the water molecules want as little interaction with the oil as possible because they want as much interaction with each other.

Or to put it another way, they want to minimise the surface area of the interaction between the water and the oil.

We could put it this way – the octane is a pretty easy-going molecule, it has no problem with water. it has no particular agendas or issues.  But the water – well – ensure handsome agendas and issues.  It needs to balance its charges. So the water has a problem with the octane because you can’t help it balance its charges.

And this affinity or hatred actually expresses itself in chemical terms such as hydrophilic (water loving), hydrophobic (water hating), lipophilic (oil loving), and lipophobic (oil hating).

So we can see now why oil and water don’t mix.  Tomorrow, we’ll look at some molecules that kind of fall between these two categories – that is, they are partially hydrophilic and partially hydrophobic, and by their nature these molecules make excellent cleaning compounds.

And then will go on look at the wonderful world of surfactants (detergents) – how they work and how they clean your dishes and your clothes.  Stay tuned



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