Why Don’t Water and Oil Mix? Part 3 – alcohols

We have seen that water and oil don’t mix because of their fundamentally different properties.

Water, because it contains an exposed oxygen atom, is polar, and hydrocarbons (molecules containing only carbon and hydrogen) are nonpolar. They therefore don’t mix, and the liquids when mixed together will separate into two layers.

But here’s a question – are there molecules that are partially non-polar and polar, that may be able to interact with both types of molecule?

Yes, there are – but only a very small number.  To fall into this category, the structure of the molecule must be such that neither the polar nor the nonpolar part of the molecule dominate the behaviour.  For this to be the case, the molecule must have an exposed electronegative atom (typically oxygen) and a short hydrocarbon chain.  If the hydrocarbon chain is too long, it will dominate the behaviour of the molecule, and the molecule will behave as a nonpolar molecule.

There are essentially only two molecules that we can put in this category (apart from surfactants, which are a special case that we will talk about later).

Let’s look at acetone and isopropanol:

Acetone looks like this:

this is an abbreviated form of a molecule, that kind of looks like a man with no arms.  The full structure looks like this:

 

Now let’s look at isopropanol in its abbreviated form:

They are remarkably similar, aren’t they?

Each of them has a hydrocarbon chain of three carbons, and each of them has an oxygen coming off the central carbon. The difference is that the acetone has an exposed (ketone) oxygen, whereas the isopropanol has an oxygen attached to a hydrogen (which makes it an alcohol).

The combination of the single polar group, with a short hydrocarbon chain, means both of these molecules will interact with both hydrocarbons and water.  For this reason, they are both very common solvents in laboratories across the world – mainly used for rinsing glassware, as they will dissolve what ever is on the glassware, whether lipophilic or hydrophilic.  That is, they will dissolve both oils and water from the glassware.  Acetone is particularly useful, as it is also very volatile, and evaporates very quickly, so the glassware can be cleaned and dried very easily.

Both of these solvents find their way into the marketplace.  Acetone is nail polish remover, and isopropanol is “rubbing alcohol”.

In terms of the chemical properties, acetone is the more aggressive of the two.  It will chemically attack many plastics and paints, which is why it is used as nail polish remover – nail polish is essentially an acrylic paint.

Isopropanol, on the other hand, is far more useful.  It is simply the best solvent in existence for general-purpose cleaning.  As the name “rubbing alcohol” suggests, it can be used for rubbing excess oil off your skin, as a skin cleanser.  Because it is miscible with water, it is also an excellent general-purpose solvent for all other forms of cleaning.  Used on your kitchen benchtop for example, it will successfully wipe up whatever might happen to be on there, with a water-based or oil based.

For some reason, the isopropanol has not found its way into any kitchen products unlike “vanilla fresh” which is an alcohol based cleaner (ethanol) which will simply not work as well on oil based things as the isopropanol will.

So go and get yourself a bottle of “rubbing alcohol”.  Whether it’s cleaning a kitchen benchtop, leather seats, or even texta or ink, it will do a good job of removing it– is a remarkable chemical, and the second-best general-purpose cleaning compound in existence.

 

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