On April 20, an explosion occurred on the Deepwater Horizon drilling rig, in the Gulf of Mexico.
The resulting oil leak, which lasted for more than three months until the well was finally capped on August 4, discharged an enormous quantity of oil into the gulf, the environmental consequences of which have yet to be fully determined.
Had the oil and water been able to mix, then instead of the Louisiana, Mississippi and Alabama beaches being coated with a thick dark goo, they would have remained relatively pristine.
So why don't oil and water mix? And is there any way we can get them to mix?
In order to answer these questions, we need to consider the molecular structures of both water and oil.
Water is made up of three atoms, one oxygen and two hydrogen, bonded together in a bent arrangement with the oxygen atom in the middle.
Although the water molecule is overall electrically neutral, the fact that it is bent gives the molecule what is called a dipole - this is an uneven distribution of electrical charge within the molecule such that part of the molecule (the oxygen atom) is slightly negative and part of the molecule (the two hydrogen atoms) is slightly positive.
Because of this charge imbalance, water molecules tend to attract neighbouring water molecules strongly, with the positive part of one molecule coming into close contact with the negative part of a neighbouring molecule.
Any molecule which exhibits a dipole is said to be polar, and we generally find that liquids which comprise polar molecules tend to mix.
On the other hand, the molecules in oil tend to be long chains composed of carbon and hydrogen atoms.
Such molecules, while again overall electrically neutral, usually have negligible dipoles, meaning that there is an even distribution of charge within each molecule, and they are therefore non-polar.
Even though they are non-polar, they still have attractive forces between the molecules, although these are much weaker than those in water.
Non-polar liquids tend to mix only with other non-polar liquids. And therein lies the problem. Water is polar, oil is non-polar, and therefore they do not mix.
The attractive forces between water molecules are so great that they essentially preclude non-polar oil molecules from coming between them.
Yet every day we force oil and water to mix whenever we use soap, shampoo, dishwashing liquid and the like. This is because such products contain molecules called surfactants (short for surface active agents).
These are generally long chain molecules derived from oil, which have a charge at one end (the head), with the rest of the molecule (the tail) being non-polar.
The charged head, being polar, prefers to be situated in water, while the non-polar tail will orient preferentially so that it is immersed in a non-polar environment.
When we add detergent to a mixture of oil and water, small spherical aggregates of surfactant molecules called micelles are formed, in which the non-polar tails point to the inside of the sphere, and the polar heads are situated on the surface of the sphere, in contact with the water.
Because the inside of the micelle is non-polar, it can dissolve oil, and this essentially makes the oil soluble in water.
You can bet that there were more than a few tonnes of surfactants dropped on the oil slick in the Gulf of Mexico, and while they may have helped disperse the oil, they won't have broken it down. Only time will tell what real damage this oil spill has caused.
All of this brings to mind a really bad joke which my first-year students never fail to groan at. If you dropped a white bear and a brown bear into water, which one would dissolve first? The white one, because its polar.
- Allan Blackman is an associate professor in the chemistry department at the University of Otago.
