Oil and water: Why do we value oil?

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Patricia Ash explains why we take the risk of drilling in the first place.

By: Dr Patricia Ash (Department of Life, Health & Chemical Sciences)

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Dr Patricia Ash

Because oil is a mix of hydrocarbons and of varying molecular sizes it’s a huge resource for us.  We can’t actually do without it now.  We rely on it to make plastics, to make manmade fibres which we use to make clothes, furnishings, curtains and carpets; so many different things.  We also use it to manufacture plastics, toys, furniture, you name it we need oil to do it, so that’s why oil is so valuable and of course one of the prime uses is to make fuel, make all sorts of fuel.  Because oil is a mixture of hydrocarbons they are relatively easy to separate out and there are these huge plants which carry out what’s known as cracking the oil which means breaking it up into all the different sizes of hydrocarbons, so you can get the fuels such as petrol and kerosene out of it relatively easily, so that’s why we have all this stuff around.

Different oil wells produce different profiles of oil.  So, what you get out of one oil well won’t necessarily be the same as what you get out of another.  So, any oil spill includes an eclectic mix of hydrocarbons that will never be exactly the same as an oil spill that has already been experienced.

Crude oil as it’s extracted from an oil well, it consists of various compounds known as hydrocarbons.  Now what hydrocarbons are, they are made up mainly of carbon atoms and hydrogen atoms, but they might also contain other components such as sulphur, oxygen, or even chlorine, but the main thing to get from this is that they are what’s known as hydrophobic.  Hydrophobic is a term that means fear of water.  Essentially it means that oil and water don’t mix, and things such as common salt they’re hydrophilic, they like mixing in water, they’ll dissolve in water, but oil, it’s much less dense than water so it tends to float to the top.  So, if you mix oil and water you end up getting a layer of oil over the water and if you’ve ever made salad dressing, French dressing out of oil and vinegar you’ll see those two layers. 

So, why does that happen?  Well, it’s because the mix of hydrocarbons in oil, they all have one thing in common, they have particular structure of the molecule that doesn’t allow them to actually interact comfortably with the fairly highly charged atoms in water.  That’s why they’re known as hydrophobic because they can’t mix with water.

If you shake an oil/water mix you will temporarily cause the phenomenon known as emulsification so the oil will break up into loads and loads of microscopic little droplets and you can see it all spread out through the water but then gradually those droplets will all coalesce together because they really love to be together, they’re desperate to find more hydrophobic oil droplets to combine with and the oil will all rise to the top again.  So, when there’s a massive oil spill at sea exactly the same thing happens, the oil will be floating on the sea surface.

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Oil and water

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