How do emulsions keep oil and water mixed?

1. A jar of homemade salad dressing has two layers. Golden oil floats on top and watery vinegar sits underneath. Why do they not mix?
2. Look very, very close. Water molecules pull strongly on each other and hold together. An oil molecule cannot break in between them, so the water squeezes the oil out.
3. Shaking breaks the oil into droplets and spreads them through the vinegar. But whenever two droplets touch, they join into a bigger one. Big droplets float up fast, and soon the oil is back on top.
4. Now a spoon of mustard goes in. Mustard contains emulsifiers. An emulsifier molecule has two different ends: a head that loves water and a tail that loves oil.
5. Shake again. The emulsifier molecules gather around each droplet, with their tails in the oil and their heads in the water. They make a coat. Coated droplets cannot join up: they just bounce apart.
6. Now the dressing stays mixed and looks creamy. This kind of mixture is called an emulsion. Mayonnaise is one too: its emulsifier comes from egg yolk.

This is a creamy dressing, magnified many thousands of times. Tap each part to see what it does.

• Water. The watery part of the dressing: vinegar or lemon juice. Its molecules hold tightly to each other, which is why they push oil out.
• Oil droplet. A droplet of oil. It is round because a ball has the smallest surface, and oil tries to touch as little water as it can.
• Emulsifier molecule. One emulsifier molecule. Mustard and egg yolk both contain emulsifiers. Each molecule has two ends that behave very differently.
• Water-loving head. The head of the molecule is attracted to water, so it always faces outward into the watery part.
• Oil-loving tail. The tail of the molecule is attracted to oil, so it buries itself in the droplet.
• The coat. Thousands of emulsifier molecules crowd around the droplet, tails in and heads out. Together they make a coat that covers the oil.
• Bounce, not join. When two coated droplets bump, their coats touch but their oil does not. They bounce apart instead of joining, so the dressing stays mixed.

Shake both jars and watch the model droplets. The right jar has mustard. What happens in each jar after a few seconds? Try less mustard, and more shakes.

Experiment: You shake two jars of oil and vinegar. One of them has a spoonful of mustard in it. What will they look like a few seconds later?

1. Press Shake both jars twice.
2. Watch both jars for about five seconds.
3. Change the mustard to A little, shake again and compare.

What we found: The plain jar separates within seconds, because bare droplets join up whenever they touch and big droplets float up fast. In the mustard jar every droplet has a coat of emulsifier, so the droplets bounce apart and the dressing stays mixed. With too little mustard, some droplets join up until there is enough to coat the rest.

Turn on JavaScript to try it yourself.

Make a creamy dressing: small droplets that are still mixed when the 8-second timer ends. Tap ingredients to add them, shake the jar, then start the timer.

Oil and water do not mix. Water molecules pull strongly on each other and stick together, a bit like tiny magnets. Oil molecules do not have that pull, so the water squeezes them out. Oil is also lighter than water, so it ends up floating on top.

If you shake oil and water, the oil breaks into droplets. For a moment the liquid looks cloudy and mixed. But every time two droplets touch, they join into a bigger droplet. Bigger droplets float upward faster, so in a few seconds the oil is back on top.

An emulsifier changes this. Its molecules have two different ends: a head that is attracted to water and a tail that is attracted to oil. They crowd around each droplet with their tails in the oil and their heads in the water. Now each droplet wears a coat. When coated droplets bump into each other, they bounce apart. A mixture of droplets that stays mixed is called an emulsion.

Mustard and egg yolk are kitchen emulsifiers. Mayonnaise is oil, lemon juice and egg yolk whisked into an emulsion so thick you can spread it. Milk is an emulsion of tiny drops of fat in water. Soap is an emulsifier too: it coats grease so water can wash it away. The friend-holding-hands comparison only goes so far: a real droplet is surrounded by a huge number of emulsifier molecules, not just one.

Key words

Emulsion

Tiny droplets of one liquid spread through another liquid that it does not normally mix with.

Emulsifier

A helper whose molecules hold oil with one end and water with the other.

Droplet

A very small drop of liquid.

Hydrophilic

Attracted to water. The word means water-loving.

Hydrophobic

Not attracted to water. The word means water-fearing. Oil is hydrophobic.

Coalescence

Two droplets merging into one bigger droplet.

Check your thinking

1. Why does the oil float back to the top after you shake oil and vinegar?

   Show the answer

   The droplets join up, and oil is lighter than water. Bare droplets merge when they touch. Big droplets of light oil float up quickly.

2. What is special about an emulsifier molecule?

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   One end holds on to water and the other end holds on to oil. Its two different ends let it sit at the surface of a droplet and coat it.

3. Which of these is an emulsion?

   Show the answer

   Mayonnaise. Mayonnaise is tiny oil droplets spread through lemon juice, held by emulsifier from egg yolk.

4. You shake a dressing very hard, but you added only a tiny bit of mustard. Why might it still separate?

   Show the answer

   There is not enough mustard to coat all the small droplets. Lots of small droplets have a lot of surface. Droplets without a coat join up again.

5. Soap helps water wash grease off a plate. Which explanation fits what you have learned?

   Show the answer

   Soap is an emulsifier: its molecules coat grease droplets so water can carry them away. Soap molecules have an oil-loving tail and a water-loving head, exactly like the emulsifiers in mustard and egg yolk.