Immiscible liquids and emulsions

Jenny and Michael are mixing oil and vinegar to make salad dressing. The two liquids form two different layers. They are immiscible liquids. The layers are clearly visible - the salad dressing is a heterogeneous mixture. What if you shake the mixture?

The oil splits into small droplets that are dispersed in the vinegar. But after a while it separates into two layers again. Is there a way to make the oil and vinegar mix better? There is! Let's add some egg yolk, and stir it really well.

The mixture of oil, vinegar, and egg yolk combine into a smooth cream. They have formed an emulsion. What's happened here? In the egg yolk, there are substances that can be dissolved both in oil and in vinegar. One such substance is Lecithin.

The molecules of this substance have two ends: ... one that is soluble in water - and in vinegar. ... and another that is soluble in oil and other fats. This makes the molecule able to act like a bridge between the two. Keeping the oil dispersed in the vinegar.

A substance that can make two immiscible liquids combine is called an emulsifier. But it's not salad dressing anymore. Instead they have made - mayonnaise! Hey, Michael, if you're trying to remove oil from your hands, it's not enough just to use water. You need something that can dissolve oil, but that can also be dissolved in water.

Yes, you can actually use egg yolk to wash your hands, but maybe it's simpler to use - soap? Just like in the egg yolk, there are molecules in soap with a fat soluble side, and a water soluble side. The fat soluble side of the molecule dissolves the oil on your hands when you lather up. And the water soluble side means that the soap is rinsed away with the water. That means soap is also a kind of emulsifier.

But the mayonnaise will probably taste better with egg yolk.