Oils do not dissolve in water. If oil and water are shaken together, tiny droplets of one liquid spread through the other liquid, forming a mixture called an emulsion.

Emulsions are thicker than oil or water and have many uses that depend on their special properties.

They provide better texture, coating ability and appearance, for example in salad dressings, ice creams, cosmetics and paints.

There are two main types of emulsion:

Water droplets in oil - margarine, butter, skin cream, moisturising lotion
Oil droplets in water -milk, ice cream, salad cream, mayonnaise

If an emulsion is left to stand, it will separate into an oil layer and a water layer. Emulsifiers are added to emulsions to prevent them from separating out in this way

Egg yolk contains a natural emulsifier. Mayonnaise is a stable emulsion of vegetable oil and vinegar with egg yolk.

Emulsifier molecules have two different ends:

a hydrophilic end - 'water-loving' - that forms chemical bonds with water but not with oils
a hydrophobic end - 'water-hating' - that forms chemical bonds with oils but not with water.

The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the water and oil droplets become unable to separate out.

Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different substances