In the "why things dissolve" page we found that water will dissolve ionic (salt) and neutral-polar (sugar) molecules but not neutral non-polar moleucles (like olive oil).

So how can we get oil to dissovle in water?

Water molecules cannot interact well with the oil molecules, and so the water "excludes" the oil pushing it away, this causes the water and oil to seperate out into layers (with the olive oil on top).

To get oil and water to mix, or to get oil to dissolve into water we need to use molecules called surfactants. These are long skiny molecules composed of an ionic "head" and a long non-polar neutral "tail", they look a bit like sperm!

Your everyday dish-washing detergent is a surfactant solution. When you wash something, the water dissolves all the ionic and neutral-polor molecules. But the oil is left behind, and the dishes are still dirty. In orger to dissolve the oils and wash them away we add dish-washing detergent.

The surfactant allows water and oil to mix by forming an intermediate layer between the oil and water. The surfactant's tail which is neutral interacts with the surface layer of the oil and the surfactant's head which is charged forms H-bonds with water molecules.

The water and oil now mix as shown in the video clip below.

Look at the label of a dishwashing detergent bottle, you will see "15-30% Anionic surfactants". Now you know why they are there and how they work!

Next have a look at the labels of other cleaning agents in your cupboard ... you will find surfactants listed on them as well. Check out your shampoo and liquid soap, they will very likely have "ammonium lauryl sulfate", or "sodium lauryl sulfate" or "sodium laureth sulfate" these are all anionic surfactants ... there to help breakdown and wash away oils.