Oil and water do not mix – usually – but under certain circumstances they can.
If you want to make a home-made vinegrette, you vigorously shake the olive oil and vinegar together. This will break the oil layer into tiny droplets mixed throughout the vinegar. A short while later, however, the little oil blobs will start to join together (coalesce) and reform the oil layer at the top.
In commercial salad dressings, an emulsifier (such as soy lecithin) stabilises these droplets in water (or vinegar) and allows the mixed product (emulsion) to remain for longer. Mayonaisse works by utilising the natural emulsifiers found in egg yolks in the same way.
But is it possible to create a stable emulsion without using an emulsifier?
One scientist believes it is. Pashley theorised that it wasn’t an intrinsic incompatability between the two liquids but naturally dissolved gases in the water which force the oil droplets to break away from the surrounding water molecules and regroup with other oil particles. (Gases naturally become dissolved when water comes into contact with air – for example fish use dissolved oxygen to breathe).
By repetitively freezing and thawing water and removing any expelled gas at each stage, Pashley created gas-free water .
When shaken with oil, a stable emulsion was formed which required no added emulsifiers. Even when the mixture was allowed to stand, thus permitting gases to dissolve back into the solution, the emulsion remained stable.
The mechanism however is still unknown so it’ll probably be a while before this pratice becomes commercially accepted.
Moral: If you want to make perfect salad dressings become a chemist.
Pashley, R.M. (2003) Effect of Degassing on the Formation and Stability of Surfactant-Free Emulsions and Fine Teflon Dispersions, Journal of Physical Chemistry B, 107(7): 1714-1720