Normal and Inverse Emulsions

The type of emulsion with which you are most familiar is probably the oil in water emulsion (e.g. salad dressing or milk). This is sometimes called a normal or o/w emulsion.

An oil in water (o/w, or normal) emulsion.

However, it is also possible to form an inverse or water in oil (w/o) emulsion. In an inverse emulsion, the water droplets are dispersed in a continuous phase of oil. Many medicinal creams and butter are water in oil emulsions.

A water in oil (w/o, or inverse) emulsion.

It is possible to test whether an emulsion is an inverse emulsion or normal emulsion by working out what the continuous phase is. If you get a sample of your unknown emulsion, put on a watch glass and add a drop of water-soluble dye to it (e.g. food colouring), a normal emulsion will take the colour of food colouring.

The continuous phase is water and the dye can dissolve in water, so the emulsion takes on the colour of the dye.

An inverse emulsion, however, will not take-up the food colouring.

The continuous phase is oil and the dye cannot dissolve in the oil, so the dye stays as a large blob separated from the water droplets by the surfactant and the continuous phase. The inverse emulsion does not take on the colour of the water soluble dye.

The reason for this is the structure of the continuous and dispersed phases. The continuous phase is in contact with the dye, but the disperse phase is not. When adding a blue water-soluble dye to an oil in water emulsion, the water is in contact with the water-soluble dye so it dissolves in the continuous phase and the emulsion turns blue. In an inverse emulsion, however, the water-soluble dye has no physical contact with the water droplets. This means that the dye remains isolated from the water droplets so the blue colour cannot spread.

Interestingly enough, it is possible to convert an o/w emulsion into a w/o emulsion or vice versa. This is a rather difficult process, but well known to you if you have ever churned cream (o/w) to become butter (w/o).