Light micrograph of an oil in water emulsion. Image size 60 x 45 µm (Image by Mary Parker & Paul Gunning, IFR)

A Simple Emulsion Primer

Most oils are less dense in water, and if oil and water are mixed then the oil will simply float to the surface. In emulsions, the oil is dispersed as liquid droplets through the continuous phase, usually but not necessarily water. This means that an emulsion is thermodynamically unstable. Those droplets want to combine together again to form a single blob of oil. To prevent them from doing this, emulsions contain a surfactant which coats the surface of each drop and prevents the droplets from coalescing. In practice, a mixture of oil, water and added surfactant are put through a blender, for example, and the product is an emulsion.

However the oil is still less dense than the water. So each drop is prone to floating upwards. This process is called creaming - the oil droplets will gradually form a dense layer at the top of the sample. An emulsion is therefore described as unstable with respect to creaming (creaming is just upside down sedimentation). To prevent creaming, many emulsion products contain additives called stabilizers that inhibit creaming. If you look on a jar of salad cream, for example, you will see listed in the contents emulsifiers (the surfactant that preserves the droplet integrity) and stabilizers (which prevent creaming). Stabilizers work by increasing the viscosity of the continuous phase in which the oil droplets are immersed, or by inducing some kind of interaction between droplets. In food products, the stabilizer is often a gum such as guar gum, or xanthan, a microbial polysachharide that is highly favoured.

A microscope image of a flocculated emulsion

Emulsions are a subset of the subject of colloids, which refers to a dispersion of particles or droplets through a liquid. Emulsions are quite different from hard particle dispersions. Large droplets, above say 10 microns in diameter, are relatively soft and deformable. Smaller droplets, say about 2 micron diameter, are more like hard spheres (due to their Laplace pressure). In addition, emulsions have an oil-water interface with a surfactant layer to consider. Also, in a real emulsion the size of the droplets varies greatly - it has a log-normal distribution. So an emulsion prepared for an experiment may contain droplets varying in diameter from 0.2 microns to 3 microns. An emulsion is said to be polydisperse. Single size dispersions are said to be monodisperse. It is possible to make monodisperse emulsions with care but this is not usual on any industrial scale.

The properties of emulsions depend on virtually everything - the continuous phase, temperature, average droplet size, droplet size distribution, the amount of oil dispersed in the water (called the oil volume fraction), the oil itself, and additives such as xanthan and in food materials a host of other substances that are present.

Two important properties of emulsions are their stability against creaming, and their flow properties, in other words their rheology. Stability against creaming relates to shelf life of commercial products - people will not buy dressings that have a layer at the top, and paints that separate out are not good. The rheology is really important in the way a product is used. In food, it is linked to the mouthfeel (how the consumer experiences the material when they eat it) in addition to such simple things as how well it pours out of a jar. In health care products, for example, spreadable creams that you might rub on the skin are emulsions that have been carefully formulated to spread easily by hand. To runny or too stiff and they would be hard to use. Emulsions are also used in agro products (farm insecticide sprays for example), paints and photographic coatings. There is much more to their rheology than simply the viscosity. The rheology is determined using a rheometer, which (in one form at least) is two concentric cylinders, with about 2mm of sample between them, which are turned relative to one another. The force required to turn the cylinders relative to one another is used to measure the viscosity, and also other rheological parameters that determine to what extent the material is elastic-like, and what extent it is liquid like.