Emulsions

An emulsion is a colloidal solution in which one liquid is finely dispersed, as tiny droplets, throughout another liquid with which it is immiscible. Every emulsion falls into one of two types, depending on which liquid forms the continuous dispersion medium: an oil-in-water (O/W) emulsion has oil droplets dispersed in a continuous water phase (milk is a classic example), while a water-in-oil (W/O) emulsion has water droplets dispersed in a continuous oil phase (stiff greases — water dispersed in lubricating oil — are a classic example).

The process of preparing an emulsion, by dispersing one liquid as fine droplets within another, is called emulsification; a colloid mill, acting here as a homogeniser, is commonly used to mix the two liquids mechanically. However, mechanical mixing alone rarely gives a STABLE emulsion — the droplets tend to re-coalesce over time — so a small amount of an emulsifying agent (emulsifier) is normally added to keep the droplets separated. Emulsifiers fall into three broad families: most lyophilic colloids, such as glue and gelatine, act as emulsifiers; long-chain compounds carrying a polar head group, such as soaps and sulphonic acids, act as emulsifiers by orienting their polar end into the water phase and their long hydrocarbon chain into the oil phase; and insoluble, finely divided powders such as clay and lamp black can also act as emulsifiers, by physically coating the droplet surfaces. …