Emulsions

Emulsions are liquid–liquid colloids — fine droplets of one liquid dispersed in another. Shaking two immiscible (or partially miscible) liquids gives a coarse dispersion called an emulsion, and one of the two liquids is usually water. There are two types:

• Oil in water (O/W): water is the dispersion medium — e.g. milk (liquid fat in water) and vanishing cream.
• Water in oil (W/O): oil is the dispersion medium — e.g. butter and cream.

Fig. 5.15 contrasts the two: in O/W, oil droplets float in a water background; in W/O, water droplets float in an oil background.

Stabilising an emulsion

Oil-in-water emulsions are unstable and may separate on standing, so a third component — an emulsifying agent — is added to form an interfacial film between the droplets and the medium. Typical agents are proteins, gums, and natural/synthetic soaps for O/W emulsions, and heavy-metal salts of fatty acids, long-chain alcohols, and lampblack for W/O emulsions.

Behaviour …