Emulsion

What is an emulsion?

An emulsion is a dispersion of droplets of one liquid in a second liquid with which it is not fully miscible. The liquid that forms the droplets is the dispersed phase; the liquid around them is the continuous phase. Milk, mayonnaise, many paints and lotions, agricultural concentrates that turn milky when diluted, and the oil-and-water mixtures produced by oil wells are all emulsions.

Making an emulsion takes energy, usually mixing, shearing or pumping, because breaking one liquid into droplets creates a large amount of new interface between the two. The energy required depends on the interfacial tension between them, which is why surfactants make emulsions easier to form.

Types of emulsion

• Oil-in-water (O/W): oil droplets dispersed in water. The emulsion mixes with water and feels watery. Milk and most spray emulsions are this type.
• Water-in-oil (W/O): water droplets dispersed in oil. The emulsion mixes with oil. Butter and many crude oil emulsions are this type.
• Multiple emulsions: droplets that themselves contain smaller droplets, such as water-in-oil-in-water.

A rule of thumb known as Bancroft’s rule says that the liquid in which the emulsifier is more soluble tends to become the continuous phase. This is the basis for choosing emulsifiers by their hydrophilic-lipophilic balance.

Why emulsions separate

An ordinary emulsion is not thermodynamically stable: left alone, it tends to return to two separate layers, because that arrangement has less interface and therefore less energy. How long that takes ranges from seconds to years. The main routes to separation are:

• Creaming or sedimentation: droplets rise or sink because their density differs from the surrounding liquid.
• Flocculation: droplets cluster together without merging.
• Coalescence: droplets merge into larger ones, the irreversible step toward separation.
• Ostwald ripening: small droplets shrink and large ones grow as the dispersed liquid diffuses between them.

An emulsifier slows these processes by forming a film at the droplet surface that resists merging, through electrical charge, a physical barrier, or both. Fine solid particles can do the same job; emulsions stabilized this way are called Pickering emulsions.

A microemulsion is different: it forms spontaneously, has much smaller structures, and is thermodynamically stable.

Emulsions in oil production

Crude oil and water are produced together from most wells, and the turbulence of flowing through chokes, pumps and valves mixes them. Natural components of the oil, including asphaltenes, resins, organic acids and fine solids, collect at the droplet surfaces and can make these emulsions very stable. Stable emulsions raise the viscosity of the produced fluid, make separation of oil and produced water harder, and can leave water in the sales oil or oil in the water sent for disposal. Some also form in the reservoir near the well and restrict flow.

Operators break emulsions with heat, settling time, electrostatic treaters and chemical demulsifiers, which displace the natural stabilizing film from the droplet surfaces so that the droplets can coalesce.

Emulsions in formulation

Formulators use emulsions deliberately to combine oil-soluble and water-soluble ingredients in one product. Emulsifiable concentrates in crop protection are oil-based products designed to form an oil-in-water emulsion when poured into the spray tank. Coatings, lubricants and cleaners also rely on emulsions whose stability must hold for the product’s shelf life and then behave predictably in use.

Frequently asked questions