Emulsion

An emulsion is a mixture in which small droplets of one liquid, such as grease, are suspended in another liquid, such as water. Malay: emulsi · Chinese: 乳浊液.

Oil and water do not normally mix; left alone they separate into two layers. In cleaning, soap makes them stay mixed. The hydrophobic tails of the soap particles bury themselves in each grease droplet while the hydrophilic heads point outward into the water, so the droplet is coated and held in suspension. Millions of these coated droplets spread through the water form an emulsion, which is why the wash water turns cloudy. Because each droplet carries a negative surface charge, the droplets repel one another and do not join back into a layer, so the emulsion is stable long enough to be rinsed away with the dirt.

Example. When you wash oily hands with soap, the water goes milky. That cloudiness is the emulsion: the grease has been broken into tiny droplets, each surrounded by soap, and suspended in the water so it can be rinsed off instead of clinging to your skin.

Confusion to avoid. Do not call an emulsion a solution. In a solution the particles are separate molecules or ions and the mixture is clear; in an emulsion the droplets are far larger, the mixture looks cloudy, and it needs soap to keep it from separating. An emulsion is also not the same as a suspension of a solid; here both phases are liquids.

Forming an emulsion is the final stage of the cleaning action in this chapter, so it usually appears in the same SPM Chemistry explanation question as the hydrophilic head and hydrophobic tail. Emulsions matter outside cleaning too: milk is an emulsion of fat droplets in water, and salad dressings are emulsions kept from separating by an emulsifier such as egg yolk. Recognising that the same idea, one liquid held in droplets inside another, links soap, food and cosmetics helps you see why the term is worth learning precisely rather than just memorising.