emulsifying agent

Egg yolk is the secret that turns oil and vinegar into thick, stable mayonnaise that does not split. The yolk acts as an emulsifying agent: a substance that wraps around each droplet and keeps the two liquids mixed long after the whisking stops. Without one, an emulsion separates almost as fast as you make it.

An emulsifying agent works in two complementary ways. First, it lowers the interfacial tension between oil and water, making it easier to break one phase into small droplets and reducing the system's tendency to minimise its interface. Second, and often more important, it forms a coherent film around each droplet — a mechanical and sometimes electrical barrier that stops droplets from touching, draining, and coalescing.

Emulsifiers fall into a few families: synthetic surfactants such as polysorbates and sodium lauryl sulfate; natural hydrocolloids such as acacia, tragacanth, and gelatin that form thick protective films; and finely divided solids such as bentonite or colloidal silica that sit at the interface (Pickering emulsifiers). Their hydrophilic-lipophilic balance largely decides which emulsion type forms — high-HLB emulsifiers tend to give oil-in-water, low-HLB ones water-in-oil — and combining two emulsifiers often gives a tougher, more stable interfacial film than either alone.