interfacial film

Imagine wrapping every tiny oil droplet in a stable emulsion inside its own flexible shrink-wrap so it cannot fuse with its neighbours. That molecular shrink-wrap, formed by emulsifier molecules crowding onto the oil-water boundary, is the interfacial film.

When an emulsifying agent adsorbs at an oil-water interface, its molecules pack together into a coherent layer around each droplet. A good film does two things: it lowers the interfacial tension so droplets form easily, and, just as importantly, it provides a mechanical and electrical barrier that resists coalescence when droplets collide. Films can be a single condensed monolayer of small surfactants, a mixed film of two emulsifiers that pack tightly together, or a thick multilayer built by polymers or finely divided solid particles.

The strength and nature of the interfacial film often decide whether an emulsion survives on the shelf. This is why combinations such as a water-soluble and an oil-soluble emulsifier are used to build a tough, closely packed mixed film, and why proteins, gums, and solid particles (Pickering stabilisation) are exploited as robust film formers. The caveat worth remembering is that a low interfacial tension alone is not enough; without a mechanically resilient film, droplets will still coalesce and the emulsion will crack.