micelle

When you add enough soap to water, the surfactant molecules eventually run out of surface to crowd onto and start huddling together in tiny balls, tails tucked inward away from the water, heads facing out. Each of these self-assembled clusters is a micelle.

A micelle typically contains tens to hundreds of surfactant molecules arranged so that the oil-loving tails form a hidden hydrophobic core while the water-loving heads form the outer shell that faces the surrounding water. The structure forms spontaneously above a threshold called the critical micelle concentration, because hiding the tails from water is energetically favourable. Micelles are dynamic, constantly exchanging molecules with the surrounding solution.

In pharmaceutics that hidden oily core is enormously useful: it can dissolve poorly water-soluble drugs that would otherwise precipitate, a process called micellar solubilisation used in injectables, oral liquids, and some sustained-release systems. Specially engineered polymeric micelles serve as nanoscale drug carriers. A caveat is that on dilution below the critical micelle concentration the micelles disassemble and any solubilised drug can crash out, so injectable micellar products must be designed and used with this in mind.