micelle
/ my-SELL /
Imagine a crowd of people, each holding an umbrella, caught in a downpour they hate. They naturally huddle into a ball, umbrellas facing out to keep the rain off and dry backs turned inward. Soap molecules in water do something similar: each has a water-loving head and a water-hating oily tail, and when enough gather, they bunch into a tiny sphere with heads facing the water outside and tails hidden inside. That self-made ball is a micelle.
More precisely, a micelle is a cluster of surfactant molecules that spontaneously assembles in water once the surfactant concentration is high enough. The water-hating (hydrophobic) tails point inward, away from the water, forming a small oily pocket; the water-loving (hydrophilic) heads form the outer skin that faces the water. The whole structure is only a few nanometres across, so micelles are colloidal particles.
Micelles are the heart of how soap and detergent clean: grease and oil, which will not dissolve in water, get trapped inside the oily core and carried away. They also matter in drug delivery, in how our bodies absorb fats, and in nanotechnology. The caveat is that micelles only form above a sharp threshold concentration; below it the surfactant just floats around as single molecules and no clusters appear.
When you wash a greasy plate, soap micelles form and trap the grease inside their oily cores. The water-loving outer surface lets the whole grease-filled ball rinse away with the water — clean plate, no scrubbing the oil itself.
Tails inward, heads outward: a micelle hides oil in a water-friendly ball.
In water the oily tails hide inside (heads out). In an oily solvent the arrangement flips — heads tucked inside around a water droplet, tails out — and the result is called a reverse micelle. Either way, the rule is that each end seeks the medium it likes.