Micellar solutions and lipid vesicles
— Amphiphilic molecules containing a strongly polar head group attached to one or more hydrocarbon chains spontaneously form organized structures in an aqueous medium in which contact between hydrocarbon and water is minimized. Phospholipids are the most impor-. tant biological molecules of this kind. They have two hydrocarbon chains per head group and their thermodynamically preferred state is a vesicle, bounded by a single bi.Layer with a central hydrocarbon core, and enclosing an aqueous cavity. Molecules with single hydrocarbon chains form small disk—shaped micelles with a central hydrocarbon core. The hydrocarbon micro—phases in both systems are liquid, but molecular motion is restricted by geometrical constraints. Nevertheless both systems have solvent properties similar to those of bulk hydrocarbon and are thus capable of forming discrete pockets of nonaqueous solution segregated from the enveloping aqueous medium.