When phospholipids are dropped into water they self-assemble into a bilayer with all fatty acid tails pointing inward. What drives this arrangement?

Core Concept

Each phospholipid is amphipathic: a charged phosphate-bearing head interacts favorably with water, while two long hydrocarbon tails cannot hydrogen-bond with it. Forcing tails into water would order the surrounding water molecules and cost entropy, so the lowest-energy arrangement buries tails in the interior with heads facing the aqueous solutions on both sides. No covalent bonds form between neighbors or to water; the bilayer assembles purely from these weak interactions.