When phospholipids are mixed in water they spontaneously organize into bilayers. Which structural feature most directly causes this behavior?

Core Concept

Phospholipids are amphipathic: their polar head groups interact with water while their nonpolar hydrocarbon tails avoid it. In water, the hydrophobic effect favors arrangements that bury tails away from the aqueous environment while exposing heads, producing bilayers and related membrane structures. Lipid bilayers are the dominant architecture of familiar bacterial and eukaryotic membranes, but 'every cell membrane' is too absolute because archaeal membrane lipids and architectures can differ.