phospholipids form a bilayer in water because there needs to be water both inside and outside of the cell. there is an outer layer because phospholipids are amphipathic so the polar heads form hydrogen bonds with each other and the water molecules, and the nonpolar tails are excluded and are in the middle of the micelle. then because cells have an internal environment primarily made of water, another layer forms in which the polar heads are attracted to the water molecules inside of the cell and the nonpolar tails are attracted to the nonpolar tails of the outer layer through van der waals. if you put the cell in a hydrophobic environment the bilayer would reverse
Hope this helps!
-Payshence xoxo
