Why does a phospholipid bilayer naturally form when phospholipids are placed in water?
The phosphate end of the phospholipid is polar and is naturally attracted to water. The fatty aid tails are nonpolar and repel water. When the phospholipid is placed in water, they align in a way so that the nonpolar fatty acids are touching leaving the polar phospholipid head exposed to water. The resulting structure spontaneously forms.