Phospholipids are the lipids containing phosphate group. It is composed of two main units, one is the glycerol unit/fatty acid unit (non-polar unit) and another is the phosphate unit (polar unit).
When phospholipids are placed in water the two groups orient themselves to form a bilayer with phosphate group which is hydrophilic in nature facing outward and the glycerol unit/ fatty acid unit which is hydrophobic in nature facing inwards.
The cohesive nature of water that is its formation of hydrogen bonding initiates the process of formation of bilayer.
