Why do the phospholipids surrounding the cell form a bilayer?
It is the shape and amphipathic nature of the lipid molecules that cause them to form bilayers spontaneously in aqueous environments.
The most abundant membrane lipids are the phospholipids . These have a polar head group and two hydrophobic hydrocarbon tails. The tails are usually fatty acids and they can differ in length.
Hydrophilic molecules dissolve readily in water because they contain charged groups or uncharged polar groups, that can form either favourable electrostatic interactions or hydrogen bonds with water molecules
Hydrophobic molecules are insoluble in water because all or most of their atoms are uncharged and non polar. They cannot form energetically favourable interactions with water molecules.
If dispersed in water , they force the adjacent water molecules to reorganise into ice like cages that surround the hydrophobic molecules.
For the above reason, lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophobic heads to water.
Being cylindrical phospholipid molecules spontaneously form bilayer in aqueous environments. In this energetically most favourable arrangement, the hydrophilic heads , face the water at each surface of the bilayer and the hydrophobic tails are shielded from the water in the interior.