Phospholipid Bilayer Formation

In the field of biochemistry, lipids play critical roles in biological membranes. One of the most prevalent types of lipids found in cell membranes is phospholipids. Phospholipids consist of a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repelling) 'tails'. This unique structure allows phospholipids to arrange themselves into a bilayer, forming the fundamental structure of cell membranes.

Which characteristic of phospholipids primarily contributes to the formation of the lipid bilayer in cellular membranes?