What factor drives the arrangement of the phospholipid bilayer in an aqueous environment?

The hydrophobic tails of the phospholipids

Explanation:

Because the phospholipid bilayer is made up of a hydrophillic head and a hydrophobic tail, the phospholipids spontaneously form a bilayer in water. This is because the hydrophobic tails are repelled by the water, meaning they retreat into one another, whereas the phosphilipid heads are attracted to water. This means the heads form the outside of the layer, and the tails form the inside of the layer.