In a bilayer formation, phospholipids are held together by various noncovalent interactions. Hydrophobic interactions between the fatty acid tails drive the formation of the hydrophobic core, while hydrophilic interactions between the head groups face the aqueous environment. Van der Waals interactions stabilize the acyl chains, and hydrogen bonding can occur between the head groups and water molecules. Electrostatic interactions between charged head groups also contribute to membrane stability.
