Final answer:
The energetically favorable formation of a lipid bilayer is due to the polar head groups of phospholipids forming hydrogen bonds with water, which increases entropy. This organizes water molecules more efficiently than if hydrophobic tails were exposed to water, and shields the tails from the aqueous environment.
Step-by-step explanation:
The formation of a lipid bilayer is energetically favorable due to the specific properties of phospholipids and their interactions with water. When phospholipids are mixed with water, the hydrophilic (water-attracting) heads of the molecules are drawn towards the water, while the hydrophobic (water-repelling) tails hide from the water. This spontaneous orientation reduces the system's free energy because it reduces the unfavorable interactions between the hydrophobic tails and water. This process leads to an increase in the order of the water molecules surrounding the hydrophobic tails, thereby reducing the total entropy of the system, but the creation of a bilayer allows the water molecules more freedom than if they were to form cage-like structures around individual hydrophobic molecules, thus increasing overall entropy.
The correct answer is that the polar head groups form a hydrogen bonding network at the interface with water, and this interaction is energetically favorable because it allows the water molecules to form a more ordered structure around the hydrophilic heads as opposed to forming cage-like structures around hydrophobic tails, leading to an increased entropy of the system. The hydrophobic tails are indeed shielded from water, which also contributes to the stability of the lipid bilayer.