Answer: hydrophobic.
Step-by-step explanation:
The outside of a membrane bilayer mainly consists of phosphatidylcholine, sphingomyelin, and glycolipids. These make it hydrophilic. However, the inner layer contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol which make it HYDROPHOBIC. This means it is impermeable to water-soluble molecules.