Many students think water actively repels nonpolar molecules like oil, but this is a common misconception. Water molecules actually attract each other very strongly to form hydrogen bonds. Nonpolar molecules lack the proper charges to form these hydrogen bonds with water.
When a nonpolar substance enters water, it disrupts the fluid water network. This disruption forces the surrounding water molecules to reorganize themselves. The water forms a stiff, cage-like structure around the intruding nonpolar molecules.
This highly ordered water cage lowers the overall entropy of the system. Entropy measures the level of disorder, and nature always prefers higher entropy. To increase the system’s entropy, the nonpolar molecules cluster tightly together.
This clustering greatly reduces their total surface area exposed to the water. The trapped water molecules are then released to move and bond normally. This entropy-driven process is widely known as the hydrophobic effect.
It is a powerful driving force in both basic chemistry and complex biology. It explains how oil separates from water in a simple salad dressing. It also drives the specific folding of long protein chains in human cells.