A hydrophobe is a substance that appears to repel water. The term is derived from the Greek words hydro (water) and phobos (fear), literally translating to “water-fearing.” These substances lack affinity for water molecules and do not readily dissolve in an aqueous environment. The apparent repulsion occurs because the substance is chemically neutral and lacks the charged or polar structure necessary to interact favorably with water.
Why Water Repels Nonpolar Molecules
The phenomenon of water repelling nonpolar molecules is driven by the collective behavior of the water molecules, not by attraction between the nonpolar molecules themselves. Water is a highly polar molecule, capable of forming an extensive network of hydrogen bonds with neighboring water molecules. This network represents a state of high disorder, or entropy, for the liquid water.
When a nonpolar molecule, such as a hydrocarbon, is introduced into water, it cannot form hydrogen bonds and disrupts the existing water network. To minimize broken hydrogen bonds, the surrounding water molecules are forced to reorient into a highly ordered, cage-like structure known as a clathrate. This rearrangement restricts the mobility and rotational freedom of the water molecules, resulting in a substantial decrease in the system’s entropy.
A system naturally seeks to maximize its overall entropy, and the formation of these ordered water cages is thermodynamically unfavorable. The system responds by forcing the nonpolar molecules to cluster together, minimizing the total surface area exposed to the water. This clustering reduces the number of water molecules required to form ordered cages. This allows the previously ordered water molecules to return to the bulk liquid state where they resume their highly disordered motion.
The aggregation of the nonpolar substance is an indirect, entropy-driven process. The apparent repulsion is a consequence of water attempting to restore its hydrogen-bonding network and maximize its entropic freedom. This clustering of nonpolar substances, known as the hydrophobic effect, is the underlying mechanism for the separation of oil and water.
Everyday Examples of Hydrophobes
Hydrophobic substances are common in daily life. They are characterized by a chemical structure composed primarily of long chains of carbon and hydrogen atoms, known as hydrocarbons. These bonds share electrons nearly equally, resulting in the nonpolar nature that prevents them from mixing with polar water. Familiar examples include cooking oils, such as olive or vegetable oil, which are largely composed of triglycerides.
Fats, like butter or lard, also exhibit strong water-repelling characteristics. Waxes, whether natural (like beeswax) or synthetic (like car wax), are highly hydrophobic due to their long-chain hydrocarbon structures. When water contacts a waxy surface, it beads up into spheres. This occurs because the strong cohesive forces between water molecules outweigh the weak adhesive forces between the water and the nonpolar surface.
Hydrophobes and Cellular Life
Hydrophobic interactions play a fundamental role in the structure and function of living cells, particularly in the formation of biological membranes. The cell membrane is constructed from a double layer of phospholipid molecules, which have a distinctive dual nature. Each phospholipid has a hydrophilic head and two long, hydrophobic fatty acid tails.
When placed in an aqueous environment, these phospholipids spontaneously organize into a lipid bilayer. The hydrophobic tails cluster inward, facing each other in the center of the membrane, shielded from the water inside and outside the cell. The hydrophilic heads face outward, interacting favorably with the watery environment. This self-assembling structure creates a stable, water-impermeable barrier that defines the cell boundary and its internal compartments.
Hydrophobic interactions are also instrumental in determining the three-dimensional shape of proteins, a process known as protein folding. Proteins are chains of amino acids, some of which possess nonpolar side chains. When a protein folds in the cellular environment, the nonpolar amino acid side chains are driven inward, clustering to form a hydrophobic core. This clustering maximizes the entropy of the surrounding water molecules and stabilizes the unique, functional structure of the protein.
The Amphiphilic Exception
A unique class of molecules, termed amphiphilic or amphipathic, acts as an interface between water and hydrophobic substances. These molecules possess both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region. Soaps and detergents are common examples, consisting of a long hydrocarbon tail and a charged ionic head.
When used for cleaning, the hydrophobic tails of the soap molecules dissolve into nonpolar grease or oil droplets. Simultaneously, the hydrophilic heads remain exposed to the surrounding water. As more molecules surround the grease, they form a spherical structure called a micelle. The micelle traps the hydrophobic grease in the center, while the charged heads form a stable, water-soluble outer shell. This structure allows the water-insoluble grease to be suspended and carried away by the water.