Hydrophobic interaction describes the behavior of substances that do not mix with water, as the name “water-fearing” implies. It is the tendency of nonpolar substances, like oils and fats, to clump together in a water-based solution.
This interaction is not a traditional chemical bond where molecules are actively attracted to one another. Instead, it is a passive aggregation driven by the properties of the surrounding water, resulting in a separation between the nonpolar substances and the water.
The Driving Force Behind the Interaction
The driver of hydrophobic interactions is the behavior of water molecules, not an attraction between nonpolar molecules. Water molecules form a dynamic network of hydrogen bonds with one another. A nonpolar molecule, which cannot form these bonds, disrupts this network, forcing the water molecules to arrange themselves into a highly ordered, cage-like structure around the nonpolar surface.
This ordered arrangement is entropically unfavorable, as systems naturally tend toward a state of higher disorder (entropy). The formation of these water cages decreases the system’s entropy because the water molecules are more restricted in their movement. To increase the overall entropy, the system pushes the nonpolar molecules together.
By aggregating, nonpolar molecules minimize their collective surface area exposed to water. This action liberates many water molecules from their rigid structures, allowing them to return to the less-ordered bulk water. The resulting increase in the entropy of the water is significant enough to make the aggregation process spontaneous. This entropy-driven effect is the essence of the hydrophobic interaction.
Hydrophobic Interactions in Biology
This interaction underpins the structure of many biological components, such as in protein folding. Proteins are long chains of amino acids, some hydrophobic (nonpolar) and others hydrophilic (polar). During the folding process, the hydrophobic amino acids bury themselves in the protein’s core, away from the cell’s aqueous environment.
This movement guides the protein into its specific three-dimensional shape, which is necessary for its function. The structure of globular proteins features a well-defined hydrophobic core that stabilizes the folded state. Without this effect, proteins could not adopt the precise shapes required to function as enzymes, structural components, or signaling molecules.
The formation of cell membranes is another result of hydrophobic interactions. Cell membranes are composed of phospholipids, which are amphipathic molecules with a hydrophilic head and a hydrophobic tail. In water, these molecules spontaneously arrange into a bilayer, with the hydrophobic tails pointing inward, shielded from water, and the hydrophilic heads facing outward. This bilayer creates a stable barrier separating the cell’s internal contents from the external environment.
Everyday Observations of Hydrophobicity
Hydrophobic interactions are observable in everyday life, such as in a mixture of oil and water. When combined, they quickly separate into distinct layers. The oil, a nonpolar substance, coalesces into droplets to minimize its contact with the polar water molecules.
This principle also explains why water beads up on waxy or greasy surfaces. The surface of a waxed car or a water-repellent jacket is hydrophobic. Water molecules are more attracted to each other than to the nonpolar surface, causing them to form droplets that minimize their contact area.
Detergents and soaps function based on this principle. Soap molecules are amphipathic, with a hydrophobic tail that interacts with grease and a hydrophilic head that interacts with water. The hydrophobic tails surround grease particles to form structures called micelles, with the hydrophilic heads facing outward. This structure allows the trapped grease to be washed away by water.
Distinguishing from Other Chemical Bonds
Distinguishing the hydrophobic interaction from other chemical bonds is helpful. Covalent bonds involve the sharing of electrons between atoms, creating a strong link that holds molecules like water (H₂O) together. These bonds require significant energy to break and form the backbone of most organic molecules.
Ionic bonds are formed from the electrostatic attraction between oppositely charged ions, such as the sodium (Na⁺) and chloride (Cl⁻) ions in table salt. This attraction occurs when one atom transfers an electron to another, creating charged particles. Like covalent bonds, ionic bonds are true chemical bonds that directly link atoms.
The hydrophobic interaction is fundamentally different from these true chemical bonds. It is not a direct attraction between nonpolar molecules but an emergent property of the system that includes the surrounding water. Its strength is weaker than covalent or ionic bonds and is highly dependent on environmental factors like temperature. This makes it a passive but significant organizing force in biological systems, driven by thermodynamics rather than direct atomic attraction.