Hydrophobicity describes the property of a substance to repel water. Wax serves as a classic example of a material that exhibits this water-fearing characteristic. Understanding why wax behaves this way involves exploring the fundamental chemical properties of both water and wax at a molecular level. This phenomenon is a direct consequence of how these substances fail to interact.
Water’s Polarity and Hydrogen Bonds
Water (H₂O) is a unique molecule, largely due to its polar nature. The oxygen atom in a water molecule attracts electrons more strongly than the hydrogen atoms. This uneven sharing of electrons results in a slight negative charge on the oxygen atom and slight positive charges on the hydrogen atoms. These partial charges create an electrical dipole.
These partial positive and negative charges allow water molecules to form strong attractions with each other, known as hydrogen bonds. The partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of an adjacent water molecule. This extensive network of hydrogen bonds gives water its high surface tension and cohesive properties, meaning water molecules prefer to stick together. This cohesion is fundamental to water’s behavior.
The Nonpolar Nature of Wax
Waxes are a diverse group of organic compounds, but they share a common structural feature: they are predominantly composed of long chains of carbon and hydrogen atoms, known as hydrocarbons. For example, common paraffin wax consists largely of saturated alkanes, which are hydrocarbon molecules typically containing between 20 and 40 carbon atoms. In these carbon-hydrogen (C-H) bonds, electrons are shared almost equally between the atoms. This even distribution of electrons means wax molecules do not have distinct positive or negative poles, making them nonpolar. Unlike water, wax molecules do not possess the partial charges necessary to form strong electrical attractions with other molecules, particularly polar ones.
Why Water and Wax Don’t Mix
The reason water and wax do not mix can be explained by the “like dissolves like” principle, a fundamental concept in chemistry. Polar substances tend to dissolve or mix with other polar substances, and nonpolar substances tend to dissolve or mix with other nonpolar substances. Water, being highly polar, readily mixes with other polar compounds. Wax, being nonpolar, mixes with other nonpolar substances, such as oils.
When water molecules encounter nonpolar wax molecules, the strong hydrogen bonds between water molecules are far more favorable than any weak interactions they could form with the wax. This strong self-attraction of water molecules effectively excludes the wax molecules. From an energetic perspective, it requires less energy for the water molecules to maintain their hydrogen bond network and “push out” the wax, causing the wax to separate and remain distinct from the water.
Everyday Examples of Hydrophobicity
The principle of hydrophobicity is evident in numerous everyday observations. Water often beads up on a freshly waxed car, forming spherical droplets that easily roll off the surface. This is because the wax coating on the car creates a hydrophobic barrier, preventing water from spreading and adhering to the paint. The wax’s nonpolar nature repels the polar water molecules.
Another striking example is the lotus leaf, renowned for its self-cleaning properties, often referred to as the “lotus effect.” Its surface is covered with microscopic bumps coated with hydrophobic waxes. These waxes, combined with the leaf’s hierarchical roughness, significantly reduce the contact area between water droplets and the leaf. Water droplets form nearly spherical beads that roll off, carrying away dirt particles and keeping the leaf clean. Similarly, bird feathers exhibit water repellency due to an oily or waxy coating applied during preening and their intricate microstructure, which helps trap a layer of air, further enhancing their hydrophobic qualities.