The answer to whether all nonpolar molecules are hydrophobic is yes, a fundamental concept in chemistry and biology. A nonpolar molecule is defined by the uniform distribution of its electrical charge, meaning it lacks distinct positive or negative ends. Conversely, a hydrophobic substance repels or fails to mix with water. The inherent properties that make a molecule nonpolar are precisely those that cause it to be rejected by water, establishing a direct link between the two terms when water is the solvent.
How Molecular Polarity is Determined
Molecular polarity originates from the unequal sharing of electrons between atoms, a difference measured by electronegativity. When two atoms in a bond have significantly different electronegativities, electrons are pulled closer to the more attractive atom. This creates a partial negative charge (\(\delta^-\)) and leaves the less attractive atom with a partial positive charge (\(\delta^+\)). This separation of charge produces an electric dipole moment within the bond.
The overall polarity of a molecule depends not just on individual bond dipoles but on the molecule’s three-dimensional shape, known as molecular geometry. If a molecule possesses polar bonds but has a symmetrical structure, the individual dipole moments can effectively cancel each other out. For example, carbon dioxide (\(CO_2\)) has two polar carbon-oxygen bonds. Because the molecule is linear and the bonds pull in opposite directions, the net dipole moment is zero, rendering the molecule nonpolar.
In contrast, a molecule like water (\(H_2O\)) is bent, or asymmetrical, meaning the bond dipoles do not cancel. The oxygen atom pulls electrons away from the two hydrogen atoms, resulting in a net negative charge on the oxygen side and a net positive charge on the hydrogen side. This uneven distribution of charge defines the molecule as polar, giving it a measurable dipole moment. A molecule is nonpolar if its constituent atoms share electrons equally or if its symmetrical shape causes all individual bond dipoles to negate one another.
The Unique Nature of Water and Hydrophobicity
Water’s structure is the foundation of hydrophobicity because the molecule is highly polar and forms a strong, dynamic network with its neighbors. The partial positive charge on the hydrogen atoms and the partial negative charge on the oxygen atom allow water molecules to form powerful intermolecular attractions called hydrogen bonds. This constant formation and breaking of hydrogen bonds creates an energetically favorable three-dimensional liquid structure.
Hydrophobicity, often called “water-fearing,” is not about a nonpolar substance actively repelling water, but rather water actively rejecting it. This rejection occurs because nonpolar molecules cannot participate in hydrogen bonding and would force surrounding water molecules to break their existing favorable bonds. To accommodate the intrusion, water molecules must reorganize themselves into a rigid, cage-like structure, known as a clathrate cage, around the nonpolar solute.
This forced organization significantly restricts the positions and orientations water molecules can adopt, resulting in a decrease in the system’s entropy, or molecular randomness. Nature favors processes that increase entropy. Therefore, the system minimizes this unfavorable entropic cost by minimizing the surface area of contact between water and the nonpolar molecule. The nonpolar molecules are consequently driven to aggregate together, effectively “hiding” from the water.
The Governing Rule of Solubility
The relationship between nonpolar molecules and hydrophobicity is explained by the principle of solubility known as “Like Dissolves Like.” This rule states that a solute will dissolve in a solvent only if the strength and type of intermolecular forces between them are comparable to the forces within the pure solvent and pure solute. Nonpolar molecules primarily interact through weak London dispersion forces, meaning the only compatible solvents are other nonpolar substances.
When a nonpolar molecule attempts to dissolve in a polar solvent like water, the process fails. Dissolving the nonpolar solute requires energy to break the strong hydrogen bonds between water molecules. This energy expenditure is not compensated for by the formation of new, energetically comparable nonpolar-polar interactions. The water-water attractions are too powerful and favorable to be replaced by the weak attractions between water and the nonpolar molecule.
The hydrophobic effect describes the energetic and entropic consequences of this mismatch between nonpolar molecules and water. Since all nonpolar molecules lack the charge distribution necessary to form strong, compensatory interactions with water, they are rejected by water’s hydrogen-bonded network. In the context of water as a solvent, being nonpolar and being hydrophobic are functionally interchangeable properties.
Where We See Nonpolar Hydrophobic Interactions
The tendency of nonpolar molecules to aggregate in water is the driving force behind the formation of biological structures, most notably the cell membrane. Every cell is enclosed by a lipid bilayer, composed of phospholipid molecules that are amphipathic. These molecules have both a polar (hydrophilic) head and two nonpolar (hydrophobic) hydrocarbon tails.
When these phospholipids are placed in an aqueous environment, the hydrophobic effect compels the nonpolar tails to cluster together, forming the interior of the membrane, shielded from the water. Simultaneously, the polar heads arrange themselves to face the water on both the inside and outside of the cell. This spontaneous organization into a bilayer gives the cell membrane its stable structure and functions as a selective barrier.
The nonpolar core of the membrane determines which substances can cross easily. Small, nonpolar molecules like oxygen (\(O_2\)) and carbon dioxide (\(CO_2\)) are soluble in the hydrophobic interior and can readily diffuse through the membrane. Conversely, polar molecules and charged ions are blocked because they cannot pass through the nonpolar, water-excluding environment of the bilayer’s core.