Cyclohexane (\(\text{C}_{6}\text{H}_{12}\)) is a clear, colorless liquid and a nonpolar solvent used in industrial applications. When introduced to water (\(\text{H}_{2}\text{O}\)), the two substances do not readily mix. Cyclohexane is considered immiscible in water, meaning it is not soluble. At \(25 \text{°C}\), its solubility is extremely low, measuring approximately \(0.0059 \text{ grams}\) per \(100 \text{ milliliters}\) of water.
The Guiding Principle: “Like Dissolves Like”
The behavior of mixtures is predicted by the fundamental rule known as “like dissolves like.” This principle suggests that substances with similar polarity tend to dissolve into one another. A solvent is effective if its molecules can establish strong, stable attractive forces with the solute molecules.
Polarity measures how evenly electrical charge is distributed across a molecule. Molecules with a significant separation of charge are classified as polar. Conversely, molecules with an even distribution of charge are considered nonpolar. For a successful mixture to form, the energy cost of breaking existing attractions must be compensated by the energy gained from forming new attractions between the solvent and solute.
Structural Analysis: Water vs. Cyclohexane
The difference in polarity between water and cyclohexane explains their lack of solubility. Water is highly polar due to its bent molecular geometry and the significant difference in electronegativity between oxygen and hydrogen. The oxygen atom pulls shared electrons toward itself, creating a partial negative charge and giving the molecule a permanent, strong dipole moment.
Cyclohexane is a nonpolar molecule consisting only of carbon and hydrogen atoms arranged in a six-membered ring. Although the carbon-hydrogen bonds have slight polarity, the molecule’s highly symmetrical structure cancels out these individual dipoles. Cyclohexane exists primarily in a puckered “chair” conformation, maintaining a net zero dipole moment. Because it is a symmetrical hydrocarbon, cyclohexane is classified as nonpolar.
The Energetic Barrier: Intermolecular Forces
The forces acting between molecules determine whether they will mix, and these forces differ greatly between water and cyclohexane. Water molecules are held together by powerful hydrogen bonds, the strongest type of intermolecular force. These bonds create an extensive network in liquid water that requires substantial energy to disrupt.
Cyclohexane molecules, being nonpolar, are held together only by weak London Dispersion Forces (LDFs). LDFs are transient attractions much weaker than the permanent hydrogen bonds in water. For cyclohexane to dissolve, the strong water-water hydrogen bonds must be broken to create space for the cyclohexane molecules.
The resulting interactions between water and cyclohexane are only weak LDFs or minor dipole-induced dipole forces. The energy released by forming these weak attractions is insufficient to cover the high energy cost of breaking the strong hydrogen bonds. This energetically unfavorable process, known as the hydrophobic effect, drives water molecules to exclude the nonpolar cyclohexane, keeping the two liquids separate.
Real-World Consequences of Immiscibility
When water and cyclohexane are combined, they immediately separate into two distinct layers. Cyclohexane has a density of approximately \(0.77 \text{ grams}\) per \(\text{milliliter}\) at \(25 \text{°C}\), making it less dense than water. Therefore, the cyclohexane layer always floats on top of the water layer.
This immiscibility is utilized in separation techniques, such as liquid-liquid extraction, in industrial and laboratory settings. The nonpolar nature of cyclohexane makes it an excellent solvent for extracting nonpolar components from a water-based mixture. This principle also explains the behavior of environmental hazards like oil spills. Nonpolar hydrocarbons do not mix with seawater and remain as a separate surface layer, which guides cleanup strategies.