Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Hydrophobic and Hydrophilic Interactions > Page 28

Hydrophobic and Hydrophilic Interactions

The term "hydrophobic interaction" implies some form of molecular repulsion, which, of course, outside the Van der Waals radii of a molecule, is impossible. The term "hydrophobic force", literally meaning "fear of water" force, is an alternative to the well-established term, dispersive force. The term may have been provoked by the immiscibility of a dispersive solvent such as n-heptane with a very polar solvent such as water.

n-heptane and water are immiscible, not because water molecules repel heptane molecules, they are immiscible because the forces between two heptane molecules and the forces between two water molecules are much greater than the forces between a heptane molecule and a water molecule. Thus, water molecules and heptane molecules interact very much more strongly with themselves than with each other.

Water has, in fact, a small but finite solubility in n-heptane, and n-heptane has a small but finite solubility in water. Although water-water interactions and hydrocarbon-hydrocarbon interactions are much stronger than water-hydrocarbon interactions, the latter does exist and is sufficiently strong to allow mutual solubility.

The term "hydrophilic force", literally meaning "love of water" force, appears to merely be the complement to "hydrophobic". It is equivalent to the term polar, and polar solvents are hydrophilic solvents because they interact strongly with water or other polar solvents.