The Mechanism of Chromatographic Retention - Chromatographic Interactions > Ionic Interactions > Page 16
The term "hydrophobic interaction" unfortunately implies some form of molecular repulsion, which, outside the van der Waals radii of a molecule, is quite impossible. The word hydrophobic literally means "fear of water" and appears to have been provoked by the immiscibility of a dispersive solvent such as n-heptane with a very polar solvent such as water.
However, n-heptane and water are immiscible, not because water molecules repel heptane molecules but because the forces between heptane molecules and the forces between water molecules are much greater than the forces between a heptane molecule and a water molecule. Immiscibility occurs because water molecules and heptane molecules interact very much more strongly with themselves than with each other. Water, in fact, has a small but finite solubility in n-heptane, and similarly n-heptane a small but finite solubility in water. Despite water-water interactions and hydrocarbon-hydrocarbon interactions being very much stronger than water-hydrocarbon interactions, the latter do, to a limited extent, exist and water-hydrocarbon interactions are sufficiently strong to allow some solution to take place. Even accepting the strong interactive forces that exist between hydrocarbons, at normal temperatures, a small fraction will randomly acquire sufficient kinetic energy to part. Consequently, equilibrium occurs in a saturated solution of any hydrocarbon in water when the probability of a pair of hydrocarbon molecules gaining sufficient energyto part is equal to the statistical probability of a hydrocarbon molecule colliding with a water molecule and interacting. The probability of an interacting hydrocarbon pair gaining sufficient energy to part is very small, so the saturated concentration of heptane in water (which determines the probability of collision) must also be small. Consequently, the mutual solubility of water and hydrocarbons must be very low. The term hydrophilic, meaning "love of water", is the complement to hydrophobic and is equivalent to the term polar, thus, substances that interact strongly with water (i.e., are relatively soluble) are called hydrophilic substances.