|University Federico II
of Naples, Italy
Molecules bearing polar groups are not always soluble in water. Compounds bearing a non polar region formed by a rather long chain of carbon atoms (usually, longer than four carbon atoms) are partially or completely insoluble in water. For instance, ethanol, bearing a two-carbon atom alkyl chain, is very soluble in water. On the contrary, butanol, bearing a four-carbon atom chain, has a limited solubility in water.
Usually, non polar compounds are not water soluble because they contain neither ionic groups nor polar functional groups that can interact favorably with water molecules. Those compounds are called "hydrophobic" (water fearing), because they escape the contact with water. Hydrocarbons, constituted by carbon and hydrogen only, are examples of such a kind of compounds. When non polar molecules enter aqueous medium, some water-water hydrogen bonds are to be broken to create a cavity for the solute molecule. Each solute molecule is entrapped in a cage, ice-like, structure formed by highly ordered water molecules, held together by hydrogen bonds. The formed complex, with the non polar molecule in the centre encircled by the cage of water molecules, is called "hydrate".
Non-polar solutes Hydrophobic interaction
Water molecules in the cage around the non polar molecule are more ordered than in pure water. In the presence of many hydrocarbon molecules, the same number of ordered cages of water molecules should form, with the consequent large increase in the order of the system, a process naturally unfavorable. To minimize the increase in the order of the system, hydrocarbon molecules, each one encircled by its own cage of water molecules, associate together (hydrophobic interaction). If, for sake of simplicity, only two non polar molecules in aqueous solution are considered, hydrophobic interaction causes the non polar molecules to come together into a single cavity to reduce the unfavorable interaction with water.
When the number of non polar molecules becomes very large, aggregates of molecules form a second phase, causing the almost total immiscibility of hydrocarbons in water.