Oil and water do not mix—hydrophobic hydration

Many surfactants contain hydrocarbon moieties that are removed from their aqueous environment (‘dehydrated’) in, for example, adsorption and micelle formation. Hydrophobic hydration relates to the interactions between individual nonpolar solute molecules and water, and can be probed using thermodynamic quantities for the dissolution of dilute hydrocarbon vapours to form dilute aqueous solutions. Contrary to the simple expectation that the entropy of hydration of a nonpolar moiety should be positive (due to disruption of water structure), it is large and negative, giving a large positive contribution to the free energy of hydration. The hydration of nonpolar molecules in water leads to an attraction between the molecules in close proximity, which is termed hydrophobic bonding. Although the free energy of hydration of nonpolar groups in bulk aqueous solution is positive, the interaction free energy of nonpolar molecules/groups with interfacial water at an air/water interface is negative.