On the solubility of aliphatic hydrocarbons in 7 M aqueous urea

Solubility measurements proved that, at 25 degreesC, methane and ethane are more soluble in water than in 7 M aqueous urea, whereas propane, i-butane, n-butane, and neopentane are more soluble in 7 M aqueous urea than in water. No convincing explanation of these experimental data has been provided up to now. An extension of an emerging theory of hydrophobic hydration is devised to account for the solubility of aliphatic hydrocarbons in 7 M aqueous urea. The conclusions reached are: (a) the work of cavity creation is always greater in 7 M aqueous urea than in water, contrasting the transfer; (b) the solute-solvent van der Waals interaction energy is always greater in magnitude in 7 M aqueous urea than in water, favoring the transfer. The latter contribution increases in magnitude with hydrocarbon size more rapidly than the difference in the work of cavity creation, explaining the existence of a threshold size for the solubility enhancement. The reorganization of H-bonds in both the solvent systems is a compensating process that does not affect the Gibbs energy change, but determines the positive sign of the transfer enthalpy and entropy changes for all hydrocarbons.