Contrasting the hydration thermodynamics of methane and methanol

Experimental hydration thermodynamic functions of methane and methanol over a large temperature range and at 1 atm (coming from previous studies of others) are analysed by means of a theoretical approach grounded in statistical mechanics. It is shown that the hydration Gibbs energy change can be reproduced quite well for both solutes by adding the large positive reversible work of cavity creation to the solute-water van der Waals and H-bond energetic attractions. The large negative hydration entropy change of both methane and methanol comes from the decrease in translational entropy of water molecules due to the solvent-excluded volume effect caused by cavity creation in water. The reorganization of water-water H-bonds upon solute insertion is characterized by almost complete enthalpy-entropy compensation and so cannot affect the hydration Gibbs energy change. In addition, such reorganization is endothermic above 4 °C, suggesting that no significant increase in water structure occurs upon methane or methanol insertion.