Hydration thermodynamics of aliphatic alcohols

Abstract

The hydration thermodynamics of five linear aliphatic alcohols in the temperature range 5–100°C is carefully analysed using a suitably modified version of the theoretical approach developed by Lee. The hydration Gibbs energy change is determined by the balance of three contributions: the direct alcohol–water van der Waals interaction energy, the direct alcohol–water H-bond energy, and the excluded volume effect due to solute insertion. The analysis shows that the direct alcohol–water H-bond energy is fundamental in determining the negative values of the hydration Gibbs energy over the whole temperature range investigated, whereas the excluded volume effect determines the large and negative hydration entropies. The reorganization of H-bonds in the hydration shell of aliphatic alcohols proves to be a compensating process, not affecting the Gibbs energy change, as in the case of the hydration of nonpolar molecules. However, H-bond reorganization is the main molecular origin of the large and positive hydration heat capacity change, a signature of hydrophobic hydration, determining the temperature dependence of the hydration enthalpy and entropy changes. We show that H-bond reorganization can be reliably described by means of the modified Muller's model, indicating that the hydration shell is not akin to an iceberg: hydration shell H-bonds are energetically slightly stronger but more broken than those in bulk water. This finding allows the rationalization of the puzzling experimental data on the temperature dependence of the water proton NMR chemical shift in solutions of aliphatic alcohols.