Microscopic argument for the anomalous hydration heat capacity increment upon solvation of apolar substances

The contribution of the hydration heat capacity upon solvation of apolar molecules is derived by applying equilibrium statistical mechanics on a simple model, which we compare to experimental data on the linear alkanes: methane, ethane, and propane, and to the aromatic compounds: benzene and toluene. The model is based on a microscopic consideration, where we assign an effective bending energy of the hydrogen bonds between the water molecules in the solvation shell around the solute molecule. Thus, we assume that the hydrophobicity is only connected to forces between the water molecules in the solvation shell, and not to forces between the apolar molecule and the surrounding water. We find that the model, by fitting the parameters, corresponds well to the experimental data. The proportionality of the heat capacity versus surface area is also discussed.