HYDROPHOBIC HYDRATION OF SMALL APOLAR MOLECULES AND EXTENDED SURFACES - A MOLECULAR-MODEL

Hydrophobic hydration of small apolar molecules and of extended hydrophobic surfaces is analysed applying a statistical thermodynamical lattice-gas model. This model, in which the orientation-dependent interactions between water molecules (e.g. hydrogen bonds) are accounted for, reproduces the anomalous thermodynamic behaviour of water. Moreover, in this model these macroscopic phenomena can be related to underlying molecular behaviour. The temperature dependencies of the density of pure water and of the solubility of small apolar molecules in water share a common molecular basis. Dissolution of small apolar molecules leads to a relatively small enhancement of the pronounced structuring that is intrinsically present in water. Formation of special ''iceberg-like'' structures around apolar solutes does not occur. The negative solvation entropy is related to changes of the local ordering of the water molecules similar to those occurring upon expansion of pure water. Hydrogen bonding hardly decreases for these cases. The small, at lower temperatures even negative value of the solvation enthalpy is due to a reduction of the number of repulsive water-water interactions. Calculations on an extended flat hydrophobic surface indicate that the anomalous aspects of the solvation thermodynamics do not occur with bulky apolar particles. With these, a decrease of hydrogen bonding cannot be avoided and water tends to be depleted from such surfaces.