  The strength of hydrogen bonding in water is stronger than that of van der
waals interaction, therefore water may play an important role in the process of
hydrophobic effects. When a hydrophobic solute is dissolved into water, an
interface appears between the solute and water. To understand the mechanism of
hydrophobic effects, it is necessary to study the structure of water and
solute/water interface. In this study, based on the structural studies on water
and air/water interface, the hydration free energy is derived, and utilized to
investigate the physical origin of hydrophobic effects. According to the
discussion on hydration free energy, with increasing solute size, it can be
divided into the initial and hydrophobic solvation processes, respectively. In
the initial solvation process, hydration free energy is dominated by the
hydrogen bonding in interfacial water (topmost water layer at the solute/water
interface). However, in the hydrophobic solvation process, the hydration free
energy is related to hydrogen bondings of both bulk water and interfacial
water. Additionally, various dissolved behaviors of solutes can be expected for
different solvation processes. From this, hydrophobic effects can be ascribed
to the competition between the hydrogen bondings in bulk water and those in
interfacial water.
