  In our recent works, based on the structural studies on water and interfacial
water (topmost water layer at the solute/water interface), hydration free
energy is derived and utilized to investigate the physical origin of
hydrophobic interactions. In this study, it is extended to investigate the
structural origin of hydration repulsive force. As a solute is embedded into
water, it mainly affects the structure of interfacial water, which is dependent
on the geometric shape of solute. Therefore, hydrophobic interactions may be
related to the surface roughness of solute. According to this study, hydration
repulsive force can reasonably be ascribed to the effects of surface roughness
of solutes on hydrophobic interactions. Additionally, hydration repulsive force
can only be expected as the size of surface roughness being less than Rc
(critical radius), which is in correspondence with the initial solvation
process as discussed in our recent work. Additionally, this can be demonstrated
by potential of mean force (PMF) calculated using molecular dynamics
simulations.
