The Structural Origin of Hydration Repulsive Force

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.