Detailed Hydration Maps of Benzene and Cyclohexane Reveal Distinct Water Structures

The hydration structures of the hydrophobic solutes benzene and cyclohexane were investigated using molecular dynamics (MD) simulations of a single solute in water. Detailed, spatially resolved, three-dimensional maps of the density of the water O and H atoms surrounding either benzene or cyclohexane were generated from the simulation data. MD simulations were computed for each solute using either the ENCAD or GROMACS program package and one of five distinct water models: F3C, TIP3P, TIP4P, SPC, or SPC/E. For each solute, five 100 ns data sets, each representing a unique simulation condition, were analyzed. The water structure maps generated from the five data sets for each solute are remarkably similar. Our results show that water molecules in the first hydration shell around cyclohexane bind weakly. In contrast, water molecules bind more strongly and in a hydrogen-bonding orientation to the faces of the benzene ring. The high-resolution maps of the water structure surrounding benzene and cyclohexane presented in this paper reveal details of the positional preferences of water not resolved by other methods. They also show that the hydration structure at this level of detail is remarkably insensitive to the potential energy functions and simulation methods used.