Topological and spatial aspects of the hydration of solutes of extreme solvation entropy

The hydration of charged Lennard-Jones spheres by simple point charge water is considered. Molecular dynamics and expanded ensemble simulations were used to compare the hydration structures surrounding solutes with extreme solvation entropy. The variations in the solvation entropy were analyzed in terms of changes in the spatial and topological structure of the hydration shells. The solvation entropy was found to be maximal for solutes that can replace water molecules in the hydrogen-bond network. Further, using a Kirkwood-type factorization, the solvation entropy was expanded as a sum over the partial n-body distribution functions. The two-body solute-water contribution to the solvation entropy was found to exceed the full solvation entropy for solutes with low charge, whereas the converse is true for the other solutes. This is consistent with the idea that water-water correlations are enhanced by solvation of, for example, noble gases, whereas they are disrupted by solvation of ions. Further, the orientational and radial parts of the two-body solute-water entropy were calculated as functions of the charge of the solute. The orientational part has a single maximum, whereas the radial part maintains the bimodal form of the full solvation entropy.