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 u sed to compare the hydration structures surrounding solutes with extreme so lvation entropy. The variations in the solvation entropy were analyzed in t erms of changes in the spatial and topological structure of the hydration s hells. The solvation entropy was found to be maximal for solutes that can r eplace water molecules in the hydrogen-bond network. Further, using a Kirkw ood-type factorization, the solvation entropy was expanded as a sum over th e partial n-body distribution functions. The two-body solute-water contribu tion to the solvation entropy was found Co exceed the full solvation entrop y for solutes with low charge, whereas the converse is true for the other s olutes. This is consistent with the idea that water-water correlations are enhanced by solvation of, for example, noble gases, whereas they are disrup ted by solvation of ions. Further, the orientational and radial parts of th e two-body solute-water entropy were calculated as functions of the charge of the solute. The orientational part has a single maximum, whereas the rad ial part maintains the bimodal form of the full solvation entropy, [S1063-6 51X(99)13110-6].