Molecular dynamics simulation study of the negative hydration effect in aqueous electrolyte solutions

To study the structure breaking effect in aqueous electrolyte solutions (also termed negative hydration), a series of molecular dynamics simulation runs has been carried out. An originally uncharged, nonpolar spherical solute particle (“Xenon”) is charged step by step from g 1 = 0.0 e to q 1 = +0.67 e , + 1.0 e and q 1 = +2.0 e ( e is the positive elementary charge). At zero charge the occurrence of hydrophobic hydration is observed. The surroundings of the divalent cation also shows marked structuring: the normal “positive” ionic hydration. In an intermediate region of charge (at q 1 = 1.0 e ) negative hydration is detected, which proves to be a state of minimum order. Structural changes are discussed by using pair and three particle correlation functions. Hydration energies, binding energies and pair interaction energy distributions are determined. The microdynamics is studied by observing the reorientational motion and the self diffusion behaviour of the water molecules in different regions. A zone of increased mobility is located in the vicinity of the ion, which may include the innermost hydration shell. Additionally a system comprising a negatively hydrated anion has also been studied.