RECOVERING THE CONCEPT OF THE HYDRATED ION FOR MODELING IONIC-SOLUTIONS - A MONTE-CARLO STUDY OF ZN-2+ IN WATER

A [Zn(H2O)6]2+-H2O interaction potential has been developed by means of ab initio calculations using the 3-21G* basis sets. Monte Carlo simulations of zinc ion in water have been performed by using the potential for the zinc hexahydrate and for comparative purposes that of Clementi developed for the single ion. Both simulations have used the same conditions. Structural results derived from simulation of zinc hexahydrate give a more disrupted second shell, with two maxima for the Zn-H radial distribution function at the second shell. Results for the hydration energy of Zn2+ obtained from the hydrated ion approach (-518 kcal/mol) are much closer to the experimental estimation (-491 kcal/mol) than results obtained with the potential for the single ion (-845 kcal/mol). As a matter of fact, the main conclusion of this study is that the MC and MD simulations of ions in solution using the concept of a species beyond the single ion is not unrealistic and can be, under some conditions, an interesting alternative to be considered in the case of conflicting simulations of highly charged or transition metal ions. Advantages and limitations of this approximation are discussed.