A Theoretical Study of the Hydration of Li+ by Monte Carlo Simulations with Refined Ab Initio Based Model Potentials

Abstract

Four water models that have the same analytical potential but different degrees of freedom were used to examine the hydration of Li+: (a) a polarizable and flexible molecule with constraints that account for the quantal nature of the vibration, (b) a polarizable and classically flexible molecule, (c) a polarizable and rigid molecule, and finally (d) a nonpolarizable and rigid molecule. The goal was to determine how individual molecular properties affect the correct description of the hydration of ions by comparing the structural and thermodynamic predictions for the aqueous solution as made by the different models, which ranged from a very refined one to a simple effective potential. The length of the Monte Carlo runs was large enough to ensure convergence and provide statistically meaningful results; the four models attained good agreement with the experimental data available for the hydration of Li+, as well as with the results of the most refined simulations. A well-defined first hydration shell was found. It had four water molecules whose dipoles were not aligned to the electric field of the ion because of their hydrogen-bonding with water molecules in outer shells. In the case of the most refined water model, the results showed this pattern clearly. On the other hand, the rigid nonpolarizable version produced a slightly higher hydration number and an almost complete alignment of the dipoles to the ion’s electric field. Moreover, a detailed analysis of a microscopic molecular model of hydration showed that the average intramolecular geometry of the water molecules in the first hydration shell was the same as the one for those in the bulk, whereas the electric field of the ion induced a dipole 0.2 D higher in the water molecules of the first hydration shell. The value of the bulk was recovered at the second shell, which explains the good performance of the simplest model. Thus, despite the differences found in the description of the first hydration shell between the polarizable and the nonpolarizable models, the major effect on the polarization of the water molecules resulted from the water-water interaction.