UC San Diego

The influence of highly charged cations on the structure and dynamics of the aqueous phase is investigated by performing Ab Initio Molecular Dynamics (AIMD) simulations on the Fe³⁺ and Ca²⁺ cations and the CaCl₂, FeOH²⁺ and AlOH²⁺ species all solvated by up to 64 waters. A detailed comparison between results of Fe³⁺ and a previous study of the Al³⁺ ion reveal significant changes in the hydrogen bond structure of 1st and 2nd hydration shells between the two systems. Differences are also noticed in the dynamics of the 2nd hydration shell. An orbital interaction is observed between Fe³⁺ and water that is not observed for Al³⁺. The results of the Ca²⁺ AIMD simulation show a more tetrahedral H-bonding structure relative to 3+ cations. Solvent exchange between the coordinating waters of Ca²⁺ and bulk proceeds via the associative interchange mechanism. Ambient and high temperature (near critical, 650K) simulations of the CaCl₂ system show that the Ca²⁺+ and Cl- ions exist as solvent separated ion pairs under ambient conditions while at 650K the Ca-Cl contact pairs are formed. This is accompanied by a significant disruption of the hydrogen bond structure of the solvent. The [FeOH]²⁺ and [AlOH]²⁺ aqueous species are found to display a significant labilizing effect due to the presence of the hydroxide species in the 1st hydration shell. A uniform destabilizing of all 1st shell waters is observed for the AlOH²⁺ species while for the FeOH²⁺ species some hydrating waters feel a stronger effect determined by their position relative to the OH⁻ species