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Ca 2+ -Cl − Association in Water Revisited: the Role of Cation Hydration

Abstract : We investigate the dissociation of a Ca2+-Cl− pair in water using classical molecular dynamics simulations with a polarizable interaction potential, parameterized from ab initio calculations. By computing the potential of mean force as a function not only of the interionic distance but also of the coordination numbers by water molecules, we show that it is necessary to use a collective variable describing the cation hydration in order to capture the dissociation mechanism. In the contact ion pair, the Ca2+ cation has a first coordination sphere containing 5 or 6 water molecules. The minimum free-energy path for dissociation involves a two-step process: First one or two additional water molecules enter the cation coordination shell, increasing the coordination number up to 7 with an almost fixed interionic distance. Then the dissociation of the ionic pair occurs at this fixed coordination number.
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Mathieu Salanne, Sami Tazi, Rodolphe Vuilleumier, Benjamin Rotenberg. Ca 2+ -Cl − Association in Water Revisited: the Role of Cation Hydration. ChemPhysChem, Wiley-VCH Verlag, 2017, 18 (19), pp.2807 - 2811. ⟨10.1002/cphc.201700286⟩. ⟨hal-01634885⟩

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