Solvation Structures and Ion Dynamics of CaCl$_2$ Aqueous Electrolytes Using Metadynamics and Machine Learning Molecular Dynamics Simulations

Abstract: The solvation structures and ion dynamics of CaCl$_2$ aqueous electrolytes have been investigated using ab initio molecular dynamics simulations and molecular dynamics simulations with deep learning potentials. We found multiple solvation structures around the Ca$^{2+}$ ion, including fully hydrated single Ca$^{2+}$ ion, Ca-Cl contact ion pair, and Ca-2Cl bridged ion pair, could coexist. The ion-pairing condition plays an important role in the translational and orientational distribution of water molecules in the solvation shell. And the local ordering introduced by the Ca$^{2+}$ ion can extend to the second solvation shell. Furthermore, we found the lifetime of water molecules in the solvation shell is sensitive to the ion-pairing conditions. The self-diffusivities of ions and water molecules, as calculated in molecular dynamics simulations with deep learning potentials, are in good agreement with experimental measurements. Finally, we elucidate the transition of Ca$^{2+}$ ion dynamics between different regimes by analyzing angle probability distribution histograms and van Hove correlation function.