The structure,dynamics and solvation mechanisms of ions in water from long time molecular dynamics simulations: a case study of CaCl2 (aq) aqueous solutions

Systematic long time (5–20 ns) molecular dynamics (MD) simulations have been carried out to study the structural and dynamical properties of CaCl₂ aqueous solutions over a wide range of concentrations (≤9.26 m) in this study. Our simulations reveal totally different structural characteristics of those yielded from short time (≤1 ns) MD simulations [A.A. Chialvo and J.M. Simonson, J. Chem. Phys. 119, 8052 (2003); T. Megyes, I. Bako, S. Balint, T. Grosz, and T. Radnai, J. Mol. Liq. 129, 63 (2006)]. An apparent discontinuity was found at 4–5 m of CaCl₂ in various properties including ion–water coordination number and self-diffusion coefficient of ions, which were first noticed by Phutela and Pitzer in their thermodynamic modelling [R.C. Phutela and K.S. Pitzer, J. Sol. Chem. 12, 201 (1983)]. In this study, residence time was first taken into consideration in the study of Ca²⁺–Cl^? ion pairing, and it was found that contact ion pair and solvent-sharing ion pair start to form at the CaCl₂(aq) concentrations of about 4.5 and 4 m, respectively, which may be responsible for the apparent discontinuity. In addition, the residence time of water molecules around Ca²⁺ or Cl^? showed that the hydration structures of Ca²⁺ and Cl^? are flexible with short residence time (<1 ns). It needs to be pointed out that it takes much longer simulation time for the CaCl₂–H₂O system to reach equilibrium than what was assumed in previous studies.