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

Abstract

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.