Quantum chemical study of correlated motion in salt/water electrolytes

Aqueous electrolyte solutions are becoming increasingly important in energy devices such as capacitors and rechargeable ion batteries. The energy conversion process in such devices is linked to the effect of ions on the physical properties of aqueous solutions. Here, we have performed quantum chemical molecular dynamics (MD) simulations for aqueous solutions of KCl, NaCl, and MgCl₂ at various concentrations from salt in water (SIW) to water in salt (WIS) conditions, using the density-functional tight-binding (DFTB) method. DFTB is a quantum chemical method that is well known for its good tradeoff between accuracy and computational efficiency. Comparisons with experimental data and published ab initio DFT results confirm that DFTB can be reasonably used to study the solvation shell of ions in an aqueous salt solution. We then proceeded to compute the time-dependent pair correlation function, also known as the Van Hove function. Our results predict a decay of the hydration shell and salt-dependent cation-water correlated motion, with Mg-O correlation lasting longer and Na-O decaying fastest. In the SIW case, the total Van Hove Function feature is dominated mainly by water-water correlation motion. In contrast, in the case of WIS, the water-water correlation is essentially lost.