Molecular dynamics simulations of aqueous electrolyte solutions

Abstract

Structural and dynamic properties of various aqueous electrolyte solutions have been calculated from MD simulations where the ST2 and Central Force model of water were employed [1]. The structural properties of the solutions are discussed on the basis of radial distribution functions, the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. An example of the latter case in given in Fig. 1 for a MgCl 2 solution [2]. The figure shows that the oxygen atoms of the six water molecules in the first hydration shell of Mg ++ are positioned at the corners of a regular octahedron, while for Cl − an octahedral arrangement only is indicated. The four nearest neighbor water molecules around a central one show a tetrahedral structure and the distributions are sharper in the hydrogen atom directions than in the lone pair directions. Dynamic properties of the solutions such as self-diffusion and rotational diffusion coefficients, reorientation times of the dipole moment vector and residence time of the water molecules in the hydration ▪ shells of the ions are calculated from the simulation through autocorrelation functions and are compared with experimental results. The spectral densities of the hindered translational and librational motions result from Fourier transformation of the corresponding autocorrelation functions. The dynamic properties can be calculated separately for the water subsystems — hydration water of the cations, the anions and bulk water — and thus provide the possibility of understanding measured macroscopic properties of solutions on a molecular level. The self-diffusion coefficients for the three kinds of water in a LiI solution are given in Table I as an example [3]. t001 Self-diffusion Coefficients for Bulk Water (D b), Hydration Water of Li + (D +) and of I − (D − in Units of 10 −5 cm 2/s from an MD Simulation of a 2.2 Molal LiI Solution at 305°K.D O denotes pure water. i D i D iD O b 2.85 ± 0.08 0.84 + 1.33 ± 0.10 0.39 − 2.67 ± 0.10 0.78