A molecular dynamics study of an aqueous NaCl-solution

Abstract

In this molecular dynamics study on an aqueous NaCl solution a central force model for water was employed. The pair potentials between oxygen-oxygen, oxygen-hydrogen and hydrogen-hydrogen were taken from the latest version of the central force water given by Stillinger and Rahman [1]. The ion—oxygen and ion—hydrogen pair potentials were adjusted to ab initio calculations of Kistenmacher, Popkie and Clementi [2]. The basic periodic box with a sidelength of 18.42 Å contained 200 oxygen atoms, 400 hydrogen atoms, 8 sodium and 8 chloride ions equivalent to a 2.2 molal solution. The stimulation was extended over 3500 time steps of 4 × 10 −16 seconds to a total time of 1.4 picoseconds. Structural properties are obtained and discussed, e.g. the different radial distribution functions, the orientation of the water molecules in the hydration shell of the ions and the influence of the ions on water geometry. These properties are compared to those of an earlier simulation of an aqueous NaCl solution with the rigid ST2-water model. Especially the double peak in the range of the first maximum of the experimental ▪ Fig. 1. Comparison of the structure function from an X-ray investigation of a 2 molal NaCl solution (dotted) [4] with the structure functions derived from MD simulations of a 2.2 molal NaCl solution with CF-water (Full) and ST2 water (dashed) [3]. ▪ Fig 2. Normalized velocity autocorrelation function for all hydrogens in a 2.2. molal NaCl solution from a simulation with CF water. structure the velocity autocorrelation function of the hydrogens (Fig. 2) hindered translational, librational and vibrational frequencies are derived and differences in these frequencies between the hydration water of the ions and bulk water are discussed.