Ion Solvation in Polarizable Chloroform: A Molecular Dynamics Study

Abstract

The structural, thermodynamic, and dynamical properties of the alkali-metal cations and the halide anions in liquid chloroform are investigated using molecular dynamics simulation techniques. From the atomic radial distribution analysis, the chloroform molecules are found to form well-defined solvation shells around the alkali-metal cations and the halide anions. The size of the solvation cage and the coordination number both increase with increasing ion size. In liquid chloroform, all these ions are shown to induce a strong orientational order in the surrounding chloroform molecules as evidenced by the angular distribution functions. We found that the mean electric potentials induced by the chloroform molecules shifted to smaller magnitudes with increasing ion size. Because of the greater electric polarizabilities of the larger ions, the average induced dipole moments were enhanced with increasing ion size. The diffusion coefficients of the alkali-metal cations and the halide anions in liquid chloroform are estimated from the mean-square displacements and the velocity autocorrelation functions. Generally, the diffusion constants of the cations are larger than those of the anions. For the cations, the diffusion constants are of similar magnitudes and do not depend on the ion size. However, the diffusion coefficients of the halide anions show a strong dependence on the ion size. The motion of the first coordination shell chloroform molecules is examined via their velocity autocorrelation functions. These correlation functions behave very similarly, suggesting that the motion of the first solvation shell is not governed by the sizes or the charges of these ions. In addition, the residence time autocorrelation functions of the first solvation shell are evaluated. As expected, the residence time decreases as the ion size increases.