Molecular dynamics simulations of Na + and Cl − ions solvation in aqueous mixtures of formamide

Abstract

Molecular dynamics computer simulations have been performed for Na +/Cl −-water-formamide mixtures over the whole concentration range of the mixed solvent. It is shown that both Na + cation and Cl − anion are preferentially solvated by formamide even if only small amounts of formamide are present in the mixture. Translational diffusion and vibrational motions of ion solvation shell molecules correlate strongly with the local composition of solvent near the ions. Librational motion of solvate molecules is determined rather by H-bonding than spatial solvent arrangements. The ionic preferential solvation phenomena manifest themselves in the reduction of the average number of H-bonds of the solvation shell molecules compared to bulk solvent molecules. The mobility of the Na + cation depends strongly on the immediate environment of the ion, whereas for Cl − mainly solvation shell-bulk processes control the dynamics of ion movement. It is concluded that the ionic mobility is determined to a large extent by first shell processes and by molecular motions in the time scale below the time of Debye relaxation.