MOLECULAR-DYNAMICS SIMULATIONS OF NA- IONS SOLVATION IN AQUEOUS MIXTURES OF FORMAMIDE( AND CL)

Molecular dynamics computer simulations have been performed for Na+/Cl --water-formamide mixtures over the whole concentration range of the m ixed solvent. It is shown that both Na+ cation and Cl(-)anion are pref erentially solvated by formamide even if only small amounts of formami de 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 solv ate molecules is determined rather by H-bonding than spatial solvent a rrangements. The ionic preferential solvation phenomena manifest thems elves in the reduction of the average number of H-bonds of the solvati on shell molecules compared to bulk solvent molecules. The mobility of the Na+ cation depends strongly on the immediate environment of the i on, 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. (C) 199 7 Elsevier Science B.V.