Effect of solvent composition on the structural and dynamical properties of sodium chloride solutions in water-methanol mixtures

Molecular dynamics (MD) simulations have been performed on sodium chloride solutions in water-methanol mixtures under ambient conditions with methanol mole fractions (xm) ranging from 0.09 to 0.87. MD simulations are performed with the flexible SPC water model and the flexible six-site model for methanol. The ions are modeled as charged Lennard-Jones spheres. The structural properties of the solutions are discussed on the basis of radial distribution functions. A chain cluster of methanol is predominant in the mixture where xm exceeds 0.7. The number of methanol molecules are higher than water molecules in the first coordination shell of sodium and chloride ions beyond xm = 0.5 and 0.48 respectively. With increasing xm, the diffusion coefficients for water (DH2O) and for methanol (DCH3OH) initially decrease. From xm = 0.29 DCH3OH increases while DH2O rises only from about 0.7. The two diffusion coefficients are equal at xm = 0.8. The deduced first and second reorientational correlation times for the OH and the dipole moment vectors for water speed up with increasing xm. Through the τ1/τ2 ratio, the reorientational motion of water molecules can be entirely ascribed to a jump. OH methanol vector reorientational occurs by a mechanism similar to that of water.