Structure and Dynamics of NaCl Ion Pairing in Solutions of Water and Methanol

Ion pairing can have profound effects upon the ionic strength of electrolyte solutions but is poorly understood in solutions containing more than one solvent. Herein a combined density functional theory and molecular dynamics approach is used to examine the effect of both methanol concentration and interionic distance upon the structure and dynamics within successive solvation shells of Na+ and Cl- in water/methanol binary solutions. The structure and dynamics of the first and second solvation shells were studied along a reaction coordinate associated with ion pair formation using potential of mean force simulations. The lifetimes of the solvent-solvent hydrogen bonds become perturbed when the second solvation shells of the ions begin to interact. In contrast, the structural properties within the first and second solvation shells of the ions were found to be largely independent of both methanol concentration and interionic distance until a contact ion pair is formed. Thus, as the ions are brought together, the effect of the opposing ion manifests itself in the solvation dynamics before any structural changes are observed. As anticipated based upon the decreased dielectric constant of the binary solution, ion pair formation becomes energetically more favorable as the concentration of methanol increases.