Solvation structure of sodium chloride (Na+-Cl-) ion pair in acetonitrile (AN)-dimethyl formamide (DMF) mixtures

Solvation structures of Na+-Cl- ion pair are investigated in acetonitrile (AN)-dimethylformamide (DMF) isodielectric mixtures. The potentials of mean force of Na+-Cl- in the five compositions of mixtures show minima corresponding to a contact ion pair (CIP) and a solvent-shared ion pair (SShIP). The solvent-separated ion pair minima are present in lower mole fractions of AN (x(AN) <= 0.50). CIPs are found to be more stable than the SShIPs. From a thermodynamic decomposition of the potentials of mean force, we find that the formation of the ion pair is entropically driven in these compositions. The most stable CIP is in pure AN. The local solvation structures around the ion pair are analysed through the running coordination numbers, excess coordination numbers, solvent orientational distributions and density profiles. We find that both Na+ and Cl- are preferentially solvated by DMF