Solvation structures of Na⁺ Cl⁻ ion pair in DMF-water and DMF-methanol mixtures

We have studied the solvation structures of Na⁺ Cl⁻ ion pair in DMF (dimethyl formamide)-water and DMF-methanol binary mixtures using constrained molecular dynamics technique. The resulting potentials of mean force shows prominent minima corresponding to contact ion pairs (CIPs) and solvent assisted ion pairs (SAIPs) in all mole fractions of DMF except in pure DMF. The stabilities of CIPs increase with an increase in mole fraction of DMF in both the binary mixtures. The CIPs in DMF-methanol mixtures are more stable than the CIPs in DMF-water mixtures. The formation of the ion pair is found to be entropically driven in both the mixtures. Preferential solvation analysis through excess coordination numbers shows that Na⁺ and Cl⁻ are preferentially solvated by water in DMF-water and by methanol in DMF-methanol binary mixtures. The rate of dissociation of the ion pair is larger in DMF-water mixtures than in DMF-methanol mixtures.