Molecular simulations of understanding the Na⁺ ion structure, dynamic and thermodynamic behavior in ionic liquids: Butyl ammonium hydrogen bisulfate and tri-butyl ammonium hydrogen bisulfate

This manuscript presents the all-atom molecular dynamics simulations to investigate intermolecular structure and solvation thermodynamics of Na+ ion in two different ammonium-based protic ionic liquids (1) Butyl Ammonium hydrogen bisulfate [BA+][HSO4−], (2) Tri-butyl ammonium hydrogen bisulfate [TBA+][HSO4−]. The ionic liquid [BA+][HSO4−] show a more coordinated behavior when compared to [TBA+][HSO4−], which is observed over the temperature range from 278 K to 348 K. Hydrogens of the cations show a hydrogen bonding interaction with oxygens of anions. The cationic [TBA+] molecules show more solvation behavior with anions when compared to the [BA+]. The Na+ ion show a strong coordination structure with [HSO4−] in [TBA+][HSO4−] when compared to the [BA+][HSO4−]. We further calculate the detailed solvation free energy (ΔG) calculations using thermodynamic integration. We found that the ΔG of Na+ is more favorable in [TBA+][HSO4−] when compared to [BA+][HSO4−] in the temperature range varying from 278 K to 348 K. With the temperature rise, we observe the more favorable solvation of Na+ in both ionic liquids. On the other hand, the solvation of Cl− becomes less favorable. Overall, this manuscript provides detailed molecular level structural and thermodynamic origins of Na+ in protic ionic liquids useful for designing and developing sustainable electrolytes for Na+ battery applications.