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

Abstract

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.

Keywords: Bisulfate; Butyl ammonium; Electrolytes; Free energy; Ionic liquids; MD simulations; Na(+) ion; Tri-butyl ammonium.