A comparative study of ammonia solubility in imidazolium-based ionic liquids with different structural compositions

Four imidazolium-based ionic liquids (ILs) with two cations 1-pentyl-3-butylimidazolium [PBIM]⁺ and 1-benzyl-3-butylimidazolium tetrafluoroborate [BzBIM]⁺, and two anions tetrafluoroborate (BF₄⁻) and trifluoromethanesulfonate (OTf⁻) were synthesized for NH₃ solubility enhancement. The structural, thermal, and electrochemical stabilities, ionic conductivity, and viscosity of the four ILs, namely, [PBIM]BF₄, [BzBIM]BF₄, [PBIM]OTf, and [BzBIM]OTf, were investigated. Due to the intermolecular interaction of the benzyl group attached to the imidazolium ring, [BzBIM]⁺-based ILs exhibited higher thermal stability but lower ionic conductivity compared to [PBIM]⁺-based ILs. Further, the NH₃ solubility in all ILs was measured using a custom-made setup at temperatures ranging from 293.15 to 323.15 K and pressures ranging from 1 to 5 bar. The effects of the cation and anion structures of ILs, as well as pressure and temperature, on the NH₃ solubility in the ILs were also investigated. [PBIM]BF₄ showed the best solubility because of its high free volume and low viscosity. Density functional calculations validated the superior NH₃ solubility in [PBIM]BF₄, attributable to the minimal reorganization of the [cation]anion complex geometry during the solvation process, yielding a low solvation free energy. The findings of this study suggest that ILs exhibit a high NH₃ solubility capacity and cation and anion structures considerably affect the NH₃ solubility in ILs.