A new insight into ionic liquid-water mixtures used as absorbent-refrigerant pairs: Theoretical and potentiometric aspects

Abstract

The interactions and solvation features in ionic liquid-water mixtures were studied for the two ionic liquids 1-octyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride using density functional theory (DFT) methods at the B3LYP/6-311++G (d, p) level. Mulliken atomic charges were obtained for both ionic liquids in the gas and solvent phases. Results indicated that the Mulliken charges calculated in the gas phase change meaningfully in the solvent phase. Additionally, the atomic charges of the cation, especially the imidazolium ring, in the presence and absence of the anion differ from each other. Frontier molecular orbital (FMO) analysis of the isolated ionic liquids revealed that HOMO is delocalized on the Cl− anion, while LUMO is mainly delocalized on the imidazolium ring even though the anion has a small contribution to LUMO. In order to examine the microsolvation of the ionic liquids by water, 65 IL-water complexes were generated and optimized with different arrangements of water molecules around ionic liquid. Our findings show that the most stable structure is the one with five water molecules located at the top of the imidazolium ring and two water molecules resided at the bottom of the ring. The activity coefficients of the ionic liquids in water were also determined at temperatures from 298.15 to 328.15K by the emf measurements using the cell described as: Ag/AgCl|saturated KCl solution|test solution|ISE membrane|0.01molkg−1 IL solution|Cu wire. The obtained activity coefficients were used to calculate the osmotic coefficients and excess Gibbs free energies of the IL-water mixtures.