(544a) The Flory-Huggins Parameters of Binary Ionic Liquid Mixtures with Ammonia

Phase equilibrium measurements and modeling are essential for the design of chemical processes such as separations, reactions, fluids flow, to name a few. One of the industrial processes that received increased attention in the last decade involves ionic liquids (ILs), which are low melting point salts composed entirely of ions. The field of ILs requires a more in-depth understanding of the interaction between ILs and other substances so that novel applications can be developed. In this work, our goal is to investigate the non-ideality of a binary mixture of ammonia and four imidazolium-based ionic liquids using the extended Flory-Huggins model, where both entropic (combinatorial) and enthalpic (residual) contributions to non-ideality are included. We previously reported the solubility of NH₃ in four ILs using the gravimetric method (1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate). Here, the Flory parameters (χ) of these four ILs and ammonia systems were regressed with the assumption that the χ parameter is dependent on temperature and independent of the concentration. The Flory-Huggins model with the combinatorial term only, and the extended Flory-Huggins model with both combinatorial and residual terms are compared. The results showed that the NH₃+IL systems have a strong negative deviation from the ideal solubility that cannot be solely described with entropic impacts, specifically at high ammonia concentrations in the ILs. This suggests an increase in the size of the IL molecule might improve the ammonia sorption capacity of an IL at low pressures. However, the ammonia sorption capacity could be significantly improved by considering the molecular interaction between ammonia and ILs (i.e., hydrogen bonding) in addition to the variation in size of the molecule.