Structure and Dynamics of Hydrofluorocarbon/Ionic Liquid Mixtures: An Experimental and Molecular Dynamics Study

The physical properties of four ionic liquids (ILs), including 1-n-butyl-3-methylimidazolium tetrafluoroborate ([C₄C₁im][BF₄]), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁im][PF₆]), 1-n-butyl-3-methylimidazolium thiocyanate ([C₄C₁im][SCN]), and 1-n-hexyl-3-methylimidazolium chloride ([C₆C₁im][Cl]), and their mixtures with hydrofluorocarbon (HFC) gases HFC-32 (CH₂F₂), HFC-125 (CHF₂CF₃), and HFC-410A, a 50/50 wt % mixture of HFC-32 and HFC-125, were studied using molecular dynamics (MD) simulation. Experiments were conducted to measure the density, self-diffusivity, and shear viscosity of HFC/[C₄C₁im][BF₄] system. Extensive analyses were carried out to understand the effect of IL structure on various properties of the HFC/IL mixtures. Density, diffusivity, and viscosity of the pure ILs were calculated and compared with experimental values. The good agreement between computed and experimental results suggests that the applied force fields are reliable. The calculated center of mass (COM) radial distribution functions (RDFs), partial RDFs, spatial distribution functions (SDFs), and coordination numbers (CNs) provide a sense of how the distribution of HFC changes in the liquid mixtures with IL structure. Detailed analysis reveals that selectivity toward HFC-32 and HFC-125 depends on both cation and anion. The molecular insight provided in the current work will help the design of optimal ILs for the separation of azeotropic HFC mixtures.