Mixtures of hydrofluorocarbons (HFCs) are the prevalent working fluids used in refrigeration systems globally. HFCs have high global warming potential and are thus being phased out. Using ionic liquids (ILs) to separate mixtures of HFCs is a vital technology under development that supports the sustainable phaseout of HFCs. There has been ample research on the thermodynamic and transport properties of IL/HFC systems but no known published work on the interfacial phenomena of IL/HFC systems. This work presents the first molecular simulation (MS) study on interfacial phenomena in IL/HFC systems, complemented with experiments. MS of the interfaces of the pure IL [C2C1im][TFSI] with vacuum and binary mixtures of the IL with one of the HFCs, R-32, R-134a, or R-125, at 295.15 K were completed. A molecular dynamics simulation in the constant normal force (NPzzAT) ensemble was used to generate a stable liquid slab of the IL/HFC mixtures, followed by an elongation of the simulation box and insertion of HFC molecules in the spaces on both sides of the liquid slab to obtain IL/HFC interfaces, followed by a 70ns long simulation. The [C2C1im][TFSI]/vacuum interface was found to be negatively charged, leading to very long-range liquid nanostructure ordering. The presence of HFCs significantly disrupted this net-negative charge of the interface and long-range ordering, leading to a decrease in surface tension. All HFCs showed strong interfacial adsorption in the IL/HFC systems, with the interfacial adsorption intensity increasing and the thickness of the adsorbed layer growing with increasing HFC composition. The orientation of molecules and ions at the IL/HFC interfaces was also studied. The qualitative trends of the interfacial tension of IL/HFC mixtures for varying HFC mole fractions and quantitative agreement with experiments for vapor-liquid equilibria properties of interest were well captured.
