Ionic Liquid with Silyl Substituted Cation: Thermophysical and CO2/N2 Permeation Properties

Novel functionalized ionic liquid (IL) combining an imidazolium‐based cation with branched alkyl chain bearing silyl group, 1‐methyl‐3‐(2‐methyl‐3‐(trimethylsilyl)propyl)imidazolium (Si?C₁?C₃‐mim]⁺), and bis(trifluoromethylsulfonyl)imide (NTf₂]^?) anion was synthesized and its thermophysical properties (density, viscosity, surface tension, surface entropy and enthalpy, thermal stability) were studied in a wide temperature range and compared with those of ILs having linear alkyl (C_n‐mim]NTf₂]) and siloxane ((SiOSi)C₁mim]NTf₂]) side chains. It was found that at 25 °C Si?C₁?C₃‐mim]NTf₂] is a liquid with dynamic viscosity of 224 cP (224 mPa s) and density of 1.32 g cm^?3. The presence of side branched alkyl chain with trimethylsilyl end‐group prevents crystallization of IL and leads to higher viscosities and lower densities in comparison with commonly known C_n‐mim]NTf₂] (n=2–4). As surface excess enthalpy was found to be in the lower end of the usual range of values for ILs, the interactions between silyl‐functionalized cation and NTf₂] anion can be considered as relatively weak. Finally, Si?C₁?C₃‐mim]NTf₂] was used for the preparation of polymer supported ionic liquid membranes (SILMs) and their CO₂ and N₂ permeation properties at 20 °C and 100 kPa were determined: permeability PCO₂=311, PN₂=12 Barrer, diffusivity DCO₂=115×10¹², DN₂=227×10¹² m² s^?1 and CO₂/N₂ permselectivity αCO₂/N₂=25.3.