Ionic Liquid with Silyl Substituted Cation: Thermophysical and CO 2 /N 2 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−C 1 −C 3 ‐mim] + ), and bis(trifluoromethylsulfonyl)imide ([NTf 2 ] − ) 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][NTf 2 ]) and siloxane ([(SiOSi)C 1 mim][NTf 2 ]) side chains. It was found that at 25 °C [Si−C 1 −C 3 ‐mim][NTf 2 ] 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][NTf 2 ] (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 [NTf 2 ] anion can be considered as relatively weak. Finally, [Si−C 1 −C 3 ‐mim][NTf 2 ] was used for the preparation of polymer supported ionic liquid membranes (SILMs) and their CO 2 and N 2 permeation properties at 20 °C and 100 kPa were determined: permeability P CO 2 =311, P N 2 =12 Barrer, diffusivity D CO 2 =115×10 12 , D N 2 =227×10 12  m 2  s −1 and CO 2 /N 2 permselectivity αCO 2 /N 2 =25.3.