Zwitterionic imidazolium compounds containing a propylsulfonate group and an ester group at 1- or 2-position were prepared and used as the membrane materials for separating ethylene/ethane and propylene/propane mixtures. The performance of the membranes consisting of a silver salt and a zwitterionic imidazolium compound bearing an ester group was significantly affected by the location of the ester group on the imidazolium ring. The investigation of the interaction between a zwitterionic imidazolium compound and AgNO3 using the density functional theory method and FT-IR study shows that the silver ion bonded to propylsulfonate group interacts intra-molecularly with the ester group only when both ester and propylsulfortate groups are located on the same side of the imidazolium ring. (C) 2008 Elsevier B.V. All rights reserved.