Clean silver nanoparticles on sugar crystals (Ag-sugar) were prepared by the plasma-ion method and were subsequently dispersed in an ionic liquid, 1-butyl-3-methyl imidazolium tetrafluroborate (BMIM+BF4-). Membranes containing Ag-sugar dispersed in BMIM+BF4- showed a mixed gas selectivity of up to 14.4 for the separation of a propylene/propane (50/50 vol%) mixture, demonstrating facilitated olefin transport. Facilitated olefin transport occurs due to carrier action of the partially positively charged surface of the silver nanoparticles, which is induced by BMIM+BF4-. The formation of this partially positive surface charge may be due to the interactions of the silver nanoparticles with the free anions of BMIM+BF4-. The formation of a partially charged surface was made evident by an increased binding energy of silver, observed using X-ray photoelectron spectroscopy (XPS). Free anion formation was monitored by using FF-Raman spectra of BMIM+BF4- Surprisingly, an extremely small amount of the activated silver nanoparticles (663.5 ppm) were required to provide good separation performance in propylene/propane mixtures. (C) 2009 Elsevier B.V. All rights reserved.