A series of amino acid ionic liquids (AAILs) composed of different cations of different sizes, including trihexyl(tetradecyl)phosphonium glycinate, tetrabutylphosphonium glycinate and tiiethyl(pentyl)phosphonium glycinate have been synthesized for the carrier of CO2 facilitated transport membrane. Their physical properties, including their viscosity, density, molar volume and N-2 absorption amount were investigated. The amount of N-2 absorption decreased as the size of the cation of the AAIL decreased. Facilitated transport membranes containing these AAILs were prepared, and their CO2 and N-2 petmeabilities were measured. The N-2 petmeabilities were systematically investigated both experimentally and theoretically with the aim of improving the selectivity of the CO2 permeation of the AAIL-based facilitated transport membrane. As expected, the CO2 permselectivity was improved using triethyl (perityl)phosphonium glycinate, which contained the smallest cation of the AAILs investigated in this study. We have proposed a methodology for improving the CO2 selectivity of AAIL-based facilitated transport membranes based on reducing their N-2 permeabilities. (C) 2013 Elsevier B.V. All rights reserved.