Amino acid ionic liquids (AAILs) composed of different sizes and numbers of amino groups, including tetrabutylammonium glycinate, tetramethylammonium glycinate, and 1,1,1-trimethylhydrazinium glycinate, were synthesized as CO2 carriers for facilitated transport membranes. The physical properties of the fabricated AAILs, such as density, viscosity, and N-2 and CO2 absorption capacities, were measured, and the fractional free volume was calculated using molecular dynamics (MD) simulations. The results showed that the molar and fractional free volumes decreased with decreasing AAIL size, which caused increased CO2 absorption and decreased N-2 absorption. In addition, the CO2 absorption increased with higher numbers of amino groups in the AAILs. The gas permeation properties of facilitated transport membranes containing the synthesized AAILs were investigated. The small AAILs with two amino groups showed better CO2 permeability and N-2 barrier properties under humid conditions. We propose that ionic liquids optimized as CO2 carriers for facilitated transport membranes under humid conditions should have high amino-group density, which could be realized by decreasing the molecular size and introducing more amino groups in the molecule. (C) 2016 Elsevier B.V. All rights reserved.