Calcium lignosullonate (CaLS) is an organic anionic surfactant, which has been used for the first time as a charged additive to improve the performance of Pebax membranes for CO2 separation. Owing to the abundant hydrophobic groups and low charge density of lignosullonate ion (LS2), CaLS was found to efficiently interrupt the chain packing of Pebax 1657 matrix via both metal-organic complexation and hydrophobic interaction, endowing membranes with tunable fractional free volume (FFV). For dry membranes, CaLS could slightly enlarge membrane FFV without forming highly crosslinked structures. For humidified membranes, water-induced swelling led to a remarkable enhancement of FFV, while the hydrophobic interaction between CaLS and Pebax avoided excessive swelling. With the coexistence of CaLS and water, optimal FFV and moderate salting-out effect were acquired, resulting in very high CO2 permeability (3585 Barrer) and fairly good CO2/gas selectivity (29 for CO2/CH4 and 71 for CO2/N-2). The separation mechanism was discussed by analyzing the relationship between salting-out effect and the well-known solution-diffusion mechanism. Based on the performance comparison of Pebax-CaLS membrane with the previously reported Pebax-salt membranes, the guideline about the design of polymer electrolyte membranes for CO2 capture was tentatively proposed. (C) 2015 Elsevier B.V. All rights reserved.