Anionic surfactant-doped Pebax membrane with optimal free volume characteristics for efficient CO2 separation

Calcium lignosulfonate (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 CO 2 separation. Owing to the abundant hydrophobic groups and low charge density of lignosulfonate ion (LS 2− ), 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 CO 2 permeability (3585   Barrer) and fairly good CO 2 /gas selectivity (29 for CO 2 /CH 4 and 71 for CO 2 /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 CO 2 capture was tentatively proposed.