Investigating intermolecular interactions among CO(2), water and PEEK-ionene membrane using cryo ToF-SIMS and isotopic labeling.

Cryogenic time-of-flight secondary ion mass spectrometry (cryo ToF-SIMS) has emerged as a powerful tool for investigating molecular interactions, speciation, and dynamics in materials for CO(2) capture. In this study, we apply cryo ToF-SIMS to probe interactions between CO(2), water, and PEEK-ionene membranes-a promising material for direct CO(2) capture due to its selectivity, durability, and efficiency. Despite this potential, the mechanisms governing CO(2) diffusion and the influence of water vapor on CO(2) behavior remain unclear. To address this, we loaded PEEK-ionene membranes with (13)CO(2) and D(2)O and employed cryo ToF-SIMS to visualize the 3D distribution of CO(2) and water within the membrane. While prior studies suggest that (13)CO(2) is absorbed under ambient conditions, our cryo ToF-SIMS analysis revealed no enhancement of the (13)C/(12)C ratio, suggesting weak CO(2)-membrane interactions. As a result, CO(2) vaporizes even at low temperatures (-140°C) under vacuum conditions. In contrast, D(2)O displayed a relatively homogeneous distribution in the membrane, suggesting stronger water-membrane interactions via hydrogen bonding (18-20 kJ/mol). Interestingly, CO(2) was not detected in D(2)O-loaded membranes, indicating minimal interference from water vapor on CO(2) diffusion. As a comparison, the cryo ToF-SIMS data show that (13)CO(2) can readily react with a basic Na(2)CO(3) aqueous solution to form NaH(13)CO(3). These findings demonstrate cryo ToF-SIMS as a critical technique for understanding gas-water-membrane interactions, offering insights for membrane functionalization to improve CO(2) capture efficiency.