Investigating intermolecular interactions among CO2, 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 CO₂ capture. In this study, we apply cryo ToF-SIMS to probe interactions between CO₂, water, and PEEK-ionene membranes-a promising material for direct CO₂ capture due to its selectivity, durability, and efficiency. Despite this potential, the mechanisms governing CO₂ diffusion and the influence of water vapor on CO₂ behavior remain unclear. To address this, we loaded PEEK-ionene membranes with ¹³CO₂ and D₂O and employed cryo ToF-SIMS to visualize the 3D distribution of CO₂ and water within the membrane. While prior studies suggest that ¹³CO₂ is absorbed under ambient conditions, our cryo ToF-SIMS analysis revealed no enhancement of the ¹³C/¹²C ratio, suggesting weak CO₂-membrane interactions. As a result, CO₂ vaporizes even at low temperatures (-140°C) under vacuum conditions. In contrast, D₂O displayed a relatively homogeneous distribution in the membrane, suggesting stronger water-membrane interactions via hydrogen bonding (18-20 kJ/mol). Interestingly, CO₂ was not detected in D₂O-loaded membranes, indicating minimal interference from water vapor on CO₂ diffusion. As a comparison, the cryo ToF-SIMS data show that ¹³CO₂ can readily react with a basic Na₂CO₃ aqueous solution to form NaH¹³CO₃. These findings demonstrate cryo ToF-SIMS as a critical technique for understanding gas-water-membrane interactions, offering insights for membrane functionalization to improve CO₂ capture efficiency.