Ionic Liquids (ILs) have emerged as promising solvents for Carbon Capture and Utilization (CCU) due to their negligible volatility, low corrosiveness, high thermal stability, tunable CO2 selectivity, and energy-efficient regeneration. While extensively studied for concentrated CO2 sources, their potential for Direct Air Capture (DAC) is still in its early stages of exploration. This work examines a set of seven ILs identified based on CO2 chemisorption across partial pressures (pCO2) from ambient to flue gas concentrations. We present CO2 absorption isotherms as a function of CO2 partial pressure and temperature, reaction product characterization using ¹H and ¹³C NMR spectroscopy, and key thermodynamic parameters and potential structural markers that contribute towards enhanced CO2 capacity for DAC. ILs were synthesized using ion exchange and neutralization followed by solvent removal. A dynamic gas breakthrough bubbling column coupled with CO2 analyzer was designed for evaluating precise uptake measurements at lower pCO2, while a pressure decay cell apparatus was employed for higher pCO2. The isotherms were fitted using Mixed Linear-Langmuir (MLL) model, capturing both CO₂ solubility and reactivity characteristics in ILs. Thermodynamic analysis conducted through Van’t Hoff plots, enabled the calculation of absorption enthalpies, which exhibited exothermic interactions across all ILs, with more negative heats of absorptions indicating stronger chemical interactions. 13C NMR identified formation of amine-carbamates, azolate-carbamates, and imidazolium-carboxylates as the different ILs during CO2 chemisorption. The DAC suitability of these ILs - and potentially new ILs suggested from our findings in the present set of ILs - is discussed.
