Evaluating the role of anion structure in the physisorption contribution to CO2 solvation in [BMIm]-based systems: A molecular dynamics study

Abstract

The urgent need for sustainable carbon capture has established imidazolium-based ionic liquids (ILs) as revolutionary solvents. However, the specific role of physisorption in their capture mechanisms remains largely unexplored. This study uses molecular dynamics simulations and the Bennett acceptance ratio method to analyze the thermodynamic and kinetic properties of CO2 in [BMIm][HCOO], [BMIm][OAc], and [BMIm][C3H5O2] at temperatures between 300 and 400 K. Our findings reveal that [BMIm][HCOO] is the thermodynamic frontrunner, exhibiting the strongest affinity for CO2 with a Henry’s law constant of just 69 bar at 300 K and a substantial physisorption enthalpy of −15.42 kJ/mol. A key finding of this study is that physisorption accounts for around 40% of the total CO2 capture process in acetate and propionate systems, highlighting its significant role in solvation. Furthermore, our data reveal a significant kinetic trade-off: while the formate system demonstrates superior binding strength, [BMIm][OAc] exhibits enhanced diffusion and permeability rates, which are crucial for dynamic membrane applications. We also demonstrate that CO2 solubility is spontaneous below ∼350 K but decreases sharply as temperatures rise, confirming the endothermic nature of the dissolution process. By detailing how anion chain length and Coulombic interactions dictate properties, such as structural flexibility and heat capacity, this study provides a vital blueprint for the rational design of high-efficiency, sustainable ILs for industrial carbon mitigation.