Nanostructure and interactions in ionic liquids with carbon dioxide: Understanding cavity formation and solvent reorganization

Carbon dioxide (CO2) capture and storage using ionic liquids (ILs) is a promising research area within the broader issue of greenhouse-gas emissions and climate change. Here, we conducted a systematic molecular dynamics study to identify structural features that optimize CO2 solubility in ILs. A total of 24 ILs, based on 1-alkyl-3-methylimidazolium cations with varying alkyl chain lengths and paired with six different anions, were examined. Simulations were performed at 300 K and 1 atm, across 0%, 10%, and 30% CO2 mole fractions. The void probability distribution functions revealed remarkable morphological similarities among the systems, with a strong correlation between the anion molar volume and the total number of voids in the liquid. The void network topology showed that spherical anions such as [BF4]- and [PF6]- promote narrow-channel connectivity, while bulkier anions significantly modify the interstitial network connecting the voids. Radial distribution functions highlighted that CO2 is preferentially located near polar domains, particularly around anions, with minimal disruption to the overall packing. These localized perturbations in the IL structure were more pronounced for smaller anions, which are more tightly coordinated with the cations. Interaction energy analysis revealed that extending the cation alkyl chains does not enhance CO2 solubility in molar terms, as the gas interacts mainly with polar regions. Taken together, our findings highlight the importance of dispersion interactions in CO2 uptake and suggest that ILs incorporating soft, bulky anions may be promising candidates for carbon capture, offering a balance between increased free volume near the polar domains and manageable viscosity.