Computational evaluation of phosphonium ILs as CO2 absorbents for electrochemistry

Since phosphonium-based ionic liquids (ILs) are suitable electrolytes for electrochemical reduction, a computationally informed analysis of these ILs was conducted to assess their ability to absorb CO₂. In this work, theoretical and experimental approaches were mixed with a focus on gas solubility, transport properties, and structural characterization. The calculations included free energy of solvation (ΔG), the Henry constant (K_h), and short-range energies (Coulomb plus Lennard–Jones). Two ILs, [P₁₄₄₄][TFSI] and [P₁₄₄₄][FSI], were selected for further evaluation because of their lower K_h values. The enthalpy and entropy of the solvation process for these selected ILs were computationally determined; both ILs were found to have a favourable enthalpy of solvation for gas dissolution, and [P₁₄₄₄][TFSI] had a lower penalty for solvation (less negative entropy). Moreover, the analysis revealed competition among the ions interacting with the gas. The structural characterization of the [P₁₄₄₄][TFSI] and [P₁₄₄₄][FSI] with the CO₂ systems involved radial and spatial distribution functions. The gas was likely enveloped by an anion cage, and oxygen and fluorine exhibited greater interactions with the carbon atom of CO₂ than nitrogen. The viscosity and density were also calculated for the two systems, and the addition of CO₂ only caused a slight change on these values. Experiments on viscosity and density confirmed the computational results; here, compared with those of neat ionic liquids, an approximate 4 % decrease in the viscosity of CO₂-saturated systems was observed. Finally, the actual CO₂ solubility in [P₁₄₄₄][TFSI] was experimentally determined to be 120 mM; this value was nearly four times greater than those of typical aqueous bicarbonate solutions.