Thermodynamic origins of the interfacial-bulk solubility trade-off for CO₂ in ionic liquids: a molecular dynamics...

Efficient CO₂ capture is essential to industrial decarbonization. Ionic liquids (ILs) are promising solvents for this purpose due to their tuneable structures and selective interactions with CO₂. In this work, classical molecular dynamics simulations of two ILs, [BMIM][BF4] and [BMIM][NTF2], reveal an interesting trade-off between interfacial adsorption and bulk solubility. [BMIM][BF4] exhibits pronounced CO₂ surface enrichment, while [BMIM][NTF2] shows weaker interfacial adsorption but greater bulk uptake. The difference originates from distinct thermodynamic driving forces at the interface versus in the bulk. Interfacial adsorption is primarily enthalpically driven, with CO₂ experiencing stronger interactions at the [BMIM][BF4] surface (-17.7 kJ mol^-1) than at the [BMIM][NTF2] surface (-12.4 kJ mol^-1). In contrast, bulk solubility is governed by the balance between enthalpic stabilization and entropic penalty. [BMIM][NTF2] shows a reduced entropic penalty (-0.035 kJ mol^-1 K^-1vs. -0.052 kJ mol^-1 K^-1 for [BMIM][ BF4]), resulting in a slightly more favourable solvation free energy (-2.1 kJ mol^-1vs. -2.0 kJ mol^-1) and higher overall CO₂ capacity. Free volume analysis supports the greater structural adaptability of [BMIM][NTF2], with the flexible NTF2 anion enabling better structural relaxation for CO₂ accommodation. These results demonstrate that the anion structure profoundly influences CO₂ absorption characteristics: compact anions (e.g., [BMIM][BF4]) promote enthalpy-driven surface capture, whereas bulky, charge-delocalized anions (e.g., [BMIM][NTF2]) favour entropy-relieved bulk absorption with lower energy costs. Interfacial adsorption may offer advantages over bulk dissolution by circumventing viscosity-related mass transfer limitations and reducing energy requirements for solvent regeneration.