Molecular Simulation and Experimental Study of CO₂ Absorption in Ionic Liquid Reverse Micelle

Abstract

The structure and dynamics for CO₂ absorption in ionic liquid reverse micelle (ILRM) were studied using molecular simulations. The ILRM consisted of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]­[BF₄]) ionic liquid (IL) as the micelle core, the benzylhexadecyldimethylammonium ([BHD]⁺) chloride ([Cl]⁻) was the cationic surfactant, and benzene was used as the continuous solvent phase in this study. The diffusivity values of this ILRM system were also experimentally determined. Simulations indicate that there is ion exchange between the IL anion ([BF₄]⁻) and the surfactant anion ([Cl]⁻). It was also found that the [bmim]­[BF₄] IL exhibits small local density at the interface region between the IL core and the [BHD]⁺ surfactant cation layer, which leads to a smaller density for the [bmim]­[BF₄] IL inside the reverse micelle (RM) compared with the neat IL. These simulation findings are consistent with experimental results. Both our simulations and experimental results show that [bmim]­[BF₄] inside the RM diffuses 5–26 times faster than the neat IL, which is partly due to the fast <i>particle</i> diffusion for the ILRM nanodroplet (IL and surfactant) as a whole in benzene solvent compared with neat [bmim]­[BF₄] diffusion. Additionally, it was found that [bmim]­[BF₄] IL solved in benzene diffuses 2 orders of magnitude faster than the neat IL. Lastly, simulations show that CO₂ molecules are absorbed in four different regions of the ILRM system, that is, (I) in the IL inner core, (II) in the [BHD]⁺ surfactant cation layer, (III) at the interface between the [BHD]⁺ surfactant cation layer and benzene solvent, and (IV) in the benzene solvent. The CO₂ solubility was found to decrease in the order II > III ∼ IV > I, while the CO₂ diffusivity and permeability decrease in the following order: IV > III > II > I