Molecular Modeling and Simulation Study of the Structure and Dynamics of Confined Ionic Liquids and Their Performance in Gas Separations

It has been known that nanoconfinement of ionic liquids (ILs) significantly alters their thermodynamic and transport properties, and there is a lot of technological interest in exploiting molecularly confined ILs. However, there has been little research on how confinement induces changes in the structure and dynamics of ILs as well as sorption and diffusion behavior of solutes dissolved in ILs. To our knowledge, there have been no investigations on the permeability (P) defined as the product of solubility (S) and diffusivity (D), and permeability selectivity (β^P) defined as the ratio of permeabilities of gases of interest in confined ILs, which are very important properties for materials to be used for the gas separation processes.

In this work we have used molecular dynamics (MD) simulations to investigate the dynamics of the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim]⁺ [Tf₂N]^-) in bulk and under confinement in graphite slit pores of size 2 nm and 5 nm. Additionally, Gibbs ensemble Monte Carlo (GEMC) simulations were used to compute the pure and mixed gas solubilities of gases such as CO₂ , CH₄ and H₂ in different IL phase and also in empty pores. Also the dynamics of IL-gas mixtures were investigated in the bulk and confined IL systems. The results thus obtained were used to compute the pure and mixed gas solubility selectivities, diffusion selectivities and permselectivities. The confined IL systems were shown to have better gas solubilities than the bulk IL and better CO₂ solubility selectivities over CH₄ and H₂ than the bulk IL and empty pores. Additionally, CO₂ permselectivities over CH₄and H₂ in confined IL systems were either similar or marginally better than those in the bulk IL. The simulations suggest that the concept of using confined ILs for gas separation has merit and further research should be directed at systems that have higher permselectivities.