(484f) Molecular Modeling of Gases Absorption in 1-n-Hexyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)Imide ([hmim][Tf2N]) Confined in Silica Slit Pores

An experimental group [1] recently in 2012 for the first time verify our theoretical predictions in 2010 [2] that ionic liquid under confinement could exhibit about 100 times faster self-diffusivity relative to the bulk IL.  The encouraging experimental data strongly suggest that ionic liquid properties could be modified through confinement. To further understand gases separation in IL confined in the nanoporous materials, we systematically study CO₂, H₂, N₂, and H₂O absorption in the [hmim][Tf₂N] confined in silica slit pores from molecular modeling.

Two-dimensional NP_xyT and isostress-osmotic Monte Carlo simulations were performed to compute the [hmim][Tf₂N] IL confined in silica slit pores and gases absorption in the silica confined IL at 313-373 K. A 9-3 analytical potential was used to describe the sorbate-sorbent interactions in most of the calculations. Simulations show that the apparent density of the confined [hmim][Tf₂N] in the 25 Å- 45 Å silica pores are smaller than pure bulk IL density. These results suggest that the silica slit pore confined [hmim][Tf₂N] IL is less packed relative to the pure IL. This implies that more free volume is available when the IL is confined in the silica pore. The CO₂ amounts of absorption in the 25 Å-silica confined IL is about 1.1-1.7 times larger than the CO₂ absorption in the bulk IL. This is consistent with the literature experimental data. Additionally, H₂ and N₂ amounts absorption in the IL are increased 1.6-3 times when the IL is confined. The increased CO₂, H₂, and N₂ amounts of absorption in the confined IL are partly due to the less packed IL structures in the silica pores. In contrast, the amounts of water absorption in the silica-confined IL are similar to that in the bulk IL. This is partly due to the strong hydrogen bonding like interactions between water and the [Tf₂N]- anion.

At low solute concentrations, CO₂ and H₂ molecules are absorbed close to the silica pore regions which are not accessible to the large [hmim][Tf₂N] IL molecules. This in turn leads to 2-8 times faster self-diffusivity for CO₂ and H₂ molecules in the confined IL than in the unconfined bulk IL. CO₂ Self-diffusion in the silica-confined IL using an atomistic model for the silica also exhibits similar improvement as using the 9-3 analytical potential. In contrast, water molecules are not absorbed close to the silica wall regions and the water self-diffusivity in the confined IL is only increase about 1.1-1.8 times. Water diffuses faster in the confine IL partly because the specific water-[Tf₂N]- pair interactions are broken under the confinement. Very interesting, the confined [hmim][Tf₂N] IL exhibits decreased dynamics at high temperature of 373 K, but improved dynamics at low temperature of 313 K. 

[1] Iocab, C et al. Soft Matter, 2012, 8, 289

[2.] Shi, W. et al. J. Phys. Chem. B 2010, 114, 15029