Structure and dynamics of CO2 and N2 in a tetracyanoborate based ionic liquid

To gain insight into the transport behavior of gas molecules such as CO2versus N2 through ionic liquid membranes, we performed molecular dynamics simulations to investigate the structure and dynamics of CO2 and N2 gases in a tetracyanoborate based ionic liquid recently shown to exhibit high CO2/N2 permselectivity. We found that upon addition of CO2 or N2 the liquid structure does not change. CO2 or N2 molecules occupy the voids between ions and their local environments are found to be similar. Gas diffusivity is about one order of magnitude greater than that of the cation or anion. Dissolved N2 diffuses slightly faster than CO2. We hence conclude that the high permeability selectivity of CO2versus N2 observed experimentally is mainly due to the disparity in gas solubility. In other words, the much higher solubility of CO2 in tetracyanoborate-based ionic liquids such as 1-ethyl-3-methylimidazolium tetracyanoborate [emimB(CN)4] leads to their high CO2 permeability. Based on the experimental solubility and the simulated diffusivity, we obtained a permeability of 2830 barrer for CO2 in emimB(CN)4 at 300 K, in good agreement with the experimental value of 2040 barrer.