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

Abstract

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.