Molecular mechanism of CO2 and SO2 molecules binding to the air/liquid interface of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: a molecular dynamics study with polarizable potential models.

Molecular dynamics simulations with many-body interactions were carried out to understand the bulk and interfacial absorption of gases in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). A new polarizable molecular model was developed for BMIMBF4, which was found to give the correct liquid density but which also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO(2) and SO(2) was calculated across the air-BMIMBF4 interface, and the bulk free energies were calculated with the free-energy perturbation method. A new polarizable model was also developed for CO(2). The air-BMIMBF4 interface had enhanced BMIM density, which was mostly related to its butyl group, followed by enhanced BF4 density a few angstroms toward the liquid bulk. The density profiles were observed to exhibit oscillations between high BMIM and BF4 density indicating the presence of surface layering induced by the interface. The potential of mean force for CO(2) and SO(2) showed more negative free energies in regions of enhanced BF4 density, while more positive free energies were found in regions of high BMIM density. Moreover, these gases showed free-energy minimums at the interface, where the BMIM alkyl groups were found to be most prevalent. Our results show the importance of ionic liquid interfacial ordering for understanding gas solvation in them.