Anion Effects on Interfacial Absorption of Gases in Ionic Liquids. A Molecular Dynamics Study

Molecular dynamics simulations with many-body interactions were carried out to systematically study the effect of anion type, tetrafluoroborate [BF(4)] or hexafluorophosphate [PF(6)], paired with the cation 1-butyl-3-methylimidazolium [bmim], on the interfacial absorption of gases in room temperature ionic liquids (RTILs). The potentials of mean force (PMF) of CO(2) and H(2)O at 350 K were calculated across the air-liquid interfaces of [bmim][BF(4)] and [bmim][PF(6)]. We found that the PMFs for H(2)O exhibited no interfacial minima at both interfaces, while the corresponding PMFs for CO(2) had significant free energy minima there. However, the PMFs for H(2)O showed a much higher interfacial free energy than in the bulk for [bmim][BF(4)], but only a slightly higher interfacial free energy for [bmim][PF(6)] than in bulk. The reason for this was due to the more hydrophilic nature of the [BF(4)] anion, and the fact that [BF(4)] was found to have little propensity for the interface. Our results show that H(2)O is much more likely to be found at the air-[bmim][PF(6)] interface than at the air-[bmim][BF(4)] interface. The free energies of solvation were found to be more negative for [bmim][BF(4)] than [bmim][PF(6)] for water and similar for CO(2). This observation is consistent with experimental Henry's law coefficients. Our results show that anion type, in addition to affecting the free energy of solvation into RTILs, should also significantly influence the uptake mechanism.