Molecular dynamics simulations of structures, dynamics, competitive interaction mechanisms for CH4, CO2, and SO2 in ionic liquid mixtures of [Cnmim][Nf2T] and [Cnmim][BF4]

Here, we performed a series of molecular dynamics (MD) simulations to investigate the structures, dynamics, and competitive interaction mechanisms of CH4, CO2, and SO2 in binary imidazolium-based ionic liquid (IL) mixtures of [Cnmim][Nf2T]0.5[BF4]0.5 (n = 2, 4, and 6). Our simulation results demonstrate that CH4 molecules tend to be even more localized around the alkyl chains of cations and such trend can be obviously enhanced as the chain length increases. However, both CO2 and SO2 molecules prefer to be localized around the anions and the [BF4]– anions are found to show more advantages than the [Nf2T]– ones in adsorbing CO2 and SO2 molecules due to their stronger Lewis acid–Lewis base (LA–LB) interactions. On the other hand, the addition of gas molecules is capable of accelerating the diffusions and rotations of ions in IL mixture–gas systems. Although the diffusion order of gas molecules always follows the order of CH4 > CO2 > SO2, the counterpart of both cations and anions displays an opposite trend in all IL mixture–gas systems. Our results further reveal that polar SO2 molecules have stronger LA–LB with anions and hydrogen bonds (HBs) with cations than CO2 and CH4 molecules, leading to faster diffusions of ions in IL mixture–SO2 systems. Besides, it is also found that the addition of gas molecules would reduce the cation–anion HB strength and thereby it can accelerate the rotational motions of cations and anions in IL mixture–gas systems.