Atomistic simulation of gas uptake and interface-induced disordering in solid phases of an organic ionic plastic crystal

Organic ionic plastic crystals (OIPCs) are a unique class of materials that exhibit a short-range disorder on the molecular level but are ordered at higher length scales. Recent experiments in our group have shown that the OIPC methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P122i4][PF6]) exhibits a high ideal selectivity of 30 with respect to CO2 and N2 at 35 °C. Here, we employ classical molecular dynamics simulations for studying gas uptake in the OIPC [P122i4][PF6] at different temperatures. Both adsorption and absorption of the gases CO2, N2, O2, and CH4 were estimated using a gas/solid interface model. The observed trend in gas uptake was CO2 > CH4 > O2 > N2. The CO2 uptake was found to be dependent on both the OIPC structure and temperature. Owing to phase transitions and intermolecular motions, the solid gets disordered with increasing temperatures. The study finds that such disordering effects can also be caused by interface effects, which can enhance gas absorption. The results qualitatively confirmed that the OIPC can be effective in separating CO2 not only from N2, as per the previous experimental study, but also from O2 and CH4. However, strategies to improve the free volume in the material become important.