Molecular simulation study of CO₂ and N₂ absorption in a phosphonium based organic ionic plastic crystal

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P_122i4][PF₆], was investigated for CO₂ and N₂ absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO₂ compared with N₂. The CO₂ absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N₂ showed low absorption at both temperatures. The simulations suggest that the significant increase in CO₂ absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO₂ separations from CO₂/N₂ mixture.