Atomistic Simulation of Gas Uptake and Interface-Induced Disordering in Solid Phases of an Organic Ionic Plastic Crystal

Abstract

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 ([P][PF]) exhibits a high ideal selectivity of 30 with respect to CO and N at 35 °C. Here, we employ classical molecular dynamics simulations for studying gas uptake in the OIPC [P][PF] at different temperatures. Both adsorption and absorption of the gases CO, N, O, and CH were estimated using a gas/solid interface model. The observed trend in gas uptake was CO > CH > O > N. The CO 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 CO not only from N, as per the previous experimental study, but also from O and CH. However, strategies to improve the free volume in the material become important.

Predicting gas selectivity in organic ionic plastic crystals by free energy calculations

“…Previous studies on gas uptake in different OIPC phases have shown the different behavior of gas uptake at low and high temperatures both experimentally and computationally. 15,16 At selected temperatures, CO 2 absorption is more limited at 275 K than at 325 K, with the opposite trend observed for CO 2 adsorption. This is due to the change of free volume and the result of entropic effects.…”

“…At 325 K, the OIPC [P 122i4 ][PF 6 ] is partially disordered, with higher free volume compared to the lower temperature of 275 K where the structure of the OIPC is more ordered. 16 When moving a CO 2 molecule from the gas phase into the OIPC phase, the PMF decreases initially as the CO 2 molecule approaches the OIPC interface, as shown in Fig. 1b, and it reaches a negative minimum before it increases aer the gas enters the OIPC solid.…”

“…where Dm ex is the change in free energy from state a to state b, c a and c b are gas concentrations in the two states. Therefore, according to the gas concentration proles of the equilibrium system obtained from our previous MD simulations, 16 we also calculated the free energy change (Dm ex ) in the entire OIPC range, which is compared with the result of the PMF calculations in this work.…”

“…The MD simulation procedure follows our previous studies with detailed description of the force eld. 16,26 The CHARMM force eld functional was adopted for the OIPC with the parameters described in the work by Chen 32 A cutoff of 1.5 nm was used for both Lennard-Jones and real space Coulomb potentials. The long-range electrostatic energies were treated using a particle-particle particle-mesh technique as implemented in LAMMPS.…”

“…313 K (40 C) in the case of [P 122i4 ][PF 6 ], the CO 2 absorption increases signicantly in the bulk phase. 16 To compare the absorption capability of an OIPC for different gases, one can calculate the number density of the gases permeating into the bulk OIPC phase aer a period of simulation time, through molecular dynamics (MD) simulations. However, the simulation time can affect the accuracy of this method.…”