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

Organic ionic plastic crystals (OIPCs) are molecularly disordered solids, and their potential for the development of gas separation membranes has recently been demonstrated. Here, the gas absorption capability of the OIPC, diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P_122i4][PF₆]), for four gases is predicted through potential of mean force (PMF) calculations based on two methods - average force method and adaptive biasing force method. Both methods correctly predicted the different trends of adsorption and absorption of these gases across the OIPC-gas interface. The distinct energy barriers of the PMF profiles of CO₂ and N₂ near the interface directly reflect the good selectivity of OIPC to these two gases. However, the selectivity of CH₄ and O₂ cannot be accurately reflected by the PMF curve near the interface, because the relative energy varies greatly at different positions inside the OIPC. Thus the average free energy change should be calculated over the entire OIPC box to evaluate the difference in selectivity between the two gases. This also suggests that gas absorption in OIPCs is greatly affected by the structural order and chemical environment. The adaptive biasing force method overall outperforms the average force method. The method should be able to provide a prediction of gas selectivity for a wider range of organic ionic plastic crystals and other solid materials.