Abstract:
Due to their tunable thermophysical properties, ionic liquids (ILs) have emerged as potential candidates for gas capture and separation. However, the underlying mechanisms for gas solvation and separation are still not clear. In order to provide more insight, we use multi-scale simulations to investigate the properties of ILs and their performance in gas capture and separation. We primarily focus on the physiochemical nature of the solute-solvent interactions, especially the electrostatic potential (ESP) surface properties of individual cations and anions, as well as that of the gas solutes. With our simulations, we isolate the role of two primary effects that contribute to gas solubilities in ILs: the free volume effect and the anion effect. The free volume effect can be quantified by the fractional free volume, which is found to be correlated to the cation and anion ionic polarity index, which is a newly proposed parameter based on an analysis of the molecular ESP surface. The anion effect can be quantified by the solvation affinity index (SAI), which is also a newly proposed parameter based on an analysis of the anion ESP surface. Based on these fundamental interactions, we design a new framework for prescreening IL pairs for achieving high gas capture and separation performance that we refer to as the solvation affinity index matrix. The SAI matrix is applicable to different gas solutes, as well as to a wide range of different anions.
