Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-Based Ionic Liquids

Abstract

While molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be shown to be the underlying property driving gas solubility and selecitivity. Previously published observations as to the distinct differences in solubility trends for gases such as CH4 and N2 relative to CO2 in systematically varied ILs can be attributed to positive and negative effects arising from increasing free volume with increasing alkyl chain length. Through the use of COSMOtherm as a powerful and rapid tool to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium ([Cnmim]­[X]) ILs, a previously unreported, yet speculated, critical underlying relationship between gas solubility in ILs is herein described. These results build upon previous assertions that Regular Solution Theory is applicable to imidazolium-based ILs, which appeared to indicate that a global maximum had already been observed for CO2 solubility in imidazolium-based ILs. However, the findings of this computational study suggest that the perceived maximum in CO2 solubility might be exceeded through rational design of ILs. We observe that although Henry’s constants in ILs are dependent on the inverse of molar volume and free volume, the volume-normalized solubility of CH4 and N2 are proportional to free volume, while CO2 is inversely proportional to the square root of free volume. Our free volume model is compared to experimental data for CO2/CH4 and CO2/N2 selectivity, and a nearly identical plot of selectivity relative to IL molar volume can be generated from the computational method alone. The overall implication is that large, highly delocalized anions paired with imidazolium cations that have minimally sized alkyl chains may hold the key to achieving greater CO2 solubility and selectivity in ILs.

Ionic liquids tailored amine aqueous solution for pre-combustion CO2 capture: Role of imidazolium-based ionic liquids

“…Although the cation and anion of ILs can effect the CO 2 solubility [40][41][42], the difference of the CO 2 cyclic capacity can be attributed to the anions in this work. Furthermore, it seems to be determined by free volumes within the ILs [43,44]. These results demonstrate that [Bmim][BF 4 ] as an additive can be used to increase the CO 2 cyclic capacity for saving the sensible heat (Q sens ) which can be explained by Eq.…”

Study on Gas Permeation and CO2 Separation through Ionic Liquid-Based Membranes with Siloxane-Functionalized Cations

“…The CH 4 and N 2 solubilities are always significantly lower than that of CO 2 amongst the SILMs studied, which is in agreement with reported results for other SILMs, in which the CO 2 separation is mainly governed by gas solubility differences in the IL. From Figure 3 [52][53][54] and in brief all the models recognized that CO 2 solubility increases by increasing the IL molecular weight, molar volume and free volume. 1 Looking at the CO 2 solubility plotted in Figure 3 According to Figure 5(a), the gas permeabilities significantly increase from 40 to 60 wt% of free siloxane-functionalized IL incorporated into the PIL-IL membranes.…”

Assessing the non-ideality of the CO2-CS2 system at molecular level: A Raman scattering study

“…[14][15][16] Harsh-repulsive forces which govern the short range local ordering in liquids and contribute to the packing of CS 2 molecules via the molecular shape must play a relevant role in favouring the formation of clusters. 94 The free molecular volume 95 in pure CS 2 calculated from the molecular and van der Waals volumes 96 is about 28 Å 3 , whereas it is about 38 Å 3 for CO 2 . This estimation shows that CO 2 molecules hardly penetrate the CS 2 compact structure indicating that a random mixing is un-favoured.…”