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

Due to the tunable structures and functionalities of ionic liquids (ILs) , current research efforts have introduced several unique materials for industrial applications , including solvents and media for CO2 capture.  Fundamental conclusions and aspects of these IL materials and their physical properties are imperative to understanding governing driving-forces for gas separations and underlying molecular structures.  Recently , we investigated fractional free volume (FFV)-dependence of gas solubility in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium-based ionic liquids (ILs) via COSMOtherm , a very powerful and rapid software utilized commonly now for predicting IL thermophyscial properties.  Our present study investigates the effects on gas solubility of novel functional groups by replacing n-alkyl chains with nitriles (CnH2nC≡N) and ethers or oligo(ethylene glycols) (-C2H4O-)n.  Initial computational results with these new functional groups via COSMOtherm reflect deviations in FFV from previously studied [Cnmim] ILs.  Polar substituents provide a higher CO2 binding affinity and repulsion of non-polar gases (i.e. CH4 , N2 , etc.) leading toward improved CO2 separation performance.  Initial results suggest decreased FFV values for appended nitrile and ether groups when compared to previous [Cnmim] IL data , which would also imply decreased gas solubility.  However , with this observed decrease in FFV due to chain aggregation of polar entities , ideal gas selectivity is shown to increase with these materials being more size selective within these microdomain/voids and unable to accommodate the larger CH4 and N2 molecules. Previous models presented for gas solubility in ILs consist of a molar volume-basis for families of 1-n-alkyl-3-methylimidazolium ILs.  Free volume within these ILs as well as the currently studied nitrile- and ether-based ILs was shown to be the underlying property driving gas solubility and selectivity.  For light gases (i.e. CH4 and N2) considered in CO2 separations , solubility increases with increasing FFV implying direct proportionality with free volume.  However , solubility of CO2 was observed to decrease with increasing FFV , with CO2 selectivity being a function of free volume to the (-3/2) power.  Controlling IL FFV via cation/anion combinations , as well as , cation functional groups can lead to improvements in CO2 solubility and selectivity and permeability in IL-based membranes.