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

Ind. Eng. Chem. Res., 2012, 51 (15), pp 5565–5576, DOI: 10.1021/ie202916e
Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-Based Ionic Liquids
Matthew S. Shannon †, Jason M. Tedstone ‡§, Scott P. O. Danielsen ‡, Michelle S. Hindman †, A. Christopher Irvin †, and Jason E. Bara *†
jbara@eng.ua.edu
† Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States
‡ NSF-REU Site: Engineering Solutions for Clean Energy Generation, Storage and Consumption, Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States
§ Department of Chemical & Biomolecular Engineering, Clemson University, Clemson, South Carolina 29634, United States
Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6315, United States
Abstract
Although molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be demonstrated to be the underlying property driving gas solubility and selectivity.
Previous 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. Authors use COSMOtherm to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium ([C_nmim][X]) ILs. They describe a previously unreported critical underlying relationship between gas solubility in ILs. The 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.
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