(83c) VLE of Lipidic Ionic Liquid Systems

Ionic liquids (ILs) are organic salts, which are liquids at temperatures below 100°C; they are a unique class of compounds that are virtually non-volatile and have highly tunable properties.  As one might expect, ionic liquids typically exhibit the same solvent characteristics as polarmolecular solvents, readily dissolving moderately-polar and polar solutes, but often being poor solvents for non-polar compounds.  This limits the applicability of ILs for chemical reactions and separations processes that involve non-polar compounds, including those of biological origin such as fatty acids and cholesterol. 

For the past few years, our group has been studying a new class of ILs that contains long alkyl chains, incorporated to impart non-polar-like solvent properties, which remain room temperature liquids.  Typically, ILs with long alkyl chains remain solids at room temperature due to the enhanced interactions created by the long chains; however, recent work has overcome this problem.  This was accomplished by taking cues from biological systems, i.e. the manner in which certain organisms regulate membrane fluidity in colder temperatures by including unsaturation in the alkyl chains of phospholipids.  In addition to unsaturations, replacement of a carbon atom in the chain with a sulfur atom, to produce an equivalent length thioether chain, has also proven to lower melting points.  The effect of the position of the sulfur atom on the melting point is similar to that to in pure, molecular thioethers.

In this work, we focus on our latest binary VLE data for lipidic ionic liquid in mixtures with permanent gases and vapors of other molecular species.  Specifically, we are interested in binary systems related to the separation of gases in biological systems (anesthetic gases) as well as selective separations for energy and environmental separations.  Earlier work has demonstrated that N₂O has a higher solubility   than CO₂ in ionic liquids containing a long chain with a cis-unsaturation, which is the same solubility trends as in olive oil, often used as a screening agent for potential anesthetic gases.  Here, we examine the effect of the structure and composition of the alkyl chain on the relative solubility of these gases in lipidic ionic liquids.