(195d) Binary Phase Behavior of Lipidic Ionic Liquids

Ionic liquids, molten salts which melt below 100°C, are a unique class of compounds that have been the subject of significant interest and development in recent decades.  As they are comprised of ions, they typically exhibit low to negligible volatility.  Because of this, ionic liquids are of great interest as environmentally benign process solvents which do not generate fugitive emissions. Additionally, since the cations, anions or both are typically comprised of organic ions, the full palette of synthetic organic chemistry can be utilized to tune the chemical and physical properties of these compounds, allowing for customizable solvents, catalysts and, more recently, active pharmaceutical ingredients. 

            The ionic nature of these species typically results in solvent properties similar to those of polar organic solvents, with high solubilities for polar to moderately polar compounds and low solubilities for non-polar species.  Recently, however, there has been interest in developing ionic liquids with significant non-polar content for use as solvents and gas absorbents.  To achieve “non-polar-like” solvent properties, the non-polar content of the ions can be increased; this can be accomplished by adding a long alkyl chain to the central ion.  However, this often results in significant melting point increases and compounds that are room temperature solids.

            Our group has described two types of lipidic ionic liquids that are room temperature liquids; these species are inspired by and derived from natural lipids.  The first type incorporates cis unsaturations into a C₁₆-C₂₀ alkyl chain appended to a central ion.¹  The double bond causes a “kink” in the alkyl chain which frustrates packing and decreases the melting point, analogous to natural triacylglycerols with more unsaturations having lower melting points than triacylglycerols with fewer unsaturations.  The second type, designed to be more oxidatively stable, have been synthesized using thiol-ene “click” chemistry and have thioehter linkages in place of the double bonds.²  Each type has significant non-polar content while maintaining low melting points.  Previously, we have measured and correlated the pure component thermophysical properties of several representative examples of these lipidic ionic liquids.³

            In the current work, we describe the binary liquid/liquid equilibria of these species with a variety of molecular compounds ranging in polarity from methanol to hexane, including several homologous series.  Of particular interests are the trends observed with respect to the size and shape of the molecular species.  We postulate that the structures that arise in the hetereogeneous nanodomains (segregated polar and non-polar domains that have been described experimentally⁴ and through molecular dynamic simulations⁵) play a key role in the overall solubility of the nonpolar compounds.  In addition to liquid/liquid equilibrium, vapor/liquid equilibrium of these systems will also be presented.

1.         (a) Murray, S. M.; O'Brien, R. A.; Mattson, K. M.; Ceccarelli, C.; Sykora, R. E.; West, K. N.; Davis, J. H., Jr., The Fluid-Mosaic Model, Homeoviscous Adaptation, and Ionic Liquids: Dramatic Lowering of the Melting Point by Side-Chain Unsaturation. Angew. Chem., Int. Ed. 2010, 49 (15), 2755-2758; (b) Mirjafari, A.; Murray, S. M.; O'Brien, R. A.; Stenson, A. C.; West, K. N.; Davis, J. H., Structure-based tuning of Tm in lipid-like ionic liquids. Insights from Tf2N- salts of gene transfection agents. Chem. Commun. 2012, 48 (60), 7522-7524; (c) Mirjafari, A.; O'Brien, R. A.; Murray, S. M.; Mattson, K. M.; Mobarrez, N.; West, K. N.; Davis, J. H., Jr., Lipid-inspired ionic liquids containing long-chain appendages: novel class of biomaterials with attractive properties and applications. ACS Symp. Ser. 2012, 1117 (Ionic Liquids), 199-216; (d) Kwan, M.-L.; Mirjafari, A.; McCabe, J. R.; O'Brien, R. A.; Essi, D. F. I. V.; Baum, L.; West, K. N.; Davis, J. H., Jr., Synthesis and thermophysical properties of ionic liquids: cyclopropyl moieties versus olefins as Tm-reducing elements in lipid-inspired ionic liquids. Tetrahedron Lett. 2013, 54(1), 12-14.

2.         Mirjafari, A.; O'Brien, R. A.; West, K. N.; Davis, J. H., Jr., Synthesis of New Lipid-Inspired Ionic Liquids by Thiol-ene Chemistry: Profound Solvent Effect on Reaction Pathway. Chem. - Eur. J. 2014, Ahead of Print.

3.         Murray, S. M.; Zimlich, T. K.; Mirjafari, A.; O'Brien, R. A.; Davis, J. H., Jr.; West, K. N., Thermophysical Properties of Imidazolium-Based Lipidic Ionic Liquids. J. Chem. Eng. Data 2013, 58(6), 1516-1522.

4.         Triolo, A.; Russina, O.; Bleif, H.-J.; Di, C. E., Nanoscale segregation in room temperature ionic liquids. J Phys Chem B 2007, 111(18), 4641-4.

5.         Canongia Lopes, J. N. A.; Padua, A. A. H., Nanostructural Organization in Ionic Liquids. Journal of Physical Chemistry B 2006, 110 (7), 3330-3335.