Ionic liquids, molten salts with melting points below 100°C, are of interest to chemical engineers because of their vanishingly low vapor pressures and the ability to tune their properties by altering the chemical structures of their constituent cations and anion. Their extremely low volatility has generated great interest in their use as environmentally benign, low emission solvents for separations and reaction processes. Ionic liquids have been designed and developed to have a large variety of chemical functionality and thermophysical properties; however, the vast majority exhibit polar to moderately polar solvent characteristics and are poor solvents for non-polar solutes. As a large number of energy intensive separations processes related to petroleum refining are centered on the separation of mixtures of non-polar species, this is currently a gap in the abilities of conventional ionic liquids.
Our group has developed a class of ionic liquids with considerable non-polar content to promote non-polar-like solvent properties; these properties are introduced by attaching long alkenyl groups to the cation of 1-alkyl-3-methylimidazolium salts. While attaching long, saturated chains results in melting point increases with alkyl chain length (for alkyl chains of about 9 carbons or longer), incorporating a mid-chain cis unsaturation lowers the melting point of the C18 appended salt by 70 °C. This melting point lowering modification was inspired by biologic systems which incorporate such unsaturations to lower the melting point and increase the fluidity of lipid membranes.
In this work, we highlight liquid/liquid and solid/liquid equilibria of binary mixtures of lipidic ionic liquid and molecular species, as well as the volumetric behavior of these solutions. The solubilities of the non-polar solutes are shown to be highly dependent on both the molar volume and shape of the solute with volumetric behavior indicating negative deviations from ideal solution behavior in the dilute to moderately dilute region with a sharp turn towards positive deviations near the solubility limit. Additionally, the ionic liquid is shown to have very low solubility in the non-polar phase, reducing losses if used in extraction processes.
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