(213a) Water Solubility and Micellar Formation of Imidazolium and Ammonium Ionic Liquids

A number of proposed applications for ionic liquids involve ionic liquid/water interfaces; such as, chemical separations or drug delivery systems.  Therefore an understanding of the solubility and micellar behavior of ionic liquids in an aqueous environment is critical fundamental knowledge.  The long term goal of our study was the utilization of ionic liquids for the delivery of biological active agents.  We chose the anion, bis(trifluoromethanesulfonyl)imide (Tf₂N) since it promotes water stability and forms water immiscible RTILs. The study then paired the Tf₂N-anion with three different classes of cations. By adding apolar substituents (-CH₂-) to the cation, the size of the molecule increases in each RTIL class, rendering the RTIL less soluble in the water phase.   The three classes of RTILs examined were 1-C_n-3-methylimidazoliums, C_n-trimethylammoniums (CTA), and branched ammoniums (BAM), with the “C_n” representing an alkyl functional group (propyl, butyl, etc.). For industrial use, CTAs are attractive, since they can be synthesized from inexpensive ammonium surfactants. However, large CTAs (twelve carbon chain and higher) are solids at ambient conditions. In contrast, BAMs have larger compounds that remain in the liquid state at ambient conditions. Nevertheless, each class has potential as a delivery agent by meeting the aforementioned criterion – namely, water immiscible and water stable at ambient temperatures.  We used Total Organic Carbon (TOC) analysis to determine the RTIL content in water that is in equilibrium with the tested RTILs.  Surface tension measurements of the RTIL containing water determined if RTIL micelles existed in the RTIL saturated water.  We used Linear Free Energy Relationship (LFER) semi-empirical models to correlate the RTIL water solubility to the molecular size and structure of the RTIL cation.  Combining the LFER results with surface tension measurements and thermodynamic calculations of sub-cooled RTIL fugacities allowed us to determine that micelle formation is not significant for the tested RTILs with molar volumes less than 350 cm³/mol.  We tested a total of 15 different RTILs with the water solubilities ranging from 10^-2 to 10^-4 moles-RTIL/L-water.