Ammonium Ionic Liquid Solubilities in Water and Micellar Formation

A number of proposed applications for ionic liquids (ILs) involve IL/water interfaces, such as chemical separations or drug delivery. Therefore, an understanding of the solubility and micellar behavior ILs in aqueous environments is critical. The anion, bis(trifluoromethanesulfonyl)imide (Tf₂N) promotes water stability and forms water immiscible ILs. This study, therefore, paired the Tf₂N-anion with three different classes of IL cations. The three classes examined were 1-alkyl-3-methylimidazoliums (Rmim), alkyl-trimethylammoniums (CTA), and bulky ammoniums (BAM). CTAs can be synthesized from inexpensive ammonium surfactants; however, large CTAs are solids at ambient conditions. In contrast, large BAMs remain in the liquid state at ambient conditions. We used total organic carbon (TOC) analysis to determine the IL content in IL saturated water. Surface tension measurements of IL containing water determined if micelles existed in the IL saturated water. We used linear free energy relationship (LFER) semiempirical models to correlate the IL water solubility to the molecular volume and IL cation structure. The reported LFERs can predict the IL solubility in water before the IL is synthesized. Combining the LFER results with surface tension measurements and thermodynamic calculations allowed us to determine that micelle formation is not significant for the tested ILs with molecular weights ≤510.