Effect of protic ionic liquids (PILs) on the formation of non-ionic dodecyl poly(ethylene oxide) surfactant self-assembly structures and the effect of these surfactants on the nanostructure of PILs

Abstract

The ability of a series of non-ionic dodecyl poly(ethylene oxide) surfactants to form micelles in a variety of protic ionic liquids (PILs) was investigated using small and wide angle X-ray scattering (SAXS/WAXS). The C₁₂E_n surfactants with n = 3–8 were examined in PILs which contained either an ethyl, diethyl, triethyl, butyl, pentyl, ethanol or pentanol-ammonium cation in conjunction with either a nitrate or formate anion. The ability of the PILs to support micelles of these surfactants was highly dependent on their liquid nanostructure. The PILs containing hydroxyl groups on the cations were not nanostructured and had very low surfactant solubility (<1 wt%). The highly nanostructured PILs with butylammonium or pentylammonium cations contain large non-polar domains, and had excellent surfactant solubility, but due to the greater hydrocarbon solubility they had insufficient drive from the “solvophobic effect” to enable micelle formation. The PILs of ethylammonium nitrate (EAN), propylammonium nitrate (PAN), diethylammonium formate (DEAF) and triethylammonium formate (TEAF) had smaller non-polar domains, and all supported micelle formation below 20 wt% surfactant. The critical micelle concentration (CMC) of surfactants in EAN were two orders of magnitude greater than in water. The minimum molecular areas of the poly(ethylene oxide) head groups at the air/ionic liquid interface, A_min, were significantly larger in EAN than in water. The SAXS patterns from the micelles present in EAN fitted well to ellipsoids, whereas the micelles present in PAN fitted well to spheres. The nanostructure of select PILs was also influenced by the presence of surfactants.