Microemulsions with an Ionic Liquid Surfactant and Room Temperature Ionic Liquids As Polar Pseudo-Phase

Abstract

The authors present for the 1st time microemulsions comprising an ionic liq. as surfactant and a room-temp. ionic liq. as polar pseudo-phase. Microemulsions contg. the long- chain ionic liq. 1-hexadecyl-3-methyl-imidazolium chloride, [C16mim][Cl], as surfactant, decanol as cosurfactant, dodecane as continuous phase and room temp. ionic liqs. (ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), resp.) as polar microenvironment were formulated. The phase diagrams of both systems were detd. at a const. surfactant/cosurfactant molar ratio. EAN microregions in oil were confirmed with cond. measurements. In presence of EAN a model of dynamic percolation could be applied. Dynamic light scattering (DLS) measurements indicated a swelling of the formed nano-structures with increasing amt. of EAN, a linear dependence of the hydrodynamic radii on the EAN wt. fraction was obsd. Both systems exhibited a single broad peak in SAXS and follow a characteristic q-4 dependence of the scattering intensity at large q values. The Teubner-Strey model was successfully used to fit the spectra giving fa, the amphiphilic factor, and the two characteristic length scales of microemulsions, the periodicity, d, and the correlation length, ζ. Also, the specific area of the interface could be detd. from the Porod limit and the exptl. invariant. The amphiphilic factor clearly demonstrated structural differences between the two systems confirming that the nature of the polar ionic liq. plays an important role on the rigidity of the interfacial film. The adaptability of three different models ranging from spherical ionic liq. in oil over repulsive spheres to bicontinuous structures was checked.