Temperature-Induced Microstructural Changes in Ionic Liquid-Based Microemulsions

In the present contribution, results concerning the effect of temperature on the nonionic surfactant Triton X-100 based 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane and bmimBF4-in-toluene ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) along with freeze-fracture transmission electron microscopy (FF-TEM) measurements revealed that the sizes of single microemulsion droplets increased with increasing temperature. However, a decreased temperature led to the appearance of droplet clusters, which have also been observed previously when the single microemulsion droplets were swollen by added bmimBF4 to a certain extent (Gao, Y. A.; Vogit, A.; Hilfert, L.; Sundmacher, K. ChemPhysChem, 2008, 9, 1603-1609). Compared to traditional aqueous microemulsions, IL microemulsions revealed relatively high temperature-independence. The droplet-shaped microstructure was always kept in a large range of temperature. The temperature-independence is ascribed to the temperature-insensitive electrostatic attraction between the solubilized bmimBF4 and Triton X-100, which was considered to be the driving force for solubilizing bmimBF4 into the cores of Triton X-100 aggregates. Two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further confirmed the microstructural change with temperature.