Combined phase behavior, dynamic light scattering, viscosity and spectroscopic investigations of a pyridinium-based ionic liquid-in-oil microemulsion

Abstract

Although several studies of imidazolium-based ionic liquid-in-oil microemulsions are available in the literature, studies on pyridinium-based ionic liquid microemulsions are uncommon. Pyridinium-based ionic liquids have superior properties, but their properties in the polar part of the microemulsion have yet to be explored. 1-Butyl-4-methyl pyridinium tetrafluoroborate ([b₄mpy][BF₄])–(Tween 20 + n-pentanol)–n-heptane ionic liquid-in-oil microemulsion system has been studied by combined phase behavior, dynamic light scattering, viscosity, and UV-visible absorption and emission spectroscopic techniques. As the ratio of Tween 20 to n-pentanol (S/CS) was decreased, the stability of the microemulsions also decreased, although it was not possible to achieve a stable microemulsion without the n-pentanol cosurfactant. Dynamic light scattering and viscosity studies revealed that the size of the microemulsion droplets increased with increasing volume fraction (φ_d) of ionic liquid. Viscosity also increased with φ_d. With an increasing amount of n-pentanol, the variation was less sensitive due to the reduced polarity of the medium induced by the alkanol. Increase in the size of microemulsion droplets was overshadowed by the increase in the fluidity of the medium, for which viscosity decreased with increasing temperature, as is common for Newtonian fluids. The state of the ionic liquid in the microemulsion was monitored by fluorescence and absorption spectroscopy with and without curcumin as the molecular probe, respectively. While a continuous increase in polarity of the IL part occurred with an increasing amount of IL, the fluorescence anisotropy results revealed that the rigidity of the part passed through maxima for all S/CS combinations. A spherical morphology of the microemulsion droplets was established by transmission electron microscopy measurements.