Phase behaviors, properties and potential application of temperature-responsive microemulsions based on tropine ionic liquids

Abstract

Compared with traditional microemulsion, stimulating stimulus-responsive microemulsion is more challenging to achieve. A temperature-responsive microemulsion was formulated and optimized by using the mixture of a series of tropine-based ionic liquids (TILs) and water as a polar phase, different aliphatic oils as a nonpolar phase, and an optimal mixture of nonionic surfactant Triton X-100 (TX-100) and different short-chain aliphatic alcohols as a cosurfactant. The results revealed that cyclohexane (as the nonpolar phase) had the better ability to combine with TX-100/1-octanol (mass ratio of 7:3) to form the TILs-based microemulsion, and that the range of monophasic region increased with the length of alkyl chain attached to the TILs cation group. The classic oil-in-water (O/W), liquid crystal (L), mixture of liquid crystal and bicontinuous (B.C.), B.C. and inverse water-in-oil (W/O) subregions were clarified by the conductivity method and rheological measurements. Adding the TILs helps the microemulsion form. The microemulsion was temperature-responsive, and with the increase of mass ratio m(TILs to H2O) and the side chain length of TILs, the sensitivity of the microemulsion to temperature decreased. Dynamic light scattering and electron electron microscopy revealed the formation of spherical droplets of approximately 2 to 5 nm in size in the W/O subregions with droplet size increasing with the increase of molar ratio R (TILs + H2O to mixed surfactant), mass ratio m(TILs to H2O) and TILs side chain length. Measurements of polarity and particle size showed that the TILs were dissolved into the core of the aqueous phase and, along with water, became the polar phase of the microemulsion. The microemulsion formation was driven by the hydrogen bonding between TX-100 and the polar phase. The TILs-based microemulsion could be used to prepare nanoparticles of silica with dimensions of 50–73 nm. This comprehensive study provides useful information for the development of analogous TILs-based microemulsions, and these temperature-responsive microemulsions have promising applications simply by treating the mass ratio m(TILs to H2O) and side chain length of TILs.