Phase behaviors and characterization of magnetic microemulsions containing pentaalkylguanidinium-based magnetic room-temperature ionic liquids (MRTILs)

Abstract

Microemulsions with magnetic properties were formulated and optimized at 25 °C by using a series of pentaalkylguanidinium-based magnetic room-temperature ionic liquids (MRTILs) 1,1,3,3-tetramethyl-4-alkylguanidinium bromotrichloroferrate(III) [C_nTMG][FeCl₃Br] (n = 2, 4, 6, and 8) 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 phase behaviors of these microemulsions were investigated by a cloud titration method. The results revealed that cyclohexane (as the nonpolar phase) coupled with TX-100/1-pentanol (mole ratio of 1 : 3) showed the best ability to formulate MRTILs-based microemulsions and the range of monophasic region was expanded with an increase in the length of alkyl chain attached to the NRH⁺ group of MRTILs cation. Based on conductivity measurements, the classical structural sequence of oil-in-MRTILs (O/MRTILs), bicontinuous sponge structure (BC), and inverse MRTILs-in-oil (MRTILs/O) microemulsions was observed with an increase of m(Oil)/m(MRTILs) ratio. Dynamic light scattering (DLS) and cryo-high resolution scanning electron microscopy (Cryo-SEM) measurements showed that microemulsion droplets with the size of about 3 to 7 nm were formed in the MRTILs/O subregions and the droplet size increased with an increase in the concentrations of MRTILs. Magnetic susceptibility and rheological measurements revealed that these MRTILs-based microemulsions had strong magnetic susceptibility and low viscosity over the entire optically clear range of composition. This comprehensive investigation provides useful information for formulating analogous MRTILs-based microemulsions and these magnetic microemulsions should have interesting potential applications by simply manipulating the external magnetic field.