Thermally stable microemulsions comprising imidazolium based surface active ionic liquids, non-polar ionic liquid and ethylene glycol as polar phase

Abstract Ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C 2 mim] [Tf 2 N], as a non-polar phase in conjunction with polar ethylene glycol (EG) forms microemulsions stabilized by surface active ionic liquids (SAILs), N-methyl-N-alkylimidazolium chlorides, [C n mim] [Cl], where n = 8, 12 and 16 in conjunction with decanol as co-surfactant. The phase behaviour of the ternary systems has been investigated and three regions of microemulsions – polar-in-ionic liquid, bicontinuous and ionic liquid-in-polar, have been identified using electrical conductivity measurements. The effect of alkyl chain length on the phase behavior has been discussed in detail. The one-phase microemulsion region is found to decrease with the increase in the alkyl chain length of the SAILs. The microstructural characteristics have been investigated by using FTIR and NMR spectroscopy. The micropolarity of reverse micelles present in the microemulsions has been investigated using UV–Vis spectroscopy employing methyl orange as a polarity probe. The dynamics of solvent relaxation in microemulsions have been investigated by steady-state and time-resolved fluorescence spectroscopy using coumarin 153 (C-153) as fluorescence probe at different compositions of microemulsions. The dynamic light scattering measurements (DLS) reveals the expansion of reverse micelles formed by [C n mim][Cl] in non polar [C 2 mim] [Tf 2 N] upon the addition of polar component. Interestingly, the microemulsions have been found to be thermally stable in a wide temperature range as revealed from temperature dependence UV–Vis, fluorescence and DLS measurements.