Preparation and characterization of ionic liquid based microemulsions and their application in catalysis

Abstract

Series of ionic liquid (IL) based microemulsions were prepared with ionic and nonionic surfactants. Hydrophilic and hydrophobic ILs were used instead of water and the oil phase, respectively. The structure of the ILs were systematically varied by varying the structure of the cation and the anion. The influence of both hydrophilic and hydrophobic ILs on the physicochemical properties of IL based microemulsions were extensively studied. Different microregions in IL based microemulsions depended on the polar phase (IL or water) to surfactant mole ratio (R or Wo) or the volume fraction of the polar phase(ϕIL or ϕW) in microemulsions were critically analyzed with the help of percolation theory. The catalytic activity of lipase in IL based microemulsions was also investigated. The lipase-catalyzed hydrolysis of 4-nitrophenyl butyrate (p-NPB) was used as a model reaction to characterize the catalytic behavior of lipase in the IL microemulsion. IL based microemulsions were prepared by dissolving surfactants in ionic liquids. The surfactants used were nonionic surfactant, Triton X-100 (TX-100), anionic surfactant, sodium dodecyl sulfate (SDS), and cationic surfactant, cetyltrimethylammonium bromide (CTAB). Hydrophilic ILs, such as1-ethyl-3-methylimidazolium ethyl sulfate ([C2mim][EtSO4]), 1-butyl-3-methylimidazolium methyl sulfate ([C4mim][MeSO4]), 1-ethyl-3-methylimidazolium methyl sulfate ([C2mim][MeSO4]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate([C4mim][BF4]) served as polar phases; while hydrophobic ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][TFSI]), 1-ethyl-3-methylimidazolium hexafluorophosphate ([C2mim][PF6]) and 1-butyl-3-methylimidazolium hexafluorophosphate([C4mim][PF6]) were used as non-polar phases. Conventional microemulsions with TX-100 and CTAB were also prepared to compare with IL based microemulsions. 1-Butanol was used as the cosurfactant to prepare CTAB based microemulsions.The IL to surfactant mole ratio (R) or water to surfactant mole ratio (Wo) was varied to prepare microemulsions of different compositions. Different types of microemulsions such as IL-in-oil (IL/o), oil-in-IL (o/IL), IL-in-water (IL/w), water-in-IL (w/IL) and bicontinuous were prepared by changing the composition and proportion of polar and nonpolar phases. Physicochemical properties of TX-100 microemulsions based on hydrophilic and hydrophobic ILs and CTAB microemulsions based on hydrophilic ILs were studied with the variation of the anion or the cation of the IL. . The conductivity, viscosity, density, and the droplet size of microemulsions changed interestingly with change in the structure of the ILs. The viscosity and density decreased with increasing temperature at R = 0.2255 and 1.5424 for TX-100 and CTAB microemulsions based on hydrophilic ILs respectively. The refractive index changed with R and temperature (at R = 0.2255) for TX-100 microemulsions based on hydrophilic ILs. The refractive index increased with R and temperature (at R = 1.5424) for CTAB based hydrophilic IL microemulsions. Surface tension changed with R for TX-100 and hydrophilic IL based microemulsions of CTAB. The conductivity first increased with Wo upto Wo ≈ 8.0 and then decreased with increasing Wo for [C2mim][TFSI] based microemulsion, decreased with increasing Wo for [C2mim][PF6] based microemulsion and first increased upto Wo ≈ 8.0 and then decreased with increasing Wo for [C4mim][PF6] based microemulsion. The viscosity followed the reverse trend. The micelle droplet size increased with decreasing Wo for hydrophobic IL based TX-100 microemulsions. The density and refractive index decreased and the surface tension changed with increasing Wo for hydrophobic IL based microemulsions with TX-100. Absorption peaks in ultraviolet region was observed in the absorption spectra of Reichardt's Betaine dye (RBD) in microemulsions with TX-100 and CTAB at a fixed wavelength. If the ions of ILs in these microemulsions are weakly bonded, the conductivity shows the higher values and viscosity shows the lower values compared to the microemulsions where ions of ILs are strongly bonded. The weakly bonded ions of ILs can penetrate the surfactant layer easily and thus such microemulsions showed larger droplet size. Although ILs with spherical anions have strong Coulombic interaction between cations and anions, inductive and dispersive forces gradually increase and dominate the system with increasing cationic and anionic side chain length. Different microregions were also established by applying percolation theory on conductivity results on TX-100 based microemulsions based on ILs with increasing volume fraction of water (ϕW) or volume fraction of IL (ϕIL) respectively. Composition percolation thresholds were obtained from the derived data of the density and refractive index. The phase transitions correspond to the structural change from IL/o to o/IL via bicontinuous microemulsions with increasing ϕIL and from w/IL to IL/w via bicontinuous microemulsions with increasing ϕW. For [C2mim][MeSO4] and [C2mim][PF6] based microemulsions with TX-100 only reverse micelle and micelle dominated microemulsions exist. Microstructures such as IL/o and bicontinuous microemulsions were identified by applying percolation theory on conductivity results for CTAB based hydrophilic IL microemulsions with increasing R which are in agreement with the change of the diameter of microemulsion droplets as a function of R. Comparative study of physicochemical properties based on the viscosity, conductivity, density, and refractive index of different microemulsions was also made by varying polar phase, nonpolar phase, and surfactant . Gel formation occured for conventional TX-100 based microemulsions at Wo ranging from 6.9385 to 12.3903. For CTAB based conventional microemulsions only micelle and reverse micelle dominated microemulsions exist. Viscosity increased with increasing Wo for both w/o and o/w microemulsions for TX-100/cyclohexane/water system.The viscosity slightly increased with increasing Wo for w/o microemulsions and viscosity initially increased and then decreased with increasing Wo for o/w microemulsions in CTAB/1-butanol/cyclohexane/water system. The conductivity and viscosity of TX-100/cyclohexane/[C2mim][EtSO4] microemulsions showed larger values compared to CTAB/1-butanol/cyclohexane/[C2mim][EtSO4] microemulsions. The catalytic behavior of lipase in aqueous medium, w/o, w/C4mim][PF6] and bicontinuous [C4mim][PF6] microemulsions under optimum conditionswas investigated using lipase catalyzed hydrolysis of p-NPB as a model reaction. Kinetic parameters of lipase catalyzed hydrolysis of p-NPB in aqueous medium were evaluated from Michaelis-Menten and Lineweaver-Burk plots.The bicontinuous [C4mim][PF6] microemulsion at Wo = 4.20 showed maximum catalytic activity among different reaction media. Microemulsions with hydrophilic IL in place of water and hydrophobic IL instead of oil phase showed significantly different properties depending on their structure and selectivity, which may be fine tuned for desirable catalytic activity like enzyme catalysis.