Studies On Micelle And Lyotropic Liquid Crystal In Ionic Liquids

As a "green" chemical product,ionic liquids(ILs) have currently attracted significant attention.In the field of colloid and interface science,ionic liquid can be used as a novel solvent.In this dissertation,the detailed aggregation behavior of a nonionic surfactant in the imidazolium ILs has been investigated,which differs the suitation in the aqueous solution.On the other hand,using the long-chained imidazolium ILs as a surfactant,the lyotropic liquid crystalline phases formed in the ternary system of long-chained imidazolium ILs/imidazolium ILs/water has been discussed.Furthermore,it is compared with the similar system containing common cationic surfactant.The outline and contents of this dissertation are as follows:Chapter one is a brief introduction of the research background of this work,in which a review of surfactant and ILs are present,and then the history and recent progress in the Ordered Molecular Assembly associated with ILs are reviewed from a worldwide angle of view.The objective and the scientific significance of this dissertation are also pointed out at the end of this part.In Chapter two,the aggregation behavior of a nonionic surfactant Tween20,the trade name of polyoxyethylene(20) sorbitan monolaurate,in 1-butyl-3-methyl imidazolium tetrafluoroborate(bmimBF₄) and hexafluorophosphate(bmimPF₆) is investigated in detail.Firstly,the surface tension measurements are carried out for the solutions of Tween20 in bmimBF₄ and bmimPF₆ at various temperatures.Two transition points are found in the surface tension-concentration curves at each temperature.The freeze-fracture transmission electron microscopy reveals that two kinds of aggregates with different sizes are formed at the concentrations of each transition point.Thus,the surfactant concentrations of the two transition points are regarded as critical aggregation concentrations,CAC₁ and CAC₂.From the CAC values and their temperature dependence,we estimate the thermodynamic parameters of the aggregate formation,â–³G_agg⁰,â–³H_agg⁰ andâ–³S_agg⁰.The thermodynamic parameters related to CAC₁ are almost independent of temperature.On the other hand,as for the aggregate formation at CAC₂,a positiveâ–³S_agg⁰ contributes to a negativeâ–³G_agg⁰ at low temperature,while a negativeâ–³H_agg⁰ contributes to a negativeâ–³G_agg⁰ at high temperature.In addition,as a powerful tool to obtain the information about solute-solvent interactions at the molecular level,¹H NMR is used to clarify the formation mechanism of the aggregates.From the chemical shift of bmim⁺,it is found that,as for the aggregates formed at CAC₁,the hydrocarbon chains of Tween20 in the aggregates are not solvated by bmim⁺ and even the POE chains are only weakly solvated,while for the aggregates formed at CAC₂,the POE chains of Tween20 in the aggregates are solvated by bmim⁺ more or less similarly to the Tween20 monomers, while the hydrocarbon chains have much less solvating bmim⁺.The behavior of the thermodynamic parameters as a function of temperature,combined with the variation of ¹H NMR chemical shifts of the bmim⁺ protons as a function of the surfactant concentration,demonstrate that the aggregates formed at CAC₁ are nanodroplets of Tween20 segregated from the solution phase,while those formed at CAC₂ are similar to the usual surfactant micelles formed in aqueous solution.In chapter three,the lyotropic liquid crystalline phases formed of long-chained imidazolium ILs in the concentrated aqueous solution of the imidazolium ILs are studied.The phase diagram of ternary mixtures of 1-dodecyl-3-methylimidazolium bromide(C₁₂mimBr)/1-butyl-3-methylimidazolium tetrafluoroborate(bmimBF₄)/H₂O is constructed at 25℃.The formed hexagonal liquid crystalline phase in the mixtures is investigated by the polarized optical microscopy(POM),small-angle X-ray scattering (SAXS),and rheology measurements in comparison with the hexagonal phase formed in the ternary system,dodecyltrimethylammonium bromide(DTAB)/bmimBF₄/H₂O. The structural parameters of the hexagonal liquid crystalline phase are found to be dependent on the component content.The increase of the lattice parameter with the increase of the bmimBF₄ content is attributed to the location region of bmimBF₄ in the water layer of the hexagonal phase.While as the water layer in the hexagonal phase swells due to the increase of the bmimBF₄ content,the cylindrical micelle is compressed,so the hydrophobic chain of the surfactant molecule shrinks.On the other hand,a few bmimBF₄ molecules could also exist at the hydrophilic/hydrophobic interface due to theÏ€-Ï€stacking interaction between the imidazolium rings in the C₁₂mimBr and bmimBF₄ molecules,together with the hydrogen-bond interaction between Br^- and the imidazolium ring in bmimBF₄ molecule.As a result,the value of effective cross-sectional area per surfactant molecule at the interface increases with the increase of bmimBF₄ content.Meanwhile,the rheological properties of the liquid crystalline phases are related to the phase structures.It is found,the zero-shear-rate viscosity and the zero-shear-rate stress both increase,while the relaxation time decreases as the bmimBF₄ content increases.Furthermore,the denser aggregation of the cylindrical units in the hexagonal liquid crystalline phase formed in the C₁₂mimBr system than in the DTAB system could be concluded from the SAXS and rheological measurements.