Studies On The Self-organized Molecular Assembly Based On Ionic Liquids

The self-assembly of surfactant molecules is of fundamental interest and is important in many applications such as nanomaterial synthesis,drug delivery,separation,pharmaceutical formulation,and other dispersant technologies.Recently,aggregations of amphiphilic molecules in ionic liquids(ILs) have received more and more attention due to the attractive properties of ILs.ILs have special physical and chemical properties such as low volatility, wide electrochemical window,nonflammability,high thermal stability,and wide liquid range. Compared to traditional volatile organic solvents,ILs are regarded as environmentally benign solvents,since their nonvolatility can prevent environmental pollution.ILs have been widely applied in organic synthesis,chemical separation,nanomaterial preparation,and polymer gel electrolytes.Even though their properties can be modified to satisfy the requirements by suitable selection of cation,anion,and cation substituent,molecular assemblies formed in ILs are of great interest because the aggregates are expected to solubilize many insoluble substances in ILs and widen the IL applications.This dissertation is focused on the investigations of the physical and chemical properties of ionic liquid based surfactant aggregates.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 ILs and surfactant are present,and then the history and recent progress in the surfactant aggregates associated with ILs are reviewed from a worldwide angle of view.The objective and the scientific significance of this doctoral dissertation are also pointed out at the end of this part.In Chapter two,the micro-properties of IL microemulsions were investigated in detail. Firstly,the phase behavior of the 1-butyl-3-methylimidazolium tetrafluoroborate(bmimBF₄), p-octylpolyethylene glycol phenyl ether(TX-100) and toluene system was investigated and micro-regions of bmimBF₄-in-toluene(IL/O),bicontinuous and toluene-in-bmimBF₄(O/IL) were identified by traditional electrical conductivity measurements.Dynamic light scattering (DLS) revealed the formation of the IL microemulsions.The micropolarities of the IL/O microemulsions were investigated by the UV-Vis spectroscopy using the methyl orange(MO) and methylene blue(MB) as absorption probes.The results indicated that the polarity of the IL/O microemulsion increased only before the IL pools were formed,whereas a relatively fixed polar microenvironment was obtained in the IL pools of the microemulsions.Moreover, UV-Vis spectroscopy has also shown that ionic salt compounds such as Ni(NO₃)₂,COCl₂, CuCl₂ and biochemical reagent riboflavin could be solubilized into the IL/O microemulsion droplets.Then the second virial coefficient of the IL/ cyclohexane microemulsion was obtained using microcalorimetry.The dilution heat of the microemulsion solutions was measured by isothermal titration microcalorimetry(ITC) and the second virial coefficient was derived from the dilution heat and the number density of the IL microemulsion solutions, based on a hard-sphere interaction potential assumption and a function of the second-order polynomial.The validity of the second virial coefficient was confirmed by the percolation behavior of different ionic liquid microemulsion solutions of Triton X-100 in cyclohexane with or without added salts.The information of second virial coefficient shows that the interactions between ionic liquid microemulsion droplets are much stronger than those of traditional microemulsions,which may be attributed to the relatively larger size of the microemulsion droplets.In chapter three,the effects of polar solvents on the microstructure of IL microemulsions were studied.A novel IL microemulsion consisting of bmimBF₄ and Triton X-100 was prepared and triethylamine was used either as organic solvent or a Lewis base.The dispersed microdroplets of bmimBF₄ were surrounded by a base environment and thus are potentially useful as the microreactors for base-catalyzed reactions.The addition of small amounts of water to the IL microemulsions was intensively investigated.UV/Vis spectra first showed that the added water molecules did not appear in the IL pools of the microemulsions,which was then confirmed by FTIR spectra.¹H NMR spectroscopic analysis further indicated that the added water binds to the continuous triethylamine to form a surrounding OH^- base environment.Small amounts of the OH^- thus formed entered the palisade layers of the IL microemulsions and a continuous base interface was provided.The possibility of using the IL microemlusions to prepare metal materials was also discussed and the results indicated that it is possible for the microemulsions to prepare their metal hydroxides and additionally their oxide materials in the microemulsions,while it is impossible for these to be obtained in the traditional microemulsions.Then the effect of a common polymer,polyethylene glycol with molecular weight of 400(PEG-400) on the microstructure of bmimBF₄/TX-100/cyclohexane IL reverse microemulsion has been investigated.The addition of PEG-400 leaded to the linear increase of the microemulsion droplet size,in accordance with the observation of dispersed phase,showing that PEG-400 was only solubilized into the polar interior of the IL microemulsions.FTIR spectroscopic analysis indicated that the addition of PEG-400 decreased the electrostatic interaction between the oxygen atoms of OE units and the positive electrical charged imidazolium cation of bmimBF₄.At the same time,the oxygen atoms of PEG-400 can also interact with the imidazolium cation.These results suggested that small amounts of PEG-400 entered the palisade layers of the IL microemulsion.The conductivity of the IL reverse microemulsions was decreased owing to the dilution of conducting polar cores by the addition of insulative PEG-400,indicating that PEG-400 was only solubilized into the reverse IL microemulsion interior.The conclusion was further supported by viscosity measurement.In chapter four,aggregation behaviors of a fluorinated surfactant(FC-4) in 1-butyl-3-methylimidazolium ionic liquids were investigated.Firstly,the aggregation behavior of FC-4 was studied by surface tension measurements in bmimBF₄ and bmimPF₆ at various temperatures.A series of surface properties,including adsorption efficiency(pC₂₀), effectiveness of surface tension reduction(Π_cmc),maximum surface excess concentration (Γ_max) and minimum surface area/molecule(A_min) at the air-water interface were estimated. By comparing the fluorinated surfactant with traditional surfactants,we deduced that the surface activity of the fluorinated surfactant in ILs was superior to the activity of traditional surfactants.From the CAC values and their temperature dependence,we estimated the thermodynamic parameters of aggregate formation.The thermodynamic parameters indicate that the aggregate of FC-4 in bmimBF₄ is a traditional micelle,while the aggregate of FC-4 in bmimPF₆ is nano-droplets composed of FC-4 molecules segregated from the solution phase. These results were further confirmed by ¹H NMR measurements.Next,the aggregation behavior of FC-4 in bmimTf₂N was studied.Surface tension,freeze-fracture transmission electron microscopy,¹⁹F-NMK,¹H-NMR and FT-IR measurements revealed thatâ…°) the FC-4 cation forms an ion-pair with the Tf₂N anion,â…±) the ion-pairs undergo association to form pre-micellar aggregates,andâ…²) the pre-micellar aggregates transform into micelles at the critical micelle concentration(CMC).The thermodynamic parameters for micelle formation derived from the temperature dependence of the CMC demonstrated that the solvophobic interaction between the solvophobic tails of the surfactant molecules is rather weak in bmimTf₂N compared with other ionic liquids,in accordance with the observation that surfactants do not readily form micelles in bmimTf₂N.The fact that FC-4 forms micelles in such an inconvenient solvent is attributed to the ion-pair formation between the surfactant cation and the ionic liquid anion.