New Lyotropic Liquid Crystals With Ionic Liquids And Their Applications In Templating Synthesis Of Nanomaterials

Surfactants,with the formation of various ordered molecular assemblies,are playing active roles in areas of life,information,energy,and materials.As an important type of assemblies,lyotropic liquid crystals(LLC)have been widely used in nanomaterial synthesis,reaction,drug delivery,lubricant,and biochemistry. Traditional LLC are always composed of surfactants and water or organic solvents and might be limited in application when water is undesirable.In addition,they should be very carefully manipulated under high temperature or electric field for the evaporation and electrolysis of water or other solvents.Therefore,exploring new types of LLC is required to meet their above potential use.Ionic liquids,so-called neoteric solvents and materials,have not only changed our knowledge on solvent,but also provided opportunities for new self-assemblies in solvents other than water.This dissertation is focused on the use of ionic liquids in ordered molecular assemblies.The main purpose is to construct new LLC systems with ionic liquids as solvents and surfactants.The application of LLC including the new systems in templating synthesis of nanomaterials is also discussed.There are three main experimental studies in this dissertation.1.New LLC systems with ionic liquids as solvents.Aggregation behavior of triblock copolymer Pluronic P123(EO₂₀PO₇₀EO₂₀)in an ionic liquid,ethylammonium nitrate(EAN)has been characterized by small angle X-ray scattering(SAXS)and polarized optical microscopy(POM).With the techniques of Fourier transform infrared spectra(FTIR),differential scanning calorimetry(DSC),and thermogravimetric analysis(TGA),the formation mechanism of ordered assemblies has been discussed.We have also studied the phase behavior of the F127 (EO₁₀₀PO₇₀EO₁₀₀)-EAN binary system for comparison.Then,research on aggregation of P123 in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆)and tetrafluoroborate([Bmim]BF₄)has been given with the influence from alkyl chains of ionic liquids.Besides,effect of the third component(n-butanol) on the P123 binary systems has been analyzed.Following results are obtained.(1)A series of lyotropic mesophases including normal micellar cubic(I₁),normal hexagonal(H₁),lamellar(L_α),and reverse bicontinuous cubic(V₂)are identified in the P123-EAN binary system.Owing to hydrogen bonds formed between the ethylammonium cations and oxygen atoms of the PEO blocks,EAN has much stronger interaction with PEO.This induces the further segregation of PEO and PPO blocks and leads to the formation of ordered assemblies.(2)Compared to water,EAN has weak solvation of the PEO block but high affinity for the PPO block.The segregation of PEO and PPO blocks in EAN is less obvious than that in water and hence the assembly structures in the EAN system are less ordered.This is confirmed from the SAXS curves and POM graphs.(3)In the F127-EAN binary system,I₁ phase is found to be the dominant mesophase formed.This is different from the P123 system,in which H₁ and L_αphases occupy large composition regions.The reason is that the high PEO content in F127 favors normal structures.(4)P123 can assemble into LLC phases in imidazolium ionic liquids,such as [Bmim]PF₆,[Bmim]BF₄,and[C₂mim]BF₄,the formation of which also originates from the strong interaction between ionic liquids and the PEO blocks.It is presumed that if a solvent has distinct solvation of PEO and PPO blocks,the copolymer might self-assemble in this solvent.Long alkyl chains of ionic liquid are not favorable for copolymer assembly due to the enhanced interaction between alkyl chains and PPO blocks.(5)LLC phases in the P123-ionic liquid binary systems can accommodate some n-butanol molecules without changing structures.These n-butanol molecules are located among copolymer molecules and participate in the construction of the interface.With the existence of n-butanol,butyl groups of imidazolium ionic liquids are helpful for the stability of ordered assembly structures.Above results enrich studies on block copolymer self-assembly in non-aqueous medium and provides useful information for further research on aggregation of amphiphilic molecules in ionic environment. 2.New LLC systems with ionic liquids as surfactants.With the help of POM and SAXS,we have investigated LLC phases formed in the ternary system of 1-octyl-3-methylimidazolium chloride([C₈mim]Cl),n-decanol(C₁₀H₂₁OH),and water and have mapped the phase diagram.The formation mechanism of the mesophases has been analyzed.Besides,we have also studied the effect of alcohols (C₆H₁₃OH,C₈H₁₇OH,and C₁₂H₂₅OH),counter-ions(BF₄^- and PF₆^-),alkyl chains ([C₁₆mim]Cl),and head groups([C₁₆mim]Cl and CTAB)of ionic liquids on the phase behavior.Following results are obtained.(1)Amphiphilc ionic liquid[C₈mim]Cl can form L_αphase with C₁₀H₂₁OH and water.The formation of the LLC phase is considered as a synergetic result of the hydrophobic force and the hydrogen-bonded network comprising an imidazolium ring, Cl^-,water,and alcohol.(2)In this imidazolium ionic liquid system,alcohols like C₆H₁₃OH,C₈H₁₇OH,and C₁₂H₂₅OH,act as co-surfactants and participate in the construction of hydrophobic bilayers in the LLC phases,thus providing a simple approach to modulate the mesophase structures by changing alcohols.However,C₄H₉OH does not lead to the formation of LLC phase in[C₈mim]Cl aqueous solution for its too short alky chains.(3)The counter-ions of ionic liquids have different interaction strength with C₁₀H₂₁OH molecules:Cl^-＞BF₄^-＞PF₆^-,which results in the following discrepancy on macroscopic phase behavior of the three corresponding ternary systems.Only the [C₈mim]Cl system exists the region of liquid crystalline phase.From[C₈mim]Cl to [C₈mim]BF₄ to[C₈mim]PF₆,the region of isotropic solution in the ternary phase diagrams shrinks while the bi-phasic or multi-phasic region enlarges.(4)[C₁₆mim]Cl show much higher surface activity for its longer alkyl chain.In the [C₁₆mim]Cl-C₁₀H₂₁OH-H₂O ternary system,there are a L_αphase with much larger region than that in the[C₈mim]Cl system and a newly formed H₁ phase.[C₁₆mim]Cl is similar to CTAB in molecular structure except for head groups.Interaction among the imidazolium head groups of ionic liquid is much stronger than that of CTAB. Furthermore,due to "Ï€-Ï€stacking" interaction,the disk-like imidazolium head groups have a strong tendency to arrange themselves mutually in a parallel fashion. Both of the two factors lead to no reversed LLC phases in the[C₁₆mim]Cl ternary system.This work pays attention to the phase behavior of ternary systems formed by amphiphilic ionic liquids and succeeds in modulating the LLC phase by the third component(alcohols).The results not only expand the research on aggregation behavior of amphiphilic ionic liquids,but also increase the types of ordered molecular assemblies.3.Application of LLC in templating synthesis of nanomaterials.LLC templates from the sodium bis-(2-ethylhexyl)sulfosuccinate(AOT)-isooctane-H₂O ternary system,the P123-EAN binary system,and the[C₈mim]Cl-C₁₀H₂₁OH-H₂O ternary system have been used to prepare nanomaterials through electrophoretic deposition,in situ chemical reduction,and ultraviolet irradiation,respectively.All of these experiments demonstrate that LLC is an excellent soft template for material synthesis.(1)Partly organized nanoparticle arrays are obtained from the electrophoretic deposition of oleate-stabilized silver nanoparticles in lamellar LLC template composed of AOT,isooctane,and water.Lamellar structure of hybrid makes nanoparticles arranged densely and ordered,for the reason that nanoparticles are restricted in the water layer of the liquid crystal.Though higher electric fields are favorable to overcome stronger diffusion barriers for nanoparticles in LLC,this is limited in aqueous media because of the electrolysis of water.One possible way to solve this problem may be realized by using ionic liquids to substitute water for construction of novel LLC template.This work supplies a promising approach for the assembly of ordered nanoparticle arrays.Motivated on increasing importance for nanoparticles confined in the nanoscale domain as diagnostic and therapeutic tools in medicine or cellular biology, research on electrophoresis of nanoparticles in LLC is valuable and helpful for potentially biochemical and biomedical applications.(2)Tie-like or truncated wedge-shaped Ag micro-and nanomaterials are successfully synthesized through in situ chemical reduction of AgNO₃ in the hexagonal phase of the P123-EAN binary system.The final materials do not replicate the template structure,and are produced by stacking of Ag single-crystalline slices, formation of which indicates the greater importance of single-crystal growth than the action of hexagonal template.In this deposition process,the ionic liquid EAN not only supplies the solvent,but also absorbs on specific facet of Ag nuclei for their extending growth.These materials might have potential applications for catalysis and electrode.(3)Bunchiness-shaped Pt nanomaterials or nanofibers are obtained through ultraviolet irradiation of H₂PtCl₆ in the lamellar phase of the [C₈mim]Cl-C₁₀H₂₁OH-H₂O ternary system.H₂PtCl₆ in the water domain has some influence on the LLC phase structure for its electrostatic interaction with the head groups of[C₈mim]Cl.The lower the H₂PtCl₆ concentration is,the less obvious the influence would be.With the fluctuation of the bilayer and the effect of the deposits, the lamellar template inevitably show some local distortion and lead to the formation of short thread-like deposits,which further stack into the final bunchiness-shaped materials.Obtained results suggest novel,simple and effective ways to produce nanomaterials with controllable shape and size,as well as a clue to understand the mechanism for crystal growth.Thanks for the supports from the National Natural Science Foundation of China (No.20573066 and 20773080).