Preparation And Characterization Of New Vesicles And Foams

Porous materials have attracted considerable attention in the areas such as separation of large molecules,biosensors,catalysis,adsorption,microelectronics, optics,and fabrication of novel nano-objects because of their uniform and adjustable properties.Generally,porous materials are fabricated by the template approach.By utilizing a liquid-crystal templating(LCT) method organized mesostructured inorganic materials can be directly or cooperatively assembled.Hubert firstly reported the vesicle templating(VT) synthesis of unilamellar silica vesicles by directly coating the pre-formed unilamellar vesicles of ionic surfactants.Although the phase behavior of block copolymers has been extensively studied,the block-copolymer VT method to synthesize inorganic unilamellar vesicular structure has not been reported. Furthermore,a large-scale,high-yield,cheap synthesis of unilamellar vesicles with sub-micrometer diameters and ultrahigh pore volumes is currently a challenge in both block-copolymer and hybrid material studies.Macroporous materials with large pore size of＞50 nm have been extensively studied and exhibited significant applications in different areas.However,ordered macroporous materials are usually synthesized by means of emulsion,microemulsion, or colloidal sphere templating methods.For future applications of functional and economic macroporous materials,it is important to search a facile and green approach to fabricate ordered macroporous materials without adding any organic solvents or hard templates to produce large pores.The current contribution concerns the synthesis of unilamellar siliceous vesicles and nanofoams with large pore diameters and high pore volumes by employing commercial triblock copolymer as templates in the absence of organic cosolvents.A new cooperative vesicle templating(CVT) approach is proposed.Novel inorganic siliceous materials with various morphologies and hierarchical pore structures are obtained by varying the reaction temperature,pH,inorganic salts and ionic strength. The transformation between the LCT and CVT approaches is revealed.In chapter 2,we report the synthesis of unilamellar siliceous vesicles and nanofoams with ultrahigh pore volumes(＞3 cm³g^-1) by using EO₂₀PO₇₀EO₂₀(P123) as a template in near neutral aqueous solutions.At controlled pH,a structural transformation from tubules to unilamellar vesicles then to nanofoams is observed by increasing the reaction temperature.It is proposed that the siliceous vesicles are synthesized via a co-operative block-copolymer vesicle templating approach,while the siliceous nanofoams are obtained by the fusion of vesicles at increased ionic strength.Compared to literature methods to synthesize siliceous vesicles and foams, our method is convenient,cheap,and produces a high yield.Siliceous nanofoams synthesized by using our approach show superior bioimmobilization capacity over other porous materials for biomolecules with large molecular weight.In chapter 3,macroporous ordered siliceous foams(MOSFs) with cage sizes on the order of～110 nm have been successfully synthesized via a facile and green approach.The synthesis is carried out under mild condition in the absence of organic cosolvent,similar to the biosilica formation process in the slightly acidic environment in silica deposition vesicles(SDV) in diatoms.The fusion of soft unilamellar composite vesicles with relatively uniform size finally results in MOSFs,and the electron tomography(ET) characterization shows that MOSFs have well ordered and defined honeycomb structures at the macroscale,which is energetically beneficial. The formation of MOSFs is influenced by the reaction time,the hydrothermal treatment process,the reaction pH,and the concentration of sodium sulfate(Na₂SO₄) during the synthesis.When the concentration of Na₂SO₄ is adjusted between 0.1 and 0.45 M when the other reaction conditions are not changed,a structural transition from vesicles to tightly aggregated vesicles and then MOSFs and finally large compound vesicles(LCV) has been observed.In chapter 4,the influences of reaction temperature,pH,different inorganic salts and the amount of the silica source have been carefully investigated,and the transformation between the LCT and CVT approaches has been revealed.In buffer solution at pH=5.0,a limited variation of temperature from 15 to 35℃leads to a structural evolution from ordered hexagonal mesostructure with a pore size of 10 nm to MOSFs with the pore size of～110 nm,inducing a pore expansion of～10 times. Under acidic conditions(pH=2-7 HCI solution),a structural transformation from ordered hexagonal mesostructure(pH＜3) to vesicles(pH=3-4) is also observed.With increasing ion strength,a structural transformation from vesicle to tightly aggregated vesicle and then MOSFs can be achieved.When different types of salts with the same ion strength are used,it is found that not only the anions,but also the cations have great influence on the final structures.Finally,by increasing the molar ratio of silica precursor to the organic template,a continuous structural transformation from vesicles to MOSFs and finally to vesicles with mesoporous walls can be achieved.In chapter 5,TiO₂-SiO₂ porous materials with tunable Si/Ti molar ratio(R) have been successfully prepared through a one-pot method under a near neutral condition. With decreasing Si/Ti R,a phase transition from a macroporous foam-like structure to mesostructure is observed.The resultant TiO₂-SiO₂ porous materials possess large surface areas and high pore volumes.In addition,the titania species are homogenously dispersed in silica matrix when Si/Ti R≥10.Our preliminary results show that such functional porous materials have good performance in the enrichment of phosphopeptides for proteomic research.