Study on the morphological regulation mechanism of hollow silica microsphere prepared via emulsion droplet template

The morphology regulation of hollow silica microspheres is significant for their properties and applications. In this paper, hollow silica microspheres were formed through the hydrolysis and condensation reaction of tetraethyl orthosilicate (TEOS) at the interface of the emulsion droplet templates composed of liquid paraffin and TEOS, followed by dissolving paraffin with ethanol. The effects of various factors including the emulsifier structure and content, TEOS content, catalyst type, and the ethanol content in the continuous water phase on the particle size, shell thickness and morphology of the prepared hollow silica microspheres were studied in detail. The results show that the diffusion and contact of TEOS and water molecules as well as the hydrolysis condensation reaction of TEOS at the oil-water interface are two critical processes for the synthesis and morphological regulation of hollow silica microspheres. Cationic emulsifier with a hydrophobic chain of appropriate length is the prerequisite for the successful synthesis of hollow silica microspheres. The ethanol content in water phase is the dominant factor to determine the average diameter of hollow microspheres, which can vary from 96 nm to 660 nm with the increase of the volume ratio of alcohol-water from 0 to 0.7. The silica wall thickness varies with the content and the hydrophobic chain length of the emulsifier, TEOS content, and the activity of the catalyst. The component of the soft template will affect the morphology of the silica wall. When the liquid paraffin is replaced by cyclohexane, hollow microspheres with fibrous mesoporous silica wall are fabricated. This work not only enriches the basic theory of interfacial polymerization in the emulsion system, but also provides ideas and methods for expanding the morphology and application of hollow silica microspheres.     

Two-dimensional ordered arrays of monodisperse silica particles grafted with concentrated polymer brushes

Hybrid particles which have a core of monodisperse silica particle and a shell of well-defined poly(methyl methacrylate) chains end-grafted on the core surface with a surface density as high as 0.8 chains/nm² were prepared by surface-initiated atom transfer radical polymerization of methyl methacrylate with an initiator-fixed silica particle. Monolayers of the hybrid particles were formed at the air-water interface by depositing a defined amount of the particle suspension onto water surface. Transmission electron microscopic and atomic force microscopic observations of these monolayers showed that the hybrid particles formed a two-dimensional hexagonally ordered lattice with a wide controllability of interparticle distance. This lattice structure was utilized as a template for the fabrication of a negatively patterned surface of poly(dimethylsiloxane) elastomer.     

反相微乳液法制备纳米SiO2的研究

在壬基酚聚氧乙烯5醚(NP-5)/环己烷/氨水的反相微乳液体系中，进行正硅酸乙酯(TEOS)的水解、缩合反应，得到粒径在30～50 nm的单分散纳米SiO₂胶体。红外光谱法(FTIR)及透射电子显微镜(TEM)观察证明了纳米SiO₂粒子的生成。反相微乳液体系相图的研究表明，水相为氨水比纯水有较窄的W/O型微乳区。氨水微乳液是碱催化TEOS水解、缩合制备纳米SiO₂粒子的适宜体系。当体系中TEOS的浓度增大时，粒子的粒径随之增大。降低NP-5     

Preparation and characterization of silica aquasols

Aquasols containing silica nanoparticles with diameters of 75 to 95 nm were obtained directly by hydrolysis of 2 wt.% tetraethoxysilane (TEOS) in water in the presence of a non-ionic surfactant. The reaction was catalyzed by hydrochloric acid, ammonia, or sodium hydroxide. The particle size, which mainly depends on the concentration of TEOS in water, was determined by dynamic light scattering (DLS). Whereas the catalysts have almost no influence on the particle size, they very strongly affect the morphology of the silica particles formed. The dried SiO(2) particles obtained via the HCl-catalyzed reaction have film-forming properties and show no measurable BET surface area. SiO(2) particles prepared with ammonia as catalyst form nanoporous films on glass, and the BET surface area of the freeze-dried particles is 540 m(2)/g. Using sodium hydroxide as catalyst results in some agglomeration of uniform spherical particles with a BET surface area of 237 m(2)/g. (29)Si MAS NMR investigations of the freeze-dried particles provide information about the degree of condensation and the ratio of "free" hydroxyl groups. The silica aquasols described have a surprisingly high hydrophilizing effect on hydrophobic fibers (PP, PET). Silica nanoparticles of comparable diameters, prepared by the "St?ber method", dispersed in alcohol do not show any hydrophilizing properties worth to mention.     

单分散聚丙烯酸丁酯-二氧化硅核壳粒子的制备

近年来,有机-无机核壳材料因其具有可调的光、电、磁等特性而备受关注．无机物外壳可以增强粒子的热力学稳定性、机械强度和抗拉性能．高分子乳胶粒内核具有弹性,且易成膜,外部包覆无机物的乳胶粒可结合两者特性并产生协同效应．     

单分散酸性纳米二氧化硅的合成新方法

非极性有机溶剂中,乙酸和醇在没有酸性催化剂的情况下发生酯化反应,酯化生成的水水解TEOS(硅酸乙酯)合成单分散酸性纳米二氧化硅,粒径从数十纳米到数百纳米。TEM研究表明,溶剂的极性影响二氧化硅的形态,只有在非极性溶剂中才可以得到球形粒子,醇的种类和TEOS的浓度影响粒子的大小和粒径分布,利用FTIR和GC对TEOS的水解和二氧化硅形成过程进行研究。同时,文中提出了有机相在TEOS的酯化水水解、晶核的形成和生长的过程模型。     

单分散二氧化硅球形颗粒的制备与形成机理

在醇水混合溶剂中以氨作催化剂，正硅酸乙酯为硅源，通过溶胶—凝胶工艺制 备单分散二氧化硅球形颗粒，通过透视电镜进行研究各种反应条件如溶剂类型、氨 和水的浓度、水解温度等对二氧化硅的颗粒大小和形貌的影响．结果显示：以甲醇 和乙醇为溶剂可以形成单分散的二氧化硅球形颗粒，以丙醇和丁醇为溶剂，二氧化 硅球形颗粒容易聚集；在其它条件不变的情况下，球形颗粒的粒径随水和硅源的浓 度增加而增大；而且水解温度的升高，生成的颗粒粒径也逐渐增大，仔细研究和讨 论了二氧化硅颗粒在不同反应条件下的形成机理．     

Nucleation of silica Stöber particles in the presence of methacryloxypropyltrimethoxysilane

The effect of 3-methacryloxypropyltrimethoxysilane (MPTMOS) on the nucleation of silica particles synthesized in a water?ethanol?ammonia?tetraethoxysilane (TEOS) mixture by the Stöber?Fink?Bohn method has been studied. It has been shown, using atomic force microscopy, that, as the content of MPTMOS in a TEOS + MPTMOS precursor mixture is increased from 0 to 12.5 mol %, the final silica particle size decreases from 470 to 10 nm, because the number of nucleation centers increases by several orders of magnitude. In contrast to TEOS, hydrolysis of MPTMOS yields a smaller amount of deprotonated orthosilicic acid monomers, the condensation of which is hindered by electrostatic repulsion. The polycondensation of electrically neutral products of MPTMOS hydrolysis gives rise to a larger number of nucleation centers in the reaction mixture.     

Particle formation in the hydrolysis of tetraethyl orthosilicate in pH buffer solution

Particle formation in the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) was studied by varying pH (9.5-11) with the basic catalysts NH3, methylamine (MA), and dimethylamine (DMA) in the presence of 5 mol/m3 CH3COOH, which was chosen to suppress time variations of pH and ionic strength during the reaction. Spherical particles were formed for MA and DMA at catalyst concentrations of 0.02-0.2 kmol/m3 and for NH3 at catalyst concentrations of 0.1-1.5 kmol/m3. In a common range of catalyst concentrations for spherical particle formation, average particle size was largest for DMA and smallest for NH3. Hydrolysis rate of TEOS could be quantified by the use of buffer systems as a function of TEOS and OH- concentrations. A specific relation was not found between the hydrolysis and the particle size. The zeta potential of silica particles measured in the reaction solvent was in the order DMA < MA < NH3, and ionic strength, estimated from pH in the reactions, was in the order DMA approximately equal to MA > NH3. This suggested that the particle sizes were controlled by electrostatic particle interactions.     

Preparation and characterization of some unusually transparent poly(dimethylsiloxane) nanocomposites

A technique was developed for preparing poly(dimethylsiloxane) nanocomposites having unusually high transparencies as quantitatively judged by ultraviolet–visible spectroscopy. The method was the in situ generation of silica particles by a two‐step sol–gel procedure in which the required water of hydrolysis was simply absorbed from the air, and the catalyst was generated in situ from a tin salt. Electron microscopy showed that the phase‐separated silica domains were very small (30–50 nm in diameter) and well dispersed, as expected from the transparency of the composites. Stress‐strain measurements in tension indicated that the particles provide very good reinforcement. Ultra‐small‐angle X‐ray scattering data showed that the domain morphology depends strongly on catalyst, but weakly on loading level. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1897–1901, 2003     

Synthesis of monodisperse fluorescent core-shell silica particles using a modified Stober method for imaging individual particles in dense colloidal suspensions

Core-shell silica particles, with a diameter of 1.5 mum, containing a dye fluorescein isothiocyanate (FITC), are synthesized by the hydrolysis and condensation of tetraethylorthosilicate (TEOS). Sodium dodecyl sulfate (SDS) is added to synthesize fluorescent core particles with the diameter of approximately 1 mum. In the addition of SDS, the surface charge reduced by counterions (Na+) of the surfactant leads to a higher degree of aggregation of the primary particles and the formation of larger secondary particles. The particle growth kinetics confirms the aggregation growth model for the synthesis of monodisperse silica particles, and also shows the dependence of final particle size on colloidal stability resulting from the addition of SDS. Light and X-ray scattering data reveal that the final particles have compactly packed structures with smooth surfaces. The seeded growth technique is then used to form a silica shell layer on the fluorescent core. The added amount of water and NH4OH has significant effects on shell formation. Finally, the final core-shell silica particles are modified by chemisorption of octadecanol at the surface to be dispersed in organic solvents. Octadecyl-coated silica particles are sterically stabilized in silica index-matching solvents such as chloroform and hexadecane to directly image separate particles using confocal microscopy. In chloroform, the organophilic silica particles disperse well, whereas in hexadecane they form a volume-filling gel structure at room temperature.     

Tapping mode AFM evidence for an amorphous reticular phase in a condensation-cured hybrid elastomer: alpha,omega-dihydroxypoly(dimethylsiloxane)/poly(diethoxysiloxane)/fumed silica nanoparticles

A new surface phenomenon is reported for hybrid nanocomposites comprising (1) a low Tg poly(dimethylsiloxane) (PDMS) phase cross-linked by (2) a siliceous phase (SP) generated by in situ hydrolysis/condensation of poly(diethoxysiloxane) (PDES), and (3) fumed silica nanoparticles (FSN). After ambient temperature cure, tapping mode atomic force microscopy (TM-AFM) easily reveals near-surface FSN. For nanocomposites with higher PDES content, FSN surprisingly "disappear" after a further cure at 100 degrees C. The observation is explained by further condensation of extant siliceous fragments creating an amorphous reticular phase, which acts as a mechanical barrier between the FSN and the AFM tip.     

由硅酸酯合成单分散二氧化硅中碳的化学形态

单分散二氧化硅是指尺寸分布十分狭窄的二氧化硅颗粒．单分散颗粒在科学研究及工业应用中得到了广泛的应用［1］．单分散二氧化硅由正硅酸有机酯在氨催化下于醇溶液中水解缩合得到．硅酸酯的水解和缩合反应可用如下反应描述．总的反应式为：nSi（OR）4＋2nH20→nSiO2＋4nROH1956年Kolbe［2］发现正硅酸乙酯（TEOS）在碱催化下于乙醇溶剂中水解反应有时会形成均一颗粒二氧化硅以来，许多学者对这一反应体系进行了较为广泛的研究，提出了双分子缩合成核机理、单分子叠加生长机理、表面反应控制生长机理、扩散控制生长机理和微晶核团聚生…     

Silica-surfactant-polyelectrolyte film formation: evolution in the subphase

We have previously reported that robust mesostructured films will grow at the surface of alkaline solutions containing cetyltrimethylammonium bromide (CTAB), polyethylenimine (PEI), and silica precursors. Here we have used time-resolved small-angle X-ray scattering to investigate the structural evolution of the micellar solution from which the films form, at several different CTAB-PEI concentrations. Simple models have been employed to quantify the size and shape of the micelles in the solution. There are no mesostructured particles occurring in the CTAB-PEI solution prior to silica addition; however, after the addition of silicate species the hydrolysis and condensation of these species causes the formation of mesophase particles in a very short time, much faster than ordering observed in the film at the interface. The mesophase within the CTAB-PEI-silica particles finally rearranges into a 2D hexagonal ordered structure. With the aid of the previous neutron reflectivity data on films formed at the air/water interface from similar solutions, a formation mechanism for CTAB-PEI-silica films at the air/water interface has been developed. We suggest that although the route of mesostructure evolution of the film is the same as that of the particles in the solution, the liquid crystalline phase at the interface is not directly formed by the particles that developed below the interface.     

ABC copolymer silicone surfactant templating for biomimetic silicification

Using the ABC copolymer silicone surfactant polydimethylsiloxane (PDMS)-graft-(polyethylene oxide (PEO)-block-propylene oxide (PPO)) (PSEP, Scheme 1a) as a template and tetraethoxysilane (TEOS) as a silica source, silica particles with various structures and morphologies (i.e., disordered spherical micellar aggregation, two-dimensional p6mm mesostructure, asymmetric multi-layer non-equilibrium vesicles and symmetric monolayer vesicles) were synthesized by changing the synthesis temperature from 30 to 80 °C. Increasing the hydrophobicity of the surfactant by increasing the temperature resulted in an increase in the surfactant packing parameter g, which led to the mesophase transformation from micellar to cylinder and later to a lamellar structure. The good compatibility between the PDMS and the TEOS, the different natures of the hydrophobic PDMS and PPO segments, and the hydrolysis and condensation rates of TEOS enabled the variation of silicification structures. This novel silicone surfactant templating route and a new type of materials with highly ordered mesostructures and asymmetric morphologies provide a new insight into the molecular factors governing inorganic-organic mesophase and biosilicification for fabricating functionalized materials.     

Preparation of mesoporous submicrometer silica capsules via an interfacial sol-gel process in inverse miniemulsion

Mesoporous silica capsules with submicrometer sizes were successfully prepared via the interfacial hydrolysis and condensation reactions of tetraethoxysilane (TEOS) in inverse miniemulsion by using hydrophilic liquid droplets as template. The inverse miniemulsions containing pH-controlled hydrophilic droplets were first prepared via sonication by using poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)-PEO) or SPAN 80 as surfactant. TEOS was directly introduced to the continuous phase of an inverse miniemulsion. The silica shell was formed by the deposition of silica on the surface of droplets. The formation of capsule morphology was confirmed by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mesoporous structure was verified by nitrogen sorption measurements. The specific surface area could be tuned by the variation of the amount of cetyltrimethylammonium bromide (CTAB) and TEOS, and the pore size by the amount of CTAB. The influences of synthetic parameters on the particle size and morphology were investigated in terms of the amount of CTAB, pH value in the droplets, TEOS amount, surfactant amount, and type of solvent with low polarity. A formation mechanism of silica capsules was proposed.     

Ammonia Catalyzed Hydrolysis-Condensation Kinetics of Tetraethoxysilane/Dimethyldiethoxysilane Mixtures Studied by <Superscript><Emphasis Type="Bold">29</Emphasis></Superscript>Si NMR and SAXS

In-situ ²⁹Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the ammonia catalyzed hydrolysis and condensation of the mixed systems of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) dissolved in methanol. With ammonia catalysis, the hydrolysis reaction orders for TEOS and DDS in the mixed systems remained first order, which is similar to that observed for their corresponding single silane component precursor systems. The hydrolysis rate constant for TEOS in the mixed systems was larger than that of TEOS in the single silane component precursor systems. Meanwhile, the hydrolysis rate constants of DDS in the mixed precursor systems were smaller than those of DDS in the single silane component precursor systems. The hydrolysis and condensation kinetics showed more compatible hydrolysis-condensation relative rates between TEOS and DDS, which remarkably affected the final microstructure of the resulting silica particles. Small angle X-ray scattering (SAXS) experiments showed a typical double fractal structure in the particulate networks.     

Dependence of silica particle sizes on network chain lengths,silica contents,and catalyst concentrations in in situ‐reinforced polysiloxane elastomers

Poly(dimethylsiloxane) networks were prepared by tetrafunctionally end‐linking hydroxyl‐terminated chains with tetraethoxysilane (TEOS). Molecular composites were then prepared by in situ sol–gel reactions on additional TEOS swelled into the networks, resulting in the formation of reinforcing silica fillers within the host elastomers. The amount of filler generated generally increased linearly with an increase in the TEOS swelling ratio, as expected. The silica particles formed were examined by small‐angle X‐ray scattering. Of particular interest were the relationships between particle size and molecular weight M_c of the network chains (mesh sizes), amount of filler introduced, and catalyst concentration. Particle sizes were smallest for the smallest values of M_c, possibly demonstrating constraining effects from the very short network chains. At fixed M_c and filler concentrations, higher catalyst concentrations gave larger particles. Increase in filler concentration generally had little effect on particle size at low and high loadings, but markedly increased sizes at intermediate levels (10–20 wt %), presumably caused by coalescence of the scattering entities into considerably larger aggregates. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1421–1427, 1999     

Preparation and microstructure of sol-gel derived silver-doped silica

Silver-doped silica was prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄) in the presence of a silver nitrate (AgNO₃) solution by two different synthesis methods. In the first synthesis route, sol-gel mixtures were prepared using an acid catalyst. In the second synthesis route, silver-doped silica gels were formed by two-step acid/base catalysis. For the same concentration of silver dopant [AgNO₃]/[TEOS] = 0.015 acid-catalyzed sol-gel formed a microporous silica with an average pore size of <25 Å whereas the two-step catalyzed silica had an average pore size of 250 Å and exhibited a mesoporous structure when fully dried. The differences in the pore size affected the silver particle formation mechanism and post-calcination silver particle size. After calcination at 800 °C for 2 h the acid-catalyzed silica contained metallic silver particles size with an average particle size of 24 ± 2 nm whereas two-step catalyzed silica with the same concentration of [AgNO₃]/[TEOS] = 0.015 contained silver nanoparticles with an average size of approximately 32 ± 2 nm. Mechanisms for silver particle formation and for silica matrix crystallization with respect to the processing route and calcination temperature are discussed.     

Synthesis of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: Effects of the Water/Surfactant Molar Ratio and Ammonia Concentration

The effect of ammonia concentration on the region of existence of single-phase water-in-oil microemulsions has been investigated for the system polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/ammonium hydroxide. The presence of ammonia decreases the size of the microemulsion region. A minimum concentration of surfactant (estimated at about 1.1 wt%) is required for solubilization of the aqueous phase; this value is not significantly affected by ammonia concentration. As indicated by fluorescence spectral data, the transition between bound and free water occurs when the water-to-surfactant molar ratio is about 1 and the presence of ammonium hydroxide does not appear to have a significant effect on this. Ultrafine (30-70 nm diameter), monodisperse silica particles produced by hydrolysis of tetraethoxysilane (TEOS) in the microemulsion show a complex dependence of the particle size on the water-to-surfactant molar ratio (R) and on the concentration of ammonium hydroxide. At relatively low ammonia concentration in the aqueous pseudophase (1.6 wt% NH3) the particle size decreases monotonically with increase in R. However, for higher ammonia concentrations (6.3-29.6 wt% NH3) a minimum in particle size occurs as R is increased. These trends are rationalized in terms of (a) the effects of the concentration, structure, and dynamics of the NP-5 reverse micelles on the hydrolysis and condensation reactions of TEOS, and (b) the effects of ammonia concentration on the stability of the microemulsion phase, the hydrolysis/condensation reactions of TEOS, and the depolymerization of siloxane bonds. Copyright 1999 Academic Press.