高比表面积窄孔分布氧化铝的制备研究 Ⅱ.加硅的影响

 采用pH摆动法，以硫酸铝为铝源，以氢氧化钠为碱沉淀剂，考察了添加SiO2对沉淀氧化铝物性的影响．通过孔结构分析、粒度测定和电镜观察等证实，加入少量SiO2可使沉淀粒子分散、变小，颗粒相对均匀，从而提高了氧化铝的比表面积和孔集中的程度．当加入2．5％的SiO时，pH仅摆动2次，即可使氧化铝粉体的孔体积高达1．2ml／g，比表面积达380m2／g．这类氧化铝的孔结构适宜，粒子小，易直接成型为孔径集中和耐压强度好的载体，故用于重油高压加氢脱氮反应具有较好的性能．沉淀时酸侧pH值降低，尽管沉淀氧化铝的孔径向较小的方面集中，但此时沉淀粒子呈紧密堆积，颗粒变大，比表面积下降，氧化铝沉淀粒子的结构发生改变．不同结构的氧化铝表现出不同的催化活性．     

Zinc oxide colloids with controlled size, shape, and structure

Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum. Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase. Depending on the experimental conditions, the size of the final polycrystalline particles formed by the aggregation of nanosize entities varied from 100 to 300 nm. The reaction temperature affected both the size of the nanosize precursors and their arrangement in the final particles. At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 degrees C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms. The aspect ratio and the internal structure of the latter could be altered by changing the nature of the zinc salt, the addition rate, and the initial concentration of the reactants. Based on the findings of the study a two-stage mechanism for the formation of uniform polycrystalline particles with well-defined geometric shapes is proposed.     

Aggregates of amphiphilic fluorinated copolymers and their encapsulating and unloading homopolymer behaviors

Amphiphilic fluorinated block copolymers synthesized via reversible addition‐fragmentation chain transfer polymerization were used for the preparation of aggregates of various morphologies. First, dissolve the copolymer in 2‐butanone; second, add a precipitant solvent, which was the mixture of water and methanol, to induce the aggregation of the hydrophobic fluorinated block. With a hydrophilic tail and a very hydrophobic segment, these copolymers are likely to self‐assemble in solution and form aggregates. Observed by TEM, spheres, rods, and vesicles can be formed by changing the precipitant mixture contents. Besides, these aggregates were found to be able to carry hydrophobic fluorinated homopolymers, and two suggested processes have been proposed to explain their morphology changes from original spheres, rods and vesicles into larger size spheres. Finally, hollow bilayer spheres and tubules can be achieved after extracting homopolymers in the center of the newly formed spheres. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1000–1006, 2008     

Variability of nanosystems

The mechanism of synchronization of molecular fluctuations operates on the surface of growing nanoparticles, increasing their scale up to macroscopic. The kinetic equation for the function of state distribution of nanoparticles therefore approaches the Fokker-Planck equation, which was established experimentally by the example of nanoparticles of different natures. In a number of suspensions and aerosols in which ordered aggregates of nanoparticles are formed, aggregation is a multistage process; at each stage, variation in the function of size distribution of the aggregates is also described by the Fokker-Planck equation. Multistage aggregation is completed when particles with multilevel hierarchical texture, which have a tendency to assume polyhedral form, appear in the system. The abovementioned features of nanosystems can be interpreted as indications of the variability and self-assembly ability of systems.     

Stability and state of aggregation of aqueous fibrinogen and dipalmitoylphosphatidylcholine lipid vesicles

The stability and state of aggregation of aqueous fibrinogen (FB) and dipalmitoylphosphatidylcholine (DPPC) vesicles in water or buffer at 25 degrees C were studied with dynamic light scattering (DLS), UV-vis spectroturbidimetry (ST), and cryo-transmission electron microscopy (cryo-TEM). In water, when 1000 ppm (0.10 wt %) DPPC dispersions were prepared with a protocol including extensive sonication, they contained mostly vesicles and were quite clear, transparent, and stable for at least 30 days. FB mixtures with water (0.075 wt %) were quite unstable and biphasic. They formed large aggregates which eventually precipitated. The addition of DPPC vesicles into these unstable FB dispersions reversed FB aggregation and precipitation and produced stable translucent microdispersions. The inferred lipid/protein aggregates were limited in size, with average diameters ranging from 200 to 300 nm. In buffer, DPPC dispersions were also clear and quite stable, with average dispersed particles diameter of ca. 90 nm. FB dissolved in aqueous buffer and formed transparent and stable solutions. Adding salt to an aggregated FB dispersion in water reversed the aggregation. FB aggregated and redissolved in the presence of the citrate and after the citrate was removed. There was no effect of citrate (present in FB initially) in the FB aggregation or redissolution. FB molecules in buffer form dimers or higher aggregates. Their average aggregation number is 2, determined with Rayleigh scattering analysis of turbidity data. The average hydrodynamic diameter of FB solutions from DLS was 30 nm. Mixing a stable FB solution in buffer and a stable DPPC dispersion in buffer produced highly unstable mixtures, in which large aggregates precipitated. These results have implications in understanding the interactions of lipids and proteins in many biological applications and food processing applications.     

Poly(1,2-dimethyl-5-vinylpyridinium methyl sulfate). IV. Flocculation of crystalline silica

Silica flocculated with a high molecular weight poly(DMVPMS) contains aggregates that are stable enough to permit size measurement with a Coulter counter. The average size of these aggregates increases up to a critical level of added polymer; the primary particles reappear at higher levels of added polymer. The aggregate size is reduced by continued mixing and the particle size distribution before flocculation is approached. Subsidence rates, equilibrium sediment volumes, and refiltration rates give somewhat different estimates of the degree of flocculation. Additional aggregation beyond that measured by the Coulter counter must be considered in the interpretation of these data. The decrease in floc strength during continued agitation is attributed to a disaggregation of the bridging polymer, to a decrease in the interparticle bonding of the bridging polymer, and to an increase in the surface coverage with polymer.     

DNA-induced size-selective separation of mixtures of gold nanoparticles

We present a novel method for size-selectively separating mixtures of nanoparticles in aqueous media utilizing the inherent chemical recognition properties of DNA and the cooperative binding properties of DNA-functionalized gold nanoparticles. We have determined that the melting temperatures (T(m)s) of aggregates formed from nanoparticles interconnected by duplex DNA are dependent upon particle size. This effect is proposed to derive from larger contact areas between the larger particles and therefore increased cooperativity, leading to higher T(m)s. The separation protocol involves taking two aliquots of a mixture of particles that vary in size and functionalizing them with complementary DNA. These aliquots are mixed at a temperature above the T(m) for aggregates formed from the smaller particles but below the T(m) for aggregates formed from the larger particles. Therefore, the aggregates that form consist almost exclusively of the larger particles and can be easily separated by sedimentation and centrifugation from the smaller dispersed particles. This unusual size-dependent behavior and separation protocol are demonstrated for three binary mixtures of particles and one ternary mixture.     

Stability and magnetic characterization of oleate-covered magnetite ferrofluids in different nonpolar carriers

This work describes the preparation and stability evaluation of suspensions consisting of hydrophobic magnetite nanoparticles dispersed in different organic solvents. The ferrite particles are covered by a shell of chemisorbed oleate ions following a procedure that is described in detail. The oleate-covered particles were dispersed in different organic solvents with dielectric constants, epsilon(r), ranging between 1.8 and 9, and the centrifugal field strength needed to remove particle aggregates formed during the synthesis was determined for the different liquid carriers used. A thermodynamic analysis demonstrated that the observed stability of the suspensions in liquids with epsilon(r) < 5 is well correlated with the very low lyophobic attraction between the particles. This can easily be surmounted by thermal agitation, since the van der Waals attraction is negligible. In contrast, for liquids with epsilon(r) > 9, the suspensions become unstable because of the combined action of the van der Waals and lyophobic attractions, the latter being dominant for very polar solvents. Finally, a complete magnetic characterization of the oleate-magnetite powder, as well as of several stable ferrofluids prepared with it, was carried out. From this characterization, the magnetic diameters and magnetic moments of the particles immersed in the different liquid carriers were estimated and compared to those corresponding to the dry magnetic particles. This made it possible to estimate the thickness of the nonmagnetic layer on the particles.     

Environmentally abundant anions influence the nucleation, growth, Ostwald ripening, and aggregation of hydrous Fe(III) oxides

The simultaneous homogeneous and heterogeneous precipitation of hydrous Fe(III) oxides was investigated in the presence of environmentally ubiquitous anions (nitrate, chloride, and sulfate). Experiments were conducted with 10(-4) M Fe(III) at acidic pH (pH = 3.7 ± 0.2), which often occurs at acid mine drainage sites or geologic CO(2) storage aquifers near injection wells. Quartz was used as a model substrate for heterogeneous precipitation. Small angle X-ray scattering (SAXS) and grazing incidence SAXS (GISAXS), atomic force microscopy (AFM), and dynamic light scattering (DLS) measurements were conducted. In situ SAXS/GISAXS quantified the size, total particle volume, number, and surface area evolutions of the primary nanoparticles formed in the nitrate and chloride systems. In both systems, the heterogeneously precipitated particles were smaller than the homogeneously precipitated particles. Compared with chloride, the volume of heterogeneously precipitated hydrous Fe(III) oxides on the quartz surface was 10 times more in the nitrate system. After initial fast heterogeneous nucleation in both nitrate and chloride systems, nucleation, growth, and aggregation occurred in the nitrate system, whereas Ostwald ripening was the dominant heterogeneous precipitation process in the chloride system. In the sulfate system, fast growth of the heterogeneously precipitated particles and fast aggregation of the homogeneously precipitated particles led to the formation of particles larger than the detection limit of GISAXS/SAXS. Thus, the sizes of the particles precipitated on quartz surface and in solution were analyzed with AFM and DLS, respectively. This study provides unique qualitative and quantitative information about the location (on quartz surfaces vs in solutions), size, volume, and number evolutions of the newly formed hydrous iron oxide particles in the presence of quartz substrate and ubiquitous anions, which can help in understanding the fate and transport of pollutants in the environment.     

Coagulation-flocculation of beech condensate: particles size distribution

Beech wood (Fagus sylvatica L.) condensate from a steaming operation was studied. The objective of our work was to study the precipitation of these wood extracts in presence of calcium ion after autoxidation at basic pH (8). The autoxidation was carried out at 250 rpm for 30 min, and flocculation was followed up for 30 min. An investigation with a laser sizer Mastersizer of Malvern has been done in order to study the influence of the agitation on the state of aggregation of the condensate. A negative correlation was observed between the mean size of particles and the agitation rate. Without stirring, flocculation rapidly occurred and the mean size of particles was high. Calcium-induced aggregation of the condensate was also found to be reversible toward agitation.     

Preparation of sustained-release chlorpyrifos particles via the emulsification coacervation method and their sustained-release performance

MMA-HEMA-MAA ternary random copolymer (PA) and CaCl₂ was used as carrier and precipitant, respectively, and emulsification coacervation was adopted to prepare sustained-release chlorpyrifos particles. The particle size, morphology, structure, and the sustained-released performance of the samples were characterized. Results showed that porous and random sustained-release chlorpyrifos particles piled up into small spherical particles. Hydrogen-bonding interactions between PA and chlorpyrifos molecules were observed, and chlorpyrifos was dispersed among the PA molecules as both crystal and noncrystal forms. Raising chlorpyrifos concentrations resulted in larger proportions of chlorpyrifos distributed in the crystalline state. Chlorpyrifos was loaded into the PA crosslinked network structure and the heat resistance of the pesticide was improved significantly. The sustained-release process of chlorpyrifos was controlled by Fick diffusion mechanism, and the release mechansim of the drug consisted with the Korsmeyer-Peppas kinetic equation.     

Redispergation of TiO2 particles in aqueous solutions

The colloidal stability of TiO2 dispersions in aqueous solutions was studied. Aqueous solutions of ATLAS G-3300 (1.57 x 10(-3) mol/l), TRITON X-100 (5 x 10(-5) mol/l), and PMAA (4 x 10(-6) and 5.81 x 10(-3) mol/l) have been used as medium for redispergation of TiO2 particles. Stability of dispersions was investigated at different pH values by two different methods. By using analytical centrifuge the sedimentation velocity of TiO2 particles was directly measured and by means of light scattering the particle size of dispersed particles has been monitored. Combination of these two methods allowed determination of the aggregation degree of TiO2 particles as well as structure of the aggregates formed in aqueous phase. It has been found that redispergation process does not provide complete separation of virgin TiO2 particles. Even in the case of stable dispersions some aggregates were found, which consisted of 2-4 virgin TiO2 particles. With increasing colloidal stability of dispersions aggregates appear to be spherically shaped. In the system where TRITON X-100 was used, formation of secondary aggregates by fusion of primary ones was observed.     

Kinetic evidence of a “matrix effect” in the bulk polymerization of acrylonitrile

The kinetics of the γ-ray-initiated polymerization of acrylonitrile in bulk are reexamined in broad ranges of temperatures and radiation dose rates. The discussion of the results coupled with an analysis of earlier data indicate that the polymerization of acrylonitrile proceeds by different mechanisms depending on the reaction temperature. Above 60°C the precipitated growing chains recombine readily; therefore, the autoaccelerated conversion curves cannot be accounted for by an “occlusion effect.” It is suggested that autoacceleration is caused by a fast propagation taking place in oriented monomer aggregates which result from dipole-dipole association of the monomer with the polymer chains formed in the early stages of the reaction (“matrix effect”). Below 10°C the precipitated growing chains are buried in the dead polymer and monomer diffusion toward the occluded chain ends is very limited (“occlusion effect”). Between 10 and 60°C the system gradually changes from one dominated by “occlusion” to one where the “matrix effect” determines the kinetic behavior. The conclusion based on kinetic data is in agreement with results obtained from studies of the postpolymerization in these various systems.     

沉淀剂对镍基甲烷化催化剂性能的影响

利用尿素、碳酸铵、氨水三种沉淀剂制备了不同的镍基甲烷化催化剂,考察了沉淀条件对催化剂性能的影响。通过XRD、H₂-TPR、BET、TPO等方法对催化剂进行表征。结果表明,使用不同沉淀剂所得催化剂性能各不相同。采用尿素沉淀剂,颗粒的比表面积达到了223.55m²/g,具有较稳定的催化活性;碳酸铵沉淀颗粒粒径较大,分散也不够均匀,催化剂更容易积炭;氨水沉淀制备的催化剂粒径小,与载体结合性强,高温下活性组分易于流失。分析认为,沉淀剂影响了催化剂前驱体的形态和结构,造成了分散度、晶相结构、对氢的吸附性质以及抗积炭性等多方面的差异,表现出甲烷化反应活性和稳定性的不同。     

Nanosize precursors as building blocks for monodispersed colloids

It is shown that many monodispersed colloid particles, precipitated in homogeneous solutions, are formed by aggregation of nanosize subunits. A model is described that specifies conditions which may yield such spherical particles of narrow size distribution by interactions of precursor singlets. A good agreement was achieved for size selection of gold and cadmium sulfide dispersions. It is illustrated that particles of other shapes may also formed by the aggregation mechanism, and the challenges facing attempts to quantify such processes are pointed out. Finally, examples are given of consequences caused by particles being composed of nanosubunits. The text was submitted by the author in English.     

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.     

Reversible aggregation Part 2. Kinetics of reversible aggregation

The aggregation of two large (5.1 and 2.6 m diameter) polymer latices was examined microscopically under perikinetic conditions. The aggregates formed were not rigid, and aggregation was reversible under some conditions. The kinetic behavior was followed by monitoring the relative concentrations of single particles and doublets during aggregation. The results were not consistent with Smoluchowski's theory of irreversible aggregation, but could be described by means of a set of mass action equations. Four simple models were used to describe the dissociation processes. The experimental results indicated that the most appropriate model depended on the electrolyte concentration and the dispersion used.     

Miniemulsion copolymerization of styrene–methyl methacrylate

Miniemulsion copolymerization of 50 : 50 weight fraction of styrene–methyl methacrylate monomer, using hexadecane as the cosurfactant, was carried out in both unseeded and seeded polymerizations. Effects of the hexadecane concentration and the ultrasonification time on the conversion–time curves and particle size of the final latex were investigated for unseeded polymerization. The kinetic and particle size distribution results showed that an increase in hexadecane concentration and ultrasonification time cause faster polymerization rate and smaller particle size. The mechanism of mass transport from miniemulsion droplets to polymer particles was also investigated for seeded polymerization. For this purpose a monomer miniemulsion was mixed with a fraction of a previously prepared miniemulsion latex particles prior to initiation of polymerization, using residual oil-soluble initiator in the seed latex. The concentration of hexadecane and a water-insoluble inhibitor (2,5 di-tert-butyl hydroquinone) in the miniemulsions were the main variables. Seeded polymerizations were also carried out in the presence of miniemulsion droplets containing a water-insoluble inhibitor and water-soluble initiator. The inhibitor concentration and the agitation speed during the course of polymerization were the experimental variables. The kinetic and particle size results from these seeded experiments suggested that collision between miniemulsion droplets and polymer particles may play a major role in the transport of highly water-insoluble compounds.     

Porous Al2O3/Al Metal Ceramics Prepared by the Oxidation of Aluminum Powder under Hydrothermal Conditions Followed by Thermal Dehydration: II. Composition and Microtexture of the Composites

The microtexture of Al₂O₃/Al composites with different oxide contents formed in the hydrothermal treatment of aluminum followed by thermal decomposition of boehmite is examined by a set of adsorption and structural analysis techniques. Two main types of alumina microtexture are found: relatively loose agglomerates of amorphous particles and highly ordered aggregates of well-crystallized primary blocks of a larger size, the fraction of which increases with the oxide content of the composite leads to an increase in the fraction of micropores and a decrease in the fraction of mesopores. The mechanism of thermal decomposition of boehmite prepared by the hydrothermal oxidation of aluminum is refined.     

Light Absorption by the Clusters of Colloidal Gold and Silver Particles Formed During Slow and Fast Aggregation

Spectra of absorption (400–800 nm) by the aggregates of colloidal gold (5, 15, and 30 nm in diameter) and silver (20 nm in diameter) particles were studied experimentally and theoretically. It was revealed that, during fast aggregation corresponding to the diffusion-limited cluster aggregation (DLCA), the pattern of spectra is dependent on the size of primary particles. Spectra with the additional absorption maximum in the red region are observed for 15 and 30 nm gold and 20 nm silver particles, while the absorption spectrum for 5 nm particles is characterized by only one maximum shifted to the red region. The slow aggregation resulted in a decrease in plasmon absorption peak with no marked shift to the red region and to the broadening of long-wave absorption wing. From data on electron microscopy, typical branched DLCA-clusters were formed during fast aggregation, whereas small compact aggregates and a noticeable number of single particles were observed in a system during slow aggregation. The computer model of the diffusion-limited cluster-cluster aggregation was used to explain these results. Optical properties of aggregates were calculated using coupled dipole method (CDM or DDA) and the exact method of a multipole expansion. Corrections for the size effect were introduced into the bulk optical constants of metals for nanosized particles. It was shown that a modified version of DDA (Markel et al.,Phys. Rev. B, 1996, vol. 53, no. 5, p. 2425) allows us to explain the pattern of experimental spectra of DLCA-aggregates and their dependence on a monomer size. The exact method was applied to calculate the extinction cross sections of metallic aggregates demonstrating strong electrodynamic interaction between particles. The number of higher multipoles that are required to adequately describe this interaction is much larger than the number of terms of an ordinary Mie series and is the main obstacle to the exact calculation of the spectra of metallic aggregates with a large number of particles.