Control of ordered structure and morphology of large-pore periodic mesoporous organosilicas by inorganic salt

Highly ordered rodlike periodic mesoporous organosilicas (PMO) were successfully synthesized using 1.2-bis(trimethoxysilyl)ethane as an precursor and triblock copolymer P123 as a template at low acid concentration and in the presence of inorganic salts (KCl). The role of acid and salt as well as the effects of synthesis temperature and reactant mole ratio in the control of morphology and the formation of ordered mesostructure was systematically examined. It was found that the addition of inorganic salt can dramatically expand the range of the synthesis parameters to produce highly ordered PMO structure and improve the quality of PMO materials. The morphology of PMOs was significantly dependent on the induction time for precipitation. The uniform PMO rods can only be synthesized in a narrow range of acid and salt concentrations. The results also show that the optimized salt concentration (I M) and low acidity (0.167 M) were beneficial to the formation of not only highly ordered mesostructure but also rodlike morphology. Increasing acidity resulted in fast hydrolysis reaction and short rod or plate-like particles. Highly ordered rod can also be prepared at low temperature (35 degrees C) with high salt amount (1.5 M) or high temperature (45 degrees C) with low salt amount (0.5 M). Optimum reactant molar composition at 40 degrees C is 0.035P123:8KCl:1.34HCI:444H(2)O:1.0bis(trimethoxysilyl)ethane. Lower or higher SiO2/PI23 ratio led to the formation of uniform meso-macropores or pore-blocking effect. (c) 2005 Elsevier Inc. All rights reserved.

Highly organized structure in the non-coding region of the psbA minicircle from clade C Symbiodinium

The chloroplast genes of dinoflagellates are distributed among small, circular dsDNA molecules termed minicircles. In this paper, we describe the structure of the non-coding region of the psbA minicircle from Symbiodinium. DNA sequence was obtained from five Symbiodinium strains obtained from four different coral host species (Goniopora tenuidens, Heliofungia actiniformis, Leptastrea purpurea and Pocillopora damicornis), which had previously been determined to be closely related using LSU rDNA region D1/D2 sequence analysis. Eight distinct sequence blocks, consisting of four conserved cores interspersed with two metastable regions and flanked by two variable regions, occurred at similar positions in all strains. Inverted repeats (IRs) occurred in tandem or 'twin' formation within two of the four cores. The metastable regions also consisted of twin IRs and had modular behaviour, being either fully present or completely absent in the different strains. These twin IRs are similar in sequence to double-hairpin elements (DHEs) found in the mitochondrial genomes of some fungi, and may be mobile elements or may serve a functional role in recombination or replication. Within the central unit (consisting of the cores plus the metastable regions), all IRs contained perfect sequence inverses, implying they are highly evolved. IRs were also present outside the central unit but these were imperfect and possessed by individual strains only. A central adenine-rich sequence most closely resembled one in the centre of the non-coding part of Amphidinium operculatum minicircles, and is a potential origin of replication. Sequence polymorphism was extremely high in the variable regions, suggesting that these regions may be useful for distinguishing strains that cannot be differentiated using molecular markers currently available for Symbiodinium.

Highly-ordered hybrid organic-inorganic isoporous membranes from polymer modified nanoparticles

atomic force microscopy (AFM); atom transfer radical polymerization (ATRP); block copolymers; self-assembly; silica nanoparticles.

Synthesis of polymer-montmorillonite nanocomposites by in situ intercalative polymerization

Two polymer-montmorillonite (MMT) nanocomposites have been synthesized by in situ intercalative polymerization. The styrene monomer is intercalated into the interlayer space of organically modified MMT, a layered clay mineral. Upon the intercalation, the complex is subsequently polymerized in the confinement environment of the interlayer space with a free radical initiator, 2,2-azobis isobutyronitrile. The aniline monomer is also intercalated and then polymerized within the interlayer space of sodium- and copper-MMT initiated by ammonium peroxodisulphate and interlayer copper cations respectively. X-ray diffraction indicates that the MMT layers are completely dispersed in the polystyrene matrix and an exfoliated structure has been obtained. The resulting polyaniline-MMT nanocomposites show a highly ordered structure of a single polyaniline layer stacked with the MMT layers. Fourier transform infrared spectra further confirm the intercalation and formation of both polymer-MMT nanocomposites.

Mixed gas equilibrium adsorption on zeolites and energetic heterogeneity of adsorption volume

A model for binary mixture adsorption accounting for energetic heterogeneity and intermolecular interactions is proposed in this paper. The model is based on statistical thermodynamics, and it is able to describe molecular rearrangement of a mixture in a nonuniform adsorption field inside a cavity. The Helmholtz free energy obtained in the framework of this approach has upper and lower limits, which define a permissible range in which all possible solutions will be found. One limit corresponds to a completely chaotic distribution of molecules within a cavity, while the other corresponds to a maximum ordered molecular structure. Comparison of the nearly ideal O-2-N-2-zeolite NaX system at ambient temperature with the system Of O-2-N-2-zeolite CaX at 144 K has shown that a decrease of temperature leads to a molecular rearrangement in the cavity volume, which results from the difference in the fluid-solid interactions. The model is able to describe this behavior and therefore allows predicting mixture adsorption more accurately compared to those assuming energetic uniformity of the adsorption volume. Another feature of the model is its ability to correctly describe the negative deviations from Raoult's law exhibited by the O-2-N-2-CaX system at 144 K. Analysis of the highly nonideal CO2-C2H6-zeolite NaX system has shown that the spatial molecular rearrangement in separate cavities is induced by not only the ion-quadrupole interaction of the CO2 molecule but also the significant difference in molecular size and the difference between the intermolecular interactions of molecules of the same species and those of molecules of different species. This leads to the highly ordered structure of this system.

Variation of the crystalline structure of coal char during gasification

The variation of the crystallite structure of several coal chars during gasification in air and carbon dioxide was studied by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. The XRD analysis of the partially gasified coal chars, based on two approaches, Scherrer's equation and Alexander and Sommer's method, shows a contradictory trend of the variation of the crystallite height with carbon conversion, despite giving a similar trend for the crystallite width change. The HRTEM fringe images of the partially gasified coal chars indicate that large and highly ordered crystallites exist at conversion levels as high as 86%. It is also demonstrated that the crystalline structure of chars can be very different although their pore structures are similar, suggesting a combination of crystalline structure analysis with pore structure analysis in studies of carbon gasification.

Observation of a cubic phase in mesoporous silicate films grown at the air/water interface

X-Ray diffraction is reported from mesoporous silicate films grown at the air/water interface. The films were studied both as powdered films, and oriented on silicon or mica sheets. At early stages of growth we observe Bragg diffraction from a highly ordered cubic phase, with both long and short d-spacing peaks. We have assigned this as a discontinuous micellar Pm3n phase in which the silica is partly ordered. Later films retain only the known hexagonal p6m peaks and have lost any order both at short d-spacings and the longer d-spacing Bragg peaks characteristic of the cubic structure. The silica framework is considerably expanded from that in bulk amorphous silica, average Si Si distances are some 30% greater. Incorporation of glycerol or polyethylene glycol preserves the earlier cubic structure. To be consistent with earlier, in situ, X-ray and neutron reflectivity data we infer that both structures are produced after a phase transition from a less-ordered him structure late in the induction phase. The structural relations between the film Pm3n and p6m phase(s) and the known bulk SBA-1 and MCM-41 phases are briefly discussed.

Influence of nano-structure on electrolytic properties in CeO2 based system

Doped ceria (CeO2) compounds are fluorite type oxides that show oxygen ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. In order to improve the conductivity, the effective index was suggested to maximize the oxygen ionic conductivity in doped CeO2 based oxides. In addition, the true microstructure of doped CeO2 was observed at atomic scale for conclusion of conduction mechanism. Doped CeO2 had small domains (10-50 nm) with ordered structure in a grain. It is found that the electrolytic properties strongly depended on the nano-structural feature at atomic scale in doped CeO2 electrolyte.

Mesoporous bioactive glasses. I. Synthesis and structural characterization

Highly ordered mesoporous bioactive glasses (MBGs) with different compositions have been synthesized by a combination of surfactant templating, sol-gel method and evaporation-induced self-assembly (EISA) processes. The texture properties and compositional homogeneity of MBGs have been characterized and compared with conventional bioactive glasses (BGs) synthesized in the absence of surfactants by evaporation method. The formation mechanism (pore - composition dependence) and compositional homogeneity in the case of MBG materials are different from those in conventional BGs. Unlike conventional sol-gel-derived BGs that shows a direct correlation between their composition and pore architecture, MBGs with different compositions may possess similar pore volume and uniformly distributed pore size when the same structure-directing agent is utilized. The framework of MBG is homogeneously distributed in composition at the nanoscale and the inorganic species generally exists in the form of amorphous phase. MBGs calcined at temperatures

Comparative Analysis of Structural and Morphological Properties of Large-Pore Periodic Mesoporous Organosilicas and Pure Silicas

A systematic study on the structural properties and external morphologies of large-pore mesoporous organosilicas synthesized using triblock copolymer EO20PO70EO20 as a template under low-acid conditions was carried out. By employing the characterization techniques of SAXS, FE-SEM, and physical adsorption of N-2 in combination with alpha(s)-plot method, the structural properties and external morphologies of large-pore mesoporous organosilicas were critically examined and compared with that of their pure-silica counterparts synthesized under similar conditions. It has been observed that unlike mesoporous pure silicas, the structural and morphological properties of mesoporous organosilicas are highly acid-sensitive. High-quality mesoporous organosilicas can only be obtained from synthesis gels with the molar ratios of HCl/H2O between 7.08 x 10(-4) and 6.33 x 10(-3), whereas mesoporous pure silicas with well-ordered structure can be obtained in a wider range of acid concentration. Simply by adjusting the HCl/H2O molar ratios, the micro-, meso-, and macroporosities of the organosilica materials can be finely tuned without obvious effect on their structural order. Such a behavior is closely related to their acid-controlled morphological evolution: from necklacelike fibers to cobweb-supported pearl-like particles and to nanosized particulates.

Adsorption in slit pores and pore-size distribution: A molecular layer structure theory

A new approach is developed to analyze the thermodynamic properties of a sub-critical fluid adsorbed in a slit pore of activated carbon. The approach is based on a representation that an adsorbed fluid forms an ordered structure close to a smoothed solid surface. This ordered structure is modelled as a collection of parallel molecular layers. Such a structure allows us to express the Helmholtz free energy of a molecular layer as the sum of the intrinsic Helmholtz free energy specific to that layer and the potential energy of interaction of that layer with all other layers and the solid surface. The intrinsic Helmholtz free energy of a molecular layer is a function (at given temperature) of its two-dimensional density and it can be readily obtained from bulk-phase properties, while the interlayer potential energy interaction is determined by using the 10-4 Lennard-Jones potential. The positions of all layers close to the graphite surface or in a slit pore are considered to correspond to the minimum of the potential energy of the system. This model has led to accurate predictions of nitrogen and argon adsorption on carbon black at their normal boiling points. In the case of adsorption in slit pores, local isotherms are determined from the minimization of the grand potential. The model provides a reasonable description of the 0-1 monolayer transition, phase transition and packing effect. The adsorption of nitrogen at 77.35 K and argon at 87.29 K on activated carbons is analyzed to illustrate the potential of this theory, and the derived pore-size distribution is compared favourably with that obtained by the Density Functional Theory (DFT). The model is less time-consuming than methods such as the DFT and Monte-Carlo simulation, and most importantly it can be readily extended to the adsorption of mixtures and capillary condensation phenomena.

Coalbed methane sorption related to coal composition

Gas sorption by coal is closely related to its physical and chemical properties, which are, in turn, governed by coal type and rank. The role of coal type (sensu maceral composition) is not fully established but it is clear that coal type may affect both adsorption capacity and desorption rate. Adsorption capacity is closely related to micropore (pores <2 nm) development, which is rank and maceral dependent. Adsorption isotherms indicate that in most cases bright (vitrinite-rich) coals have a greater adsorption capacity than their dull (often inertinite-rich) equivalents. However, no differences, or even the opposing trend, may be observed in relation to coal type. Desorption rate investigations have been performed using selected bright and dull coal samples in a high pressure microbalance. Interpretation of results using unipore spherical and bidisperse pore models indicate the importance of the pore structure. Bright, vitrinite-rich coals usually have the slowest desorption rates which is associated with their highly microporous structure. However, rapid desorption in bright coals may be related to development of extensive, unmineralised fracture systems. Both macro-and micro-pore systems are implicated in the more rapidly desorbing dull coals. Some dull, inertinite-rich coals may rapidly desorb due to a predominance of large, open cell lumina. Mineral matter is essentially nonadsorbent to coal gases and acts as a simple diluent. However, mineral-rich coals may be associated with more rapid desorption. Coal rank and type (maceral composition) per se do not appear to be the critical factors in controlling gas sorption, but rather the influence they exert over pore structure development. (C) 1998 Elsevier Science B.V.

Chemical synthesis, antibacterial activity and conformation of diptericin, an 82-mer peptide originally isolated from insects

The small amounts of antibacterial peptides that can be isolated from insects do not allow detailed studies of their range of activity, side-chain sugar requirements, or their conformation, factors that frequently play roles in the mode of action. In this paper, we report the solid-phase step-by-step synthesis of diptericin, an 82-mer peptide, originally isolated from Phormia terranovae. The unglycosylated peptide was purified to homogeneity by conventional reversed-phase high performance liquid chromatography, and its activity spectrum was compared to that Of synthetic unglycosylated drosocin, which shares strong sequence homology with diptericin's N-terminal domain. Diptericin appeared to have antibacterial activity:for only a limited number of Gram-negative bacteria. Diptericin's submicromolar potency against Escherichia coli strains indicated that, in a manner similar to drosocin, the presence of the carbohydrate side chain is not,necessary to kill bacteria. Neither the N-terminal, drosocin-analog fragment, nor the C-terminal, glycine-rich attacin-analog region was active against any of the bacterial strains studied, regardless of whether the Gal-GalNAc disaccharide units were attached. This suggested that the active site of diptericin fell outside the drosocin or attacin homology domains. In addition, the conformation of diptericin did not seem to play a role in the antibacterial activity, as was demonstrated by the complete lack of ordered structure by two-dimensional nuclear magnetic resonance spectroscopy and circular dichroism. Diptericin completely killed bacteria within I h, considerably faster than drosocin and the attacins; unlike some other, fast-acting antibacterial peptides, diptericin did not lyse normal mammalian cells. Taken together, these data suggest diptericin does not belong to any known class of antibacterial peptides.

Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves

The discovery of periodic mesoporous MCM-41 and related molecular sieves has attracted significant attention from a fundamental as well as applied perspective. They possess well-defined cylindrical/hexagonal mesopores with a simple geometry, tailored pore size, and reproducible surface properties. Hence, there is an ever-growing scientific interest in the challenges posed by their processing and characterization and by the refinement of various sorption models. Further, MCM-41-based materials are currently under intense investigation with respect to their utility as adsorbents, catalysts, supports, ion-exchangers, and molecular hosts. In this article, we provide a critical review of the developments in these areas with particular emphasis on adsorption characteristics, progress in controlling the pore sizes, and a comparison of pore size distributions using traditional and newer models. The model proposed by the authors for adsorption isotherms and criticalities in capillary condensation and hysteresis is found to explain unusual adsorption behavior in these materials while providing a convenient characterization tool.

A new approach to the synthesis of nanocrystal conjugated polymer composites

A novel one pot process has been developed for the preparation of PbS nanocrystals in the conjugated polymer poly 2-methoxy,5-(2 ethyl-hexyloxy-p-phenylenevinylene) (MEH-PPV). Current techniques for making such composite materials rely upon synthesizing the nanocrystals and conducting polymer separately, and subsequently mixing them. This multi-step technique has two serious drawbacks: templating surfactant must be removed before mixing, and co-solvent incompatibility causes aggregation. In our method, we eliminate the need for an initial surfactant by using the conducting polymer to terminate and template nanocrystal growth. Additionally, the final product is soluble in a single solvent. We present materials analysis which shows PbS nanocrystals can be grown directly in a conducting polymer, the resulting composite is highly ordered and nanocrystal size can be controlled.