Silicate Condensation Research Articles

Primordial compositions of refractory inclusions

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10−6 to 10−1bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO+Al2O3 relative to more volatile MgO+SiO2 is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.

Synthesis of silica–ammonium chloride macrofibers generated by anionic surfactant templated nanotubes

Silica-ammonium chloride nanotubes, generated by the condensation of sodium silicate onto ammonium cation-modified anionic sodium dodecyl sulfate (SDS) micellular templates, readily create hierarchical structures through the self-assembly into thicker nanotubes, belts, and ultimately into centimetre long macrofibers. The resulting fibers possess a remarkable range of tubular dimensions (i.e., nanometre to micrometre diameters, and micrometre to centimetre lengths) within a single assembly. The silica-ammonium chloride nanotubes have a high organic content as a result of the surfactant template. The nanotubes lose structural integrity in aqueous solution, but demonstrate retention of structure in organic solvents or when dried. The dependence on each of the component reagents for the generation of these nanotube structures has been investigated. The silica-ammonium chloride nanotubes have been characterized by SEM, TEM, XRD, XPS, and TGA. The creation of three-dimensional mineral structures using anionic surfactants more closely mimics biomineralization processes than prior approaches with cationic or non-ionic organic templates.

Significant improvement in the pore properties of SBA-15 brought about by carboxylic acids and hydrothermal treatment

A comparative study of the pore properties of SBA-15 samples prepared under nonhydrothermal and hydrothermal conditions, in the absence and presence of carboxylic acids such as succinic, tartaric and citric acids has been carried out. In the absence of carboxylic acid, flake-like and spheroid particles were generally obtained irrespective of the preparative procedures. On the other hand, stirring of the pre-mix induces a rod-like morphology in presence of carboxylic acids. The samples prepared under non-hydrothermal conditions exhibit a higher degree of silicate condensation compared to those synthesized under hydrothermal conditions. SBA-15 samples prepared under hydrothermal conditions show higher values of the d (100) spacing independent of the presence of carboxylic acids. Presence of carboxylic acids as well as hydrothermal treatment improves the pore properties of SBA-15.

Effect of nanosized ferric oxide on the thermostability of methylsilicone resin

Nanocomposite of methylsilicone resin/Fe2O3 (MSR/Fe2O3, metal oxide nanoparticles with average particle-size of 20 nm) was successfully synthesized by acid-catalyzed hydrolysis and condensation of methyltrimethoxy silane (MTMS). The obtained nanocomposite was characterized by Fourier-transformed infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). The FTIR and XPS results show that the nanoparticles were well dispersed in the MSR matrix. The thermogravimetric analysis (TGA) results show that Fe2O3 nanoparticles can remarkably improve the thermal stability of MSR. In comparison with methylsilicone resin, the temperature values of the resulting nanocomposite at the onset of degradation and the maximum rate of mass loss were increased 55 and 76 °C, respectively.

Mechanism of a one‐step method for preparing silane grafting and cross‐linking polypropylene

AbstractA high‐melt‐strength polypropylene (HMSPP) with partially cross‐linking, modified from a normal isotactic polypropylene (IPP), is prepared by a one‐step silane grafting and cross‐linking method in a twin‐screw reactive extruder. The melt grafting and cross‐linking of IPP is achieved by using vinyl unsaturated silane with long chain as a grafting monomer, benzoyl peroxide (BPO) as an initiator, styrene as a coagent, and a little water as a catalyst for cross‐linking. By analyzing the difference of the Fourier transformed infrared spectra between raw and modified PP, comparing the variety of the melt flow rate (MFR) and gel percentage between cured and uncured PP samples, and investigating the effect of catalyst on the MFR and gel percentage of modified PP, the partially cross‐linked polypropylene in the way of one‐step reactive extrusion has been verified. We put forward the reactive mechanism of one‐step method based on our experimental results. In this mechanism, the hydrolysis and condensation of silane occur first through the catalysis of water and a compound with two or more double‐bond is formed, which then the compound reacts with PP macroradicals to form the partially cross‐linked PP. POLYM. ENG. SCI., 47:1004–1008, 2007. © 2007 Society of Plastics Engineers

Surfactant Templating Effects on the Encapsulation of Iron Oxide Nanoparticles within Silica Microspheres

Hollow silica microspheres encapsulating ferromagnetic iron oxide nanoparticles were synthesized by a surfactant-aided aerosol process and subsequent treatment. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB) played an essential role in directing the structure of the composite. Translation from mesoporous silica particles to hollow particles was a consequence of increased loading of ferric species in the precursor solution and the competitive partitioning of CTAB between silicate and ferric colloids. The hypothesis was that CTAB preferentially adsorbed onto more positively charged ferric colloids under acidic conditions. At a critical Fe/Si ratio, most of the CTAB was adsorbed onto ferric colloids and coagulated the colloids to form larger clusters. During the aerosol process, a silica shell was first formed due to the preferred silicate condensation on the gas-liquid interface of the aerosol droplet. Subsequent drying concentrated the ferric clusters inside the silica shell and resulted in a silica shell/ferric core particle. Thermal treatment of the core shell particle led to encapsulation of a single iron oxide nanoparticle inside each silica hollow microsphere.

Hybrid titania–aminosilane platforms evaluated with human mesenchymal stem cells

The properties of hybrid aminopropyltriethoxysilane-tetraisopropylorthotitanate (APTS-TIPT) platforms prepared by a sol-gel route have been explored, and their biocompatibility was assayed after culture of human mesenchymal stem cells (hMSCs). The organic content of this material was observed to be preferably surface-oriented as indicated by microanalytical techniques. Furthermore, the surface showed characteristic amino-silane bands when explored by Raman spectroscopy as well as indications of silane and titanate condensation. Surface activity of the amino groups was probed by ultraviolet-visible spectroscopy imine derivatization and chemical force spectroscopy, showing a pH-dependent surface charge-induced potential. hMSCs cultured onto these surfaces showed relevant differences with respect to their behavior on gelatin-coated glass plates. Even if with a lower proliferative rate than controls, the cells develop long cytosolic prolongations in osteogenic differentiation medium, thus, supporting the idea of an APTS-TIPT stimulated process.

Influence of poly- l-lysine on the biomimetic growth of silica tubes in confined media

Pore channels of polycarbonate membranes were recently used as biomimetic models to study the effect of confinement on silicate condensation, leading to the formation of silica tubes exhibiting a core–shell structure. In this work, we preimmobilized poly- l-lysine on the membrane pores, leading to modification of the tube shell formation process and variation in core particle size. These data strengthen previous assumptions on the role of confinement on silica growth, i.e., interfacial interactions and perturbation of the diffusion coefficient. They also suggest that this approach is suitable to investigate in more detail the contribution of confinement effects on silica biomineralization.

Influence of Lysozyme on the Biomimetic Growth of Silica Tubes in Porous Membranes

AbstractPore channels of poly-carbonate membranes were recently used as biomimetic models to study the effect of confinement on silicate condensation, leading to the formation of silica tubes exhibiting a core-shell structure. In this work, we pre-immobilized lysozyme on the membrane pores, inducing the modification of the tube shell formation process, and variation in core particle size. These data strengthen previous assumptions on the role of interfacial interactions on the growth of the tube shell and indicate that such interactions also influence the core particle formation. Such approach therefore seems suitable to mimic the formation of silica/protein multilayers as found in several biomineralizing organisms

Dependence of amine-accelerated silicate condensation on amine structure

Diatoms are known to grow elaborate nano- and microstructured silica shells by depositing material from precursor-containing vesicles at mild temperature and pH. Oligo(1-methylazetane) and related moieties, in some cases attached to proteins, are believed to facilitate this process. To complement prior studies of more complex amines, we aim to understand why such a unique structure has evolved through a systematic study of a set of simple model compounds. The degree to which a series of diamines in solution enhances condensation of phosphate-buffered silicic acid at neutral pH increases with increasing alkylation, a factor more important than amine pKa. This suggests why diatoms often use methylated oligomers. Bis(quaternary ammonium) salts result in even greater reactivity enhancement, constituting a new class of compounds that promote condensation under mild conditions. Methods are presented for incorporation of these new moieties into artificial peptides or other template-forming molecules, which should allow for more effective production of tailored silica nanostructures.

Real time study of hybrid sol-gel photopolymerization by UV light

Hybrid organic-inorganic materials obtained via sol-gel processing using an organically modified silicon alkoxide have been investigated by liquid 29Si NMR and Real Time Fourier Transformed Infra-Red spectroscopy. Free-radical polymerization was initiated by irradiating the sample under UV-light. Significant structural modifications during irradiation were evident in both the inorganic and organic domains of the hybrid material. It appeared that the degree of condensation of the silicate network was of crucial importance, although the presence of the silicate backbone did not limit the final conversion ratio of the organic polymer. On the contrary, it was found that photopolymerization occurred more efficiently for systems with the higher degree of silicate condensation. The effects of photoinitiator concentration and the functionality of the polymerizable organic moiety on the reaction kinetics were also studied.

Oppositely charged surfactant–polyelectrolyte systems as model systems for mesoscopically ordered surfactant–silicates

Due to the strongly kinetic nature of the chemical events leading to the formation of mesoscopically ordered inorganic-surfactant composite materials, detailed studies on the interdependency between the degree of inorganic condensation and the co-operative self-assembly is difficult to perform. We have therefore undertaken a study where organic polyelectrolytes are used as models for the inorganic oligomeric/polymeric species and studied the phase behavior of negatively charged polyacrylate positively charged surfactant complexes in aqueous solution as a function of polyelectrolyte molar mass. Dodecyltrimethylammonium chloride was used as the surfactant. The toluene solubilization capacity of the complexes were also determined, and a clear dependency on the (poly)electrolyte molar mass was found. The results can be used to explain many of the observed features observed for evolving silicate–surfactant under kinetic control. It is suggested that electrostatic screening effects dominate at low degrees of silicate condensation, while the stabilizing effect of the polyelectrolyte becomes increasingly dominant upon further silicate condensation.

Detailed in Situ XRD and Calorimetric Study of the Formation of Silicate/Mixed Surfactant Mesophases under Alkaline Conditions. Influence of Surfactant Chain Length and Synthesis Temperature

The formation of mesoscopically ordered silica/surfactant composites under alkaline synthesis conditions has been studied by time-resolved in situ small-angle X-ray diffraction with synchrotron radiation. Alkyltrimethylammoniumbromide surfactants, C(n)()TAB, of different chain lengths (n = 14, 16, and 18) as well as mixtures thereof were used as structure directing agents and the measurements were carried out at two different temperatures. A linear relationship between the mean surfactant chain length and the d spacing of the hexagonal phase was observed, suggesting an ideal mixing of the surfactants in the supramolecular surfactant aggregates. It is shown that the formation of the hexagonal phase is kinetically controlled mainly by the rate of silicate condensation, while the effect of changes in the surfactant chain length on the kinetics is small under the studied conditions. Two concominant, albeit partly interlinked, processes, suggested being intra- and intermicellar condensation, followed by aggregate-aggregate condensation, govern the nucleation and growth of the hexagonal phase. The two-step mechanism is confirmed by a microcalorimetric study where the heat evolved during the hydrolysis-condensation reactions is followed as a function of time.

Extremely Brief Formation Interval for Refractory Inclusions and Uniform Distribution of 26 Al in the Early Solar System

Calcium-aluminum-rich inclusions (CAIs) are millimeter-sized refractory objects commonly found in chondritic meteorites and are the oldest solids formed in our solar system. Primary CAI formation may have occurred through condensation and/or evaporation processes near the proto-Sun or, alternatively, during localized events in the asteroid belt. As such, these objects provide us with a unique window into the earliest development of the Sun and into the evolution of the protoplanetary disk. Here we report a 26Al-26Mg isochron for bulk CAIs from four CV carbonaceous chondrites, which yields an initial 26Al/27Al of (5.85 ± 0.05) × 10-5, suggesting that primary formation of the CV CAIs may have occurred within an interval as brief as 20,000 years. This timescale is inconsistent with the secular evolution of T Tauri stars but may be consistent with CAI formation during the infall stage of the protostellar evolution of the Sun. High-precision Mg isotope measurements of samples from the Earth, Moon, Mars, and bulk chondrite meteorites show that these have identically elevated 26Mg abundances compared to the initial 26Mg abundance (δ26Mg* = -0.0317‰ ± 0.0038‰) defined by the CAI isochron. This observation unequivocally demonstrates the homogeneous distribution of 26Al within the accretion region of the terrestrial planets. However, the initial 26Mg abundance of CAIs implies a brief history of elevated Al/Mg in CAI precursor material, which may represent primary condensation of refractory silicates and oxides from the solar nebula.

Effect of close packing of octadecyltriethoxysilane molecules on monolayer morphology at the air/water interface

The acid catalyzed hydrolysis and condensation of octadecyltriethoxy silane (OTES) was examined at the air/water interface using Langmuir–Blodgett trough. The various studies include isotherm, isobar and transfer of OTES molecules in general and octadecyltrimethoxysilane molecules for comparison with some of the results. It was observed that barrier speed, initial surface area, time and pH influenced the final mean molecular area of the surface film. Finally the monolayer films obtained were transferred onto various substrates like mica [J. Wood and R. Sharma, Langmuir, 10 (1994) 2307.], silicon and gallium nitride (GaN) [J.L. Rouvière, J.L. Weyher, M. Seelmann-Eggebert, S. Porowski, Appl. Phys. Lett. 73 (1998) 668, J.L. Weyher, S. Müller, I. Grzegory, S. Porowski, J. Cryst. Growth 182 (1997) 17.]. The film on mica was prepared at two different barrier speeds and the evolution of film formation was examined by Atomic Force Microscopy. The use of silicon and GaN, as substrates for film transfer is part of ongoing studies in as potential gate dielectrics for semi-conductor materials.

Monitoring Temporal Evolution of Silicate Species during Hydrolysis and Condensation of Silicates Using Mass Spectrometry

The first stages of solid-state formation from solution can be crucial in determining the properties of the resulting solids. We are trying to approach prenucleation reactions of silicates from an aqueous solution containing tetraalkylammoniumhydroxides (TAAOH) and tetraalkoxysilanes (TAOS) by analyzing hydrolysis and condensation using electrospray mass spectrometry (ESI MS). Time-resolved measurements were performed using different reactor systems to show the stepwise hydrolysis of the silanes and subsequent condensation of silicate monomers via oligomers to form larger units. We approached the precipitation point by varying the pH and the concentrations of the reactants. The results show the evolution of different silicate species occurring during condensation. No defined molecular entities were identified at pH values close to precipitation, which suggests that under the conditions used, solids are probably not formed from defined building blocks.

Synthesis of Indolizines as Illustrated in Alkaloid Construction

A short and efficient synthesis of the alkaloid tashiromine is presented in which an intramolecular acyl imminium-allyl silane condensation plays a key role to form an indolizine heterocycle. Thus, a cross-metathesis initiates the total synthesis to give the succinimide 1 which, upon partial reduction and treatment with TFA, undergoes an intramolecular cyclization via 2 to give the indolizinone 3 in high yield and diastereoselectivity. Simple two-step oxidative unmasking with in situ reduction to the primary alcohol followed by lactam reduction gives tashiromine in 19% overall yield starting from succinimide (six steps).

Solid state NMR investigations on the role of organic admixtures on the hydration of cement pastes

Abstract The influence of different inorganic additives and organic admixtures on the hydration and hardening of Portland cement (CEM I 42.5R) were studied on a nanometer scale by advanced solid state NMR methods. Added quartz was found to be partially attacked by the alkaline media of the cement paste. Even small amounts of organic admixtures strongly influence the hydration and crystallization process of the cement paste. Methyl cellulose, poly(vinyl acetate co vinyl alcohol), poly(ethylene oxide), poly(acrylic acid) and poly(acrylamide) modify the hydration of the calcium aluminum oxides. Major changes in the inorganic structure were detected for low amounts of citric acid and tartaric acid which suppress silicate condensation and strongly alter calcium aluminum oxide hydration. Within this study several solid state NMR methods like 1D magic angle spinning (MAS), 2D exchange and 2D double quantum NMR were applied for the detection of 1 H, 27 Al and 29 Si nuclei. Thus, cement pastes, inorganic additives and organic admixtures could be monitored individually. The findings on a molecular level as provided by NMR are related to changes in the mechanical properties of the cement pastes.

Evaporation induced self-assembly of templated silica and organosilica thin films on various electrode surfaces

A new one-step method is reported for the deposition of hybrid mesoporous thin films on various electrode surfaces (gold, platinum, glassy carbon). Deposition was achieved by spin-coating sol–gel mixtures in the presence of a surfactant template to get mesostructured thin layers on the various conducting substrates. Film formation occurred by evaporation induced self-assembly (EISA) involving the hydrolysis and (co)condensation of silane and/or organosilane precursors on the electrode surface. Extraction of the surfactant from the ordered mesoporous films led to a large increase of mass transport rates into the materials and imparted high accessibility to the organic moieties in case of functionalized mesoporous overlayers. The electrochemical properties of the film-modified electrodes have been studied by cyclic voltammetry (CV), and also via the chemical accumulation of mercury ions prior to their stripping analysis by differential pulse voltammetry (i.e. for thiol-functionalized thin films). Some evidences to support the formation of self-assembled monolayers (SAMs) on electrodes, have been also discussed. The formation of well-adhering mesoporous thin films on solid electrode surfaces is expected to have a high impact on the development of new electrochemical sensors.

Layered Inorganic−Organic Clay-like Nanocomposites Rearrange To Form Silsesquioxanes on Acid Treatment

The formation of talc-like compounds by the condensation of organotrialkoxy silanes with magnesium hydroxide has been recently reported. These represent layered hybrid nanomaterials that have a layer thickness of around 1 nm, have organic moieties covalently linked to the layer surfaces, and are called "organoclays." We show that such compounds are sensitive to acid treatment. When a phenylclay is treated with hydrochloric acid, magnesium leaches out, destroying the layered structure. The extent to which magnesium is leached out is a function of the time of the acid treatment and the concentration of the acid used. Magnesium leaches out rapidly when the concentration of acid used to treat the phenyl-clay is higher, and the extent of structural magnesium that is leached out is also higher for higher acid concentrations. Removal of the magnesium rearranges the structure of the phenyl-clay to form oligomeric phenylsilsesquioxanes. FTIR and NMR suggest that the silsesquioxanes formed by acid treatment of the phenyl-clay comprise a mixture of ladderlike and cagelike structures.