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The high benzene gas selectivity of mesoporous silicate (SBA-15) was observed in the sub-nanometre micropore condensation region. The benzene/toluene ratios of the adsorbed amount were >100 and >6 in ideal and pseudo-atmospheric environments, respectively.
A mesostructured nanocomposite was fabricated by using a novel electroactive, polymerizable surfactant as a template in a sol-gel process, and the first example of well-resolved polystyrene with redoxactive functional group synthesized in silicate matrices was provided.
The synthesis and structural characterisation of a thermally stable novel three-dimensional microporous copper silicate open-framework are described; the material is capable of undergoing reversible zeolitic water removal without destruction of the framework.
The synthesis of lamellar mesophases of organosilicate-surfactant composites with periodicity within the silicate layers due to periodic arrangement of phenylene- and biphenylylene-silica moieties is reported.
The amorphous aluminum silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100 K> or =T> or =2300 K as well as glass configurations that were obtained from cooling runs from T=2300 to 300 K with a cooling rate of about 10(12) K/s. Already at temperatures as high as 4000 K, most of the Al and Si atoms are fourfold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wave number q=0.5 A(-1). A comparison of experimental x-ray data with the results from the simulation shows good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.
Using the inverse geometry spectrometer QENS at the Intense Pulsed Neutron Source of the Argonne National Laboratory, we collected quasielastic and inelastic neutron scattering spectra of hydrated tricalcium and dicalcium silicate, the main components of ordinary Portland cement. Data were obtained at different curing time, from a few hours to several months. Both the quasielastic and inelastic spectra have been analyzed at the same time according to the relaxing cage model, which is a model developed to describe the dynamics of water at supercooled temperatures. Short-time and long-time dynamics of hydration water in hydrated cement pastes as a function of the curing time have been simultaneously obtained. The results confirm the findings reported in previous experiments showing that it is possible to fit consistently the quasielastic and inelastic spectra giving insights on the effect of the curing time on the short-time vibrational dynamics of hydration water.
The central control of mineral weathering rates on biogeochemical systems has motivated studies of dissolution for more than 50 years. A complete physical picture that explains widely observed variations in dissolution behavior is lacking, and some data show apparent serious inconsistencies that cannot be explained by the largely empirical kinetic "laws." Here, we show that mineral dissolution can, in fact, be understood through the same mechanistic theory of nucleation developed for mineral growth. In principle, this theory should describe dissolution but has never been tested. By generalizing nucleation rate equations to include dissolution, we arrive at a model that predicts how quartz dissolution processes change with undersaturation from step retreat, to defect-driven and homogeneous etch pit formation. This finding reveals that the "salt effect," recognized almost 100 years ago, arises from a crossover in dominant nucleation mechanism to greatly increase step density. The theory also explains the dissolution kinetics of major weathering aluminosilicates, kaolinite and K-feldspar. In doing so, it provides a sensible origin of discrepancies reported for the dependence of kaolinite dissolution and growth rates on saturation state by invoking a temperature-activated transition in the nucleation process. Although dissolution by nucleation processes was previously unknown for oxides or silicates, our mechanism-based findings are consistent with recent observations of dissolution (i.e., demineralization) in biological minerals. Nucleation theory may be the missing link to unifying mineral growth and dissolution into a mechanistic and quantitative framework across the continuum of driving force.
Sponges (phylum Porifera) of the class of Demospongiae are stabilized by a siliceous skeleton. It is composed of silica needles (spicules), which provide the morphogenetic scaffold of these metazoans. In the center of the spicules there is an axial filament that consists predominantly of silicatein, an enzyme that catalyzes the synthesis of biosilica. By differential display of transcripts we identified additional proteins involved in silica formation. Two genes were isolated from the marine demosponge Suberites domuncula; one codes for a galectin and the other for a fibrillar collagen. The galectin forms aggregates to which silicatein molecules bind. The extent of the silicatein-mediated silica formation strongly increased if associated with the galectin. By applying a new and mild extraction procedure that avoids hydrogen fluoride treatment, native axial filaments were extracted from spicules of S. domuncula. These filaments contained, in addition to silicatein, the galectin and a few other proteins. Immunogold electron microscopic studies underscored the role of these additional proteins, in particular that of galectin, in spiculogenesis. Galectin, in addition to silicatein, presumably forms in the axial canal as well as on the surface of the spicules an organized net-like matrix. In the extraspicular space most of these complexes are arranged concentrically around the spicules. Taken together, these additional proteins, working together with silicatein, may also be relevant for potential (nano)-biotechnological applications of silicatein in the formation of surface coatings. Finally, we propose a scheme that outlines the matrix (galectin/silicatein)-guided appositional growth of spicules through centripetal and centrifugal synthesis and deposition of biosilica.
We demonstrate nanostructural evolution resulting in highly increased photoluminescence in silicates doped with Er3+ ions. High-resolution transmission electron microscopy (HRTEM) imaging, nano-energy dispersed X-ray (NEDX) spectroscopy, X-ray diffraction (XRD) and photoluminescence analysis confirm the local composition and structure changes of the Er3+ ions upon thermal annealing. We studied two types of amorphous nanopowder: the first is of the composition SiO2/18Al2O3/2Er2O3 (SAE), synthesized by combustion flame-chemical vapor condensation, and the second is with a composition of SiO2/8Y2O3/2Er2O3 (SYE), synthesized by sol-gel synthesis (composition in mol%). Electron diffraction and HRTEM imaging clearly show the formation of nanocrystallites with an average diameter of approximately 8 nm in SAE samples annealed at 1000 degrees C and SYE samples annealed at 1200 degrees C. The volume fraction of the nanocrystalline phase increased with each heat treatment, eventually leading to complete devitrification at 1400 degrees C. Further XRD and NEDX analysis indicates that the nanocrystalline phase has the pyrochlore structure with the formula Er(x)Al(2-x)Si2O7 or Er(x)Y(2-x)Si2O7 and a surrounding silica matrix.
We examined the possibility of chemical evolution in superheated hydrothermal environments and found the formation of microspheres at 250 degrees C and above from a mixture of glycine, alanine, valine, and aspartic acid. The microspheres did not form at lower temperatures and consisted of silicates and peptide-like polymers that contained imide bonds and amino acid residues having an abundance of valine. The results show the possibility of thermophilic cellular structures, which might be adopted by the extremely thermophilic organisms, if they exist, reported by Baross and Deming.
The aim of our study was to compare the safety and efficacy of a new preparation, Dysprosium-165 Hydroxide Macroaggregate (165Dy) with Yttrium-90 Silicate (90Y) for radiation synovectomy of the knee in patients with RA and OA. A multicentre double blind clinical trial with subjects randomized to receive 165Dy or 90Y was undertaken in Sydney, Melbourne and Perth. Seventy knees of 59 patients were studied, using as clinical end point measurements, pain in the knee on walking, pain in the knee at rest and stiffness in the knee after rest. Cytogenetic damage, knee retention and extra-articular spread of the radionuclide to regional lymph nodes, liver, urine and blood were evaluated. There was no significant difference in clinical response in the two treatment groups for either RA or OA. Chromosomal changes occurred with equal frequency and the knee retention and extra-articular leakage of radiocolloids to regional lymph nodes and liver were comparable in the two groups. For radiation synovectomy of the knee, 165Dy is at least as safe and as effective as 90Y and has the advantage of a short half-life (2.334 h) and hence requires a shorter period of post-injection immobilization and hospitalization.
We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power laws in time, x/(1-x) approximately (t/t(x))(psi). For early times t t(x) a parabolic behavior (psi=1/2). The crossover time is estimated as t(x) approximately 16 h. We interpret these results in terms of a global second-order rate equation indicating that (a) hydrates catalyze the hydration process for t t(x), and (c) the value of the associated rate constant is of magnitude 6 x 10(-7) - 7 x 10(-6) l mol(-1) s(-1). We argue, by considering that the hydration process actually occurs via diffusion limited precipitation, that the exponents psi=5/2 and psi=1/2 directly indicate a preferentially plate-like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.
Quasielastic neutron scattering (QENS) has been used to investigate the single-particle dynamics of interfacial water in dicalcium silicate (C2S)/water paste. Our previous neutron-scattering studies on interfacial water have focused attention on the translational dynamics of the center of mass of water molecules. In this paper, we have collected QENS data on a wider range of wave-vector transfer so that both translational and rotational motions of water molecules are detected. The data have been analyzed by models for translation and rotation we recently proposed for supercooled water. The evolution of the parameters describing the relaxational dynamics of water embedded in the C2S matrix is given at temperature T=303 K as a function of the curing time.
Synchrotron x-ray diffraction studies were performed on a synthetic layered silicate Fluorohectorite clay. Diffraction patterns along the stacking direction were obtained in surface reflection and bulk transmission geometries on bulk pressed samples under controlled temperature and relative humidity. One-dimensional structure factors modeling the positions of the intercalant atoms have been obtained for three stable hydration states. From the narrow (00l) peak widths we conclude that well-crystallized domains consist of stacks of about 100 platelets, forming crystallites of the order of 0.1 microm thick. These crystallites have an orientational angular distribution of about 24 degrees around the stacking direction and represent the solid framework for microporosity in these samples.
Self-affine dehydrated colloidal deposits on fresh mica surfaces of the synthetic layered silicate 2:1 smectite clay laponite have been studied by means of atomic force microscopy (AFM). AFM images of these prepared assemblies of sol and gel aggregates have been analyzed both by means of standard AFM Fourier software and a wavelet method. The deposited surfaces show a persistence to antipersistent crossover with a clay concentration dependent crossover length. It is concluded that the crossover length is associated with aggregate size, and further that the persistent roughness at small length scales signals near compact clusters of fractal dimension three, whereas the antipersistent roughness at large length scales signals a sedimentation process.
Using density-functional theory, we compute the optical and static dielectric constants for a set of Zr silicates modeled by various SiO2 crystals, with Zr atoms substitutional to Si, and by an amorphous structure. We then derive a microscopic scheme that relates the dielectric constants to structural units centered on Si and Zr atoms through the definition of characteristic parameters. Applied to amorphous (ZrO2)(x)(SiO2)(1-x), these schemes describe the observed dependence of the dielectric constants on the Zr concentration and highlight the role of ZrO6 units.
A nanocomposite consisting of PbTe nanocrystals embedded in a silicate glass was studied by small-angle x-ray scattering during the early stage of isothermal annealing at 793 K. A theoretical function based on a model of spherical PbTe nanocrystals surrounded by a Pb and Te depleted shell fits well to all experimental curves. The time dependences of the nanocrystal radius and size of the depleted shell agree with the prediction of the theory of nucleation and growth by the classical mechanism of atomic diffusion.
The available sites for ions in a typical disordered ionic conductor are determined. For this purpose we devised a straightforward algorithm which via cluster analysis identifies these sites from long time ionic trajectories below the glass transition. This is exemplified for a lithium silicate glass (Li2O)(x)(SiO2)((1-x)) for x=0.5 and x=0.1. We find for both concentrations that the number of sites is only slightly bigger than the number of ions. This result suggests a theoretical description of the dynamics in terms of mobile vacancies as most appropriate. Furthermore, identification of the ionic sites allows one to obtain detailed characteristics of the ionic motion, e.g., quantification of correlated forward-backward jumps.