Silicate Surface Research Articles

Continuous-flow chemoselective reduction of cyclohexanone in a monolithic silica-supported Zr(OPri)4 multichannel microreactor

Meerwein-Ponndorf-Vereley reduction of cyclohexanone using 2-butanol as hydrogen donor was studied in the multichannel monolithic silica microreactors modified with zirconium species to demonstrate their higher efficacy than of conventional batch systems with powdered catalysts. Zr(OPri)4 species were attached onto the monoliths siliceous surface to obtain a nominal Zr/Si mass ratio in the range of 0.01–0.28. The value of 0.14 gave the largest concentration of Lewis acid sites and hence also the highest conversion (88%) after 20 min reaction/residence time and productivity of 2.22 mmol/g h. The monoliths prepared by the sol-gel process combined with phase separation were characterized using nitrogen adsorption, mercury porosimetry, thermogravimetry, SEM, FTIR and UV–Vis spectroscopy, pyridine adsorption and XRD. They featured very open 3D co-continuous structure of 20 nm textural meso- and flow-through macropores 30–60 μm in diameter (4 cm3/g total pore volume) which was not appreciably changed after their modification with zirconium species. Diffraction peaks characteristic for crystalline zirconia were not detected even in materials with the highest Zr content.

Interfacial Structure and Evolution of the Water–Silica Gel System by Reactive Force-Field-Based Molecular Dynamics Simulations

Nanoporous silica gel structures and associated interfaces formed on the surface of silicate and borosilicate glasses play an important role in understanding the dissolution mechanisms of these glasses. Interfacial models that consist of bulk silica, nanoporous hydrated silica gel, and bulk water were constructed and their evolution was studied to understand the water–glass reaction fronts using the Reactive Force Field (ReaxFF)-based molecular dynamics simulations. The short- and medium-range structures of the gel and the interfaces as well as water diffusion and silica dissolution behaviors were studied in detail. It was found that the gel region exhibited an increase in Si–O network connectivity with time, consistent with recent NMR results, due to cross-linking of siloxane bonds in the gel and the dissolution of less connected [SiO4] groups into the water region. Higher network connectivity of the silica gel can regulate transport of ions and plays the role of passivation. Dissolved silica clusters in...

How hydroxylation affects hydrogen adsorption and formation on nanosilicates

Silicate dust constitutes one of the primary solid components of the Universe and is thought to be an essential enabler for complex chemistry in a number of astronomical environments. Hydroxylated silicate nanoclusters (MgO)x(SiO2)y(H2O)z, where strongly absorbed water molecules are dissociated on the silicate surface, are likely to be persistent in diffuse clouds. Such precursor species are thus also primary candidates as seeds for the formation and growth of icy dust grains in dense molecular clouds. Using density functional calculations we investigate the reactivity of hydroxylated pyroxene nanoclusters (Mg4Si4O12)(H2O)N (N= 1−4) towards hydrogen physisorption, chemisorption and H2 formation. Our results show that increased hydroxylation leads to a significant reduction in the energy range for the physisorption and chemisorption of single H atoms, when compared to bare silicate grains and bare bulk silicate surfaces. Subsequent chemisorption of a second H atom is, however, little affected by hydroxylation. The H2 reaction barrier for the recombination of two chemisorbed H atoms tends to follow a linear correlation with respect to the 2Hchem binding energy, suggestive of a general Brønsted–Evans–Polanyi relation for H2 formation on silicate grains, independent of dust grain size, composition and degree of hydroxylation.

Change in the surface OH group on soda lime silicate glass and silica glass after heat treatment in nitrogen atmosphere

The contact angle of water is measured to estimate the wettability of soda lime silicate and silica glass surfaces. The contact angle of the silica glass is increased to 33° with the heat treatment at 850°C under an N2 atmosphere, which gradually decreases with elapsed time finally reaching the similar value of 5° before the heat-treatment. This indicates that the surface OH content is reduced by the heat treatment and recovers with time to the initial value, which is qualitatively confirmed by D-SIMS measurements. On the other hand, the contact angle for the soda lime silicate glass surface is almost the same before and after the heat treatment under an N2 atmosphere. It is considered that hydration of the soda lime silicate glass surface proceeds faster than the silica glass through exchange with alkali and/or alkali-earth cations.

Hydrolytic Stability of 3-Aminopropylsilane Coupling Agent on Silica and Silicate Surfaces at Elevated Temperatures.

3-Aminopropylsilane (APS) coupling agent is widely used in industrial, biomaterial, and medical applications to improve adhesion of polymers to inorganic materials. However, during exposure to elevated humidity and temperature, the deposited APS layers can decompose, leading to reduction in coupling efficiency, thus decreasing the product quality and the mechanical strength of the polymer-inorganic material interface. Therefore, a better understanding of the chemical state and stability of APS on inorganic surfaces is needed. In this work, we investigated APS adhesion on silica wafers and compared its properties with those on complex silicate surfaces such as those used by industry (mineral fibers and fiber melt wafers). The APS was deposited from aqueous and organic (toluene) solutions and studied with surface sensitive techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), streaming potential, contact angle, and spectroscopic ellipsometry. APS configuration on a model silica surface at a range of coverages was simulated using density functional theory (DFT). We also studied the stability of adsorbed APS during aging at high humidity and elevated temperature. Our results demonstrated that APS layer formation depends on the choice of solvent and substrate used for deposition. On silica surfaces in toluene, APS formed unstable multilayers, while from aqueous solutions, thinner and more stable APS layers were produced. The chemical composition and substrate roughness influence the amount of deposited APS. More APS was deposited and its layers were more stable on fiber melt than on silica wafers. The changes in the amount of adsorbed APS can be successfully monitored by streaming potential. These results will aid in improving industrial- and laboratory-scale APS deposition methods and increasing adhesion and stability, thus increasing the quality and effectiveness of materials where APS is used as a coupling agent.

Effect of hydrophobic groups on the adsorption conformation of modified polycarboxylate superplasticizer investigated by molecular dynamics simulation

All-atom molecular dynamics (MD) simulations were used to study the adsorption conformations of hydrophobically-modified comb-shaped polycarboxylate ether-based (PCE) superplasticizer molecules on a model surface of dicalcium silicate (C2S) in vacuum and in an explicit solution, respectively. Three different hydrophobic modifying groups, namely, the ethyl group, the n-butyl group and the phenyl group, decorated to the backbone, were examined. Comparing the hydrophobically-modified PCEs to the unmodified one, differences were found in the binding energy, the adsorption conformation and the water density at the interface. The interaction between PCE molecules and C2S was weakened in a solution with explicit solvents than that obtained from vacuum-based simulations. The presence of hydrophobic groups lowered the polymer-surface binding energy, decreased the radius of gyration (Rg) of the adsorbed polymer, increased the peak position in the heavy-atom density profiles in the direction perpendicular to the surface, and also caused the adsorbed conformations to be more globular in shape. The parallel and perpendicular components (relative to the surface plane) of the geometric sizes of the adsorbed polymers were calculated, and the results showed that the presence of hydrophobically modifying groups decreased the in-plane radius while increased the adsorption layer thickness compared to the unmodified control. The presence of PCEs perturbed the dense water layer above the C2S surface and lowered the water density. Perturbations to the interfacial water density were found to correlate nicely with the adsorbed conformations of PCEs.

Nitrogen Isotopic Fractionation in Ammonia during Adsorption on Silicate Surfaces

Adsorption is a fundamental phenomenon that occurs at various interfaces; however, the isotopic fractionation in stable isotopes associated with this process has not yet been well documented for most molecules. In this study, we conducted ammonia adsorption experiments on two silicate minerals, montmorillonite and saponite, to determine the nitrogen isotopic fractionation during the process. Ammonia adsorbed on these minerals is up to +44‰ enriched in 15N relative to initial ammonia. The degree of 15N enrichment has a negative correlation with the adsorption ratio of ammonia. These enrichments are remarkably large compared to those reported in other physicochemical (e.g., evaporation) or biological (e.g., enzymatic reaction) processes. On the basis of these results, we can predict that preferential accumulation of 15NH3 occurs by adsorption on mineral surfaces, which may explain the heterogeneity of the 15N/14N ratio in the solar system.

Sputtering of sodium and potassium from nepheline: Secondary ion yields and velocity spectra

Silicates are the dominant surface material of many Solar System objects, which are exposed to ion bombardment by solar wind ions and cosmic rays. Induced physico-chemical processes include sputtering which can contribute to the formation of an exosphere. We have measured sputtering yields and velocity spectra of secondary ions ejected from nepheline, an aluminosilicate thought to be a good analogue for Mercury's surface, as a laboratory approach to understand the evolution of silicate surfaces and the presence of Na and K vapor in the exosphere. Experiments were performed with highly charged ion beams (keV/u–MeV/u) delivered by GANIL using an imaging XY-TOF-SIMS device under UHV conditions. The fluence dependence of sputtering yields gives information about the evolution of surface stoichiometry during irradiation. From the energy distributions N(E) of sputtered particles, the fraction of particles which could escape from the gravitational field of Mercury, and of those falling back and possibly contributing to populate the exosphere can be roughly estimated.

Effect of pH on adsorption and desorption of hexadecyl trimethyl ammonium bromide from silicate surface

Effect of pH on adsorption and desorption of hexadecyl trimethyl ammonium bromide from silicate surface

Interaction of organic compounds with chondritic silicate surfaces. Atomistic insights from quantum chemical periodic simulations.

The interaction of 14 different probe organic molecules with the crystalline (010) forsterite Mg2SiO4 surface has been studied at quantum chemical level by means of B3LYP-D2* periodic simulations. The probe molecules are representatives of the class of soluble organic compounds found in carbonaceous meteorites, namely: aliphatic and aromatic hydrocarbons, alcohols, carbonyl compounds, amines, amides, nitrogen heterocycles, carboxylic and hydroxycarboxylic acids, sulfonic and phosphonic acids, amino acids, and carbohydrates. With the exception of the aliphatic and aromatic hydrocarbons, the interaction takes place mainly between the O and N electron donor atoms of the molecules and the outermost Mg surface cations, and/or by hydrogen bonds of H atoms of the molecules with O surface atoms. Dispersion also contributes to the final interaction energies. Each surface/molecule complex has also been characterized by computing its harmonic vibrational spectrum, in which the most significant frequency perturbations caused by the surface interaction are described. With the calculated interaction energies, a trend of the intrinsic affinity of the probe molecules with the silicate surface has been obtained. However, this affinity scale does not correlate with the experimental abundances of the class of compounds found in the Murchison meteorite. A brief discussion of this lack of correlation and the factors that can help us to understand the abundances is provided.

Effect of silicate on the sorption properties of kaolinite: removal of U(VI) and mechanism

The surface and sorption properties of kaolinite were analyzed as a function of silicate. Batch experiments indicate that the U(VI) sorption is promoted by the addition of silicate at low pH while is depressed at high pH. The sorption was acceptably predicted by the formation of a ternary silicate surface complex under the experimental conditions. The pseudo-second order kinetic model fit the sorption kinetics better. The sorption isotherms are more in accordance with Langmuir model and the thermodynamic parameters indicate a spontaneous and endothermic sorption process.

The biological effect of asbestos exposure is dependent on changes in iron homeostasis

Functional groups on the surface of fibrous silicates can complex iron. We tested the postulate that (1) asbestos complexes and sequesters host cell iron resulting in a disruption of metal homeostasis and (2) this loss of essential metal results in an oxidative stress and biological effect in respiratory epithelial cells. Exposure of BEAS-2B cells to 50 μg/mL chrysotile resulted in diminished concentrations of mitochondrial iron. Preincubation of these cells with 200 μM ferric ammonium citrate (FAC) prevented significant mitochondrial iron loss following the same exposure. The host response to chrysotile included increased expression of the importer divalent metal transporter-1 (DMT1) supporting a functional iron deficiency. Incubation of BEAS-2B cells with both 200 μM FAC and 50 μg/mL chrysotile was associated with a greater cell accumulation of iron relative to either iron or chrysotile alone reflecting increased import to correct metal deficiency immediately following fiber exposure. Cellular oxidant generation was elevated after chrysotile exposure and this signal was diminished by co-incubation with 200 μM FAC. Similarly, exposure of BEAS-2B cells to 50 μg/mL chrysotile was associated with release of the proinflammatory mediators interleukin (IL)-6 and IL-8, and these changes were diminished by co-incubation with 200 μM FAC. We conclude that (1) the biological response following exposure to chrysotile is associated with complexation and sequestration of cell iron and (2) increasing available iron in the cell diminished the effects of asbestos exposure.

Fideliacyclus wombatiensis gen. et sp. nov. – a Paleocene non-marine centric diatom from northern Canada with complex frustule architecture

Marine diatoms began colonizing freshwater habitats in the early Cenozoic, or possibly earlier, becoming well established by the Eocene. However, because of a sparse fossil record, little is known about the earliest diatom representatives that signalled this important ecological and evolutionary event. We describe a new centric diatom genus, Fideliacyclus, from lake sediments deposited during the Paleocene in the Canadian Arctic. This organism, one of the oldest known freshwater diatoms reported to date, has a unique, highly complex wall structure composed of areolae with large, bulbous, spherical shaped chambers that are open to the external environment but rest on a solid siliceous surface referred to as the solum, and are surrounded by an open space, or hypocaust. The solum is only perforated by marginal labiate processes, which largely isolates the protoplast from ambient conditions hence limiting exchange; this may have represented an adaptive response to living in freshwater. Given the complement of microfossils found in the fossil locality, the lake was an unambiguously softwater, slightly acidic, limnic system of moderate nutrient content. Although the exact taxonomic position of Fideliacyclus remains uncertain, it appears to be most closely allied to Actinocyclus (Class: Coscinodiscophyceae) or possibly Spumorbis given the range of synapomorphies.

Hydrolytic conversion of preceramic polymers into silicate glass coatings with different wettability

In this paper, two types of preceramic polymers were used to form silicate glass coatings with different wettability via hydrolysis either under harsh or mild conditions. Allylhydridopolycarbosilane (AHPCS) and polyvinylsilazane (PVSZ) were spin-coated on Si substrate and consolidated by photo- and thermal-curing and subjected to hydrolysis for conversion to the silicate glass phase. The contact angle of the both hydrophobic cured polymers decreased from 91°–92° to 18°–36° with little difference, when hydrolyzed in strong alkaline of NaOH solution at room temperature. However, soft hydrolysis in ammonia vapor at 80 °C showed that the AHPCS derived silicate surface showed the moderate contact angle 58°, while the PVSZ derived silicate became very hydrophobic surface with the contact angle 103° presumably due to re-oriented alkyl groups. Eventually, it was demonstrated that the surface wettability of the hydrolyzed silicates was controlled with the polymer chemistry and the hydrolytic conditions upon the preceramic polymer-to-silica conversion.

H2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe+ : A Silsesquioxanes-Based Computational Model.

Cosmic siliceous dust grains are involved in the synthesis of H2 in the inter-stellar medium. In this work, the dust grain siliceous surface is represented by a hydrogen Fe-metalla-silsesquioxane model of general formula: [Fe(H7 Si7 O12-n )(OH)n ]+ (n=0,1,2) where Fe+ behaves like a single-site heterogeneous catalyst grafted on a siliceous surface synthesizing H2 from H. A computational analysis is performed using two levels of theory (B3LYP-D3BJ and MP2-F12) to quantify the thermodynamic driving force of the reaction: [Fe-T7H7 ]+ +4H→[Fe-T7H7 (OH)2 ]+ +H2 . The general outcomes are: 1) H2 synthesis is thermodynamically strongly favored; 2) Fe-H / Fe-H2 barrier-less formation potential; 3) chemisorbed H-Fe leads to facile H2 synthesis at 20≤T≤100 K; 4) relative spin energetics and thermodynamic quantities between the B3LYP-D3BJ and MP2-F12 levels of theory are in qualitative agreement. The metalla-silsesquioxane model shows how Fe+ fixed on a siliceous surface can potentially catalyze H2 formation in space.

Water Sorption Property Controlled by Nanoscale Pore Connectivity of Large-Sized Cage-Type Mesopores

Water sorption property over silicate surfaces is designed by a reasonable method to control the distance of large-sized cage-type mesopores and their connectivity through small mesoporous windows that can be fabricated using a polystyrene-block-polybutadiene-block-polystyrene triblock copolymer functionalized with hydrophilic sulfonic groups.

Effects of Al(3+) Ions on Formation of Silica Framework and Surface Active Sites for SO4(2-) Ions.

Al(3+) ions were introduced into silica framework at 318 K in order to make active Al sites for SO4(2-) by the addition of aqueous sodium silicate solution to aqueous sulfuric acid solution of Al2(SO4)3. The (27)Al and (29)Si NMR spectra of aluminosilicates were measured at 278 K with reaction time. (29)Si NMR spectra were analyzed by the multivariate curve resolution. The addition of Al(3+) ions to aqueous silicate solution promoted gel formation. Small amounts of Al(3+) ions were incorporated as a four-coordinated complex at early stage of polymerization reaction of silicates and during subsequent reaction six-coordinated Al complex increased, suggesting reversible conversion between 4- and 6-coordinated complexes. SO4(2-) ions interact with positive surfaces of aluminosilicates and are specifically adsorbed on the surface sites of 6-coordinated Al(3+) species, which may be stabilized on silicate surfaces as [Al(H2O)5SO4](+).

Catalytic activation of layered silicates for the synthesis of nanocomposite materials based on ultra-high molecular weight polyethylene

The effects of the structure of organomodified montmorillonite and the conditions of its catalytic activation by titanium and vanadium chlorides on the synthesis of nanocomposite materials based on ultra-high molecular weight polyethylene with an exfoliated structure by an in situ polymerization method were studied. It was shown that, with the use of organomodified montmorillonite with the interplanar spacing d001 = 1.6–1.8 nm, in which the alkyl radicals of a modifier are arranged in parallel to the basal silicate surfaces, the catalyst is adsorbed only on the external surface of particle, and it does not penetrate into the interlayer space (in this case, the exfoliation of a filler does not occur). With the use of montmorillonite samples with d001 > 2 nm with the packing of a modifier as paraffin-like mono- or bilayers, the catalyst is predominantly intercalated into the interlayer space of the layer silicate. As a result, in the course of polymerization, polyethylene is formed in the interlayer space of particles to facilitate the exfoliation of the filler in separate nanolayers. Conditions for the supporting of a catalyst onto organomodified montmorillonite, which prevent the transfer of the catalyst into solvent and the formation of a free polymer on the synthesis of nanocomposites under the conditions of suspension polymerization in n-heptane, were determined. The intercalation of a catalyst into the interlayer space of the particles of layered silicates and the exfoliation of filler particles in the course of the synthesis of composites were confirmed by X-ray diffraction analysis.

Wetting Effects and Molecular Adsorption at Hydrated Kaolinite Clay Mineral Surfaces

In this study, classical molecular dynamics simulations have been used to understand the key interactions and surface structure of a set of organic molecules at the hydrated surfaces of the 1:1 clay mineral kaolinite. Decane, decanoic acid, and decanamine have been modeled at both the hydroxylated and silicate surfaces of kaolinite. Additionally, the effect of pH is observed via looking at the protonated decanamine and decanoate anion forms. The key results show that relative affinity of the organic molecules to the kaolinite surface may be readily switched between the hydroxylated and the silicate surfaces according to the pH and the nature of the organic head functional group. Decane molecules readily form droplets atop the silicate surface and do not adsorb to the hydroxyl surface, as do protonated decanoic acids. In stark contrast, decanoate anions do not adsorb to the silicate surface, yet adsorb to the hydroxyl surface through an anion exchange mechanism. Decanamine readily adsorbs to both silicate ...

Influence of the Filler Nature on the Crystalline Structure of Polylactide-Based Nanocomposites: New Insights into the Nucleating Effect

This work deals with the influence of the nature of a filler on the thermally induced crystalline structure of polylactide (PLA)-based nanocomposites. Particularly two types of silicated clay both having a platelet-like shape, i.e., talc and kaolin, were melt compounded with PLA, and the crystalline structure involved during isothermal crystallization was followed in situ by means of both wide- and small-angle X-ray scattering techniques. Results indicate that even if the two types of clay are chemically similar, i.e., they both exhibit silicate surfaces, only talc plays the role of nucleating agent on the PLA crystallization. Moreover, structural analyses reveal that for PLA nanocomposites filled with talc a transcrystallization process occurs and that, in addition to the perpendicular growth of the PLA crystals toward the clay surface, a well-ordered nanometer scaled structure can be formed depending on the crystallization temperature. Particularly, for the first time, an epitaxial growth process has be...