Interaction Of Silica Research Articles

Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass

Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z 1,2 excitonic emission of AgI exhibits fine structure with spacing of ∼0.20 eV (1610 cm −1), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI–silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO 2 interface region before radiative recombination.

Relationship between the polymer/silica interaction and properties of silica composite materials

AbstractCast film composites have been prepared from aqueous polymer solutions containing nanometric silica particles. The polymers were polyvinyl alcohol (PVA), hydroxypropylmethylcellulose (HPMC) and a blend of PVA‐HPMC polymers. In the aqueous dispersions, the polymer–silica interactions were studied through adsorption isotherms. These experiments indicated that HPMC has a high affinity for silica surfaces, and can adsorb at high coverage; conversely, low affinity and low coverage were found in the case of PVA. In the films, the organization of silica particles was investigated through transmission electron microscopy (TEM) and small‐angle neutron scattering (SANS). Both methods showed that the silica particles were well‐dispersed in the HPMC films and aggregated in the PVA films. The mechanical properties of the composite films were evaluated using tensile strength measurements. Both polymers were solid materials, with a high‐elastic modulus (65 MPa for HPMC and 291 for PVA) and a low‐maximum elongation at break (0.15 mm for HPMC and 4.12 mm for PVA). In HPMC films, the presence of silica particles led to an increase in the modulus and a decrease in the stress at break. In PVA films, the modulus decreased but the stress at break increased upon adding silica. Accordingly, the polymer/silica interaction can be used to tune the mechanical properties of such composite films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1134–1146, 2006

Effect of a fatty amine on processing and physical properties of SBR compounds filled with silane–silica particles

AbstractThe use of fatty amines, obtained from natural fatty acids, in rubber compounds to improve silica dispersion was evaluated. Fatty amines can interact with the free silanol groups of the bis(3‐triethoxysilyl‐propyl) tetrasulfide (TESPT) silane‐modified silica forming an amine‐modified silica complex, which reduces the hydrophilic nature of the silica surface, minimizing the silica–silica interactions, and reducing the formation of secondary structures of silica. The effect of the addition of different amines and/or polyethylenglycol to SBR compounds loaded with silane‐modified silica was evaluated. Finally, incorporation of fatty amines improved the processability and traction properties of rubber compounds loaded with modified silica. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3222–3229, 2006

From cluster to bulk: Size dependent energetics of silica and silica-water interaction

We present our computational investigations on the energetics of clusters that consist of H2O and SiO2 using first-principles Born-Oppenheimer molecular dynamics method. Cohesive energy and hydration energy of both pure (or dry) and hydroxylated (or wet) ring-structured clusters have been investigated as functions of system size. We have found clear trends of energy as the cluster size increases. Energetics of a small silica nano-rod that contains 108 atoms is also obtained as a middle reference point for size evolution. Results from cluster and nano-rod calculations are compared with values from bulk quartz calculations using the same level of theoretical treatments.

Effect of oleyl amine on SBR compounds filled with silane modified silica

The effect of oleyl amine on processing and physical properties of SBR compounds filled with silane–silica particles has been evaluated. Two different types of silane molecules have been used as coupling agents to minimize the silica–silica interactions and reduce the formation of secondary structures of silica, in addition with an increment in the filler–rubber interactions. Significant differences between those compounds have been found according with the silane structure. However, in both cases, the dynamic, rheological, and mechanical properties of the filled rubber compounds were improved after the incorporation of oleyl amine molecules. This fact could be related with the capacity of the amine molecules to interact with the free silanol groups of silane-modified silica forming an amine-modified silica complex, which reduces the hydrophilic nature of the silica surface. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1806–1814, 2007

Study of an iron-manganese Fischer–Tropsch synthesis catalyst promoted with copper

The metal–silica interaction and catalytic behavior of Cu-promoted Fe–Mn–K/SiO 2 catalysts were investigated by temperature-programmed reduction/desorption (TPR/TPD), differential thermogravimetric analysis, in situ diffuse reflectance infrared Fourier transform analysis, and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO 2 catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions. In addition to the well-known effect of Cu promoter on easing the reduction of iron-based FTS catalysts, it is found that Cu promoter can increase the rate of carburization, but does not vary the extent of carburization during the steady-state FTS reaction. The basicity of the Cu and K co-promoted catalyst is greatly enhanced, as demonstrated by CO 2-TPD results. In the FTS reaction, Cu improves the rate of catalyst activation and shortens the induction period, whereas the addition of Cu has no apparent influence on the steady-state activity of the catalyst. Promotion of Cu strongly affects hydrocarbon selectivity. The product distribution shifts to heavy hydrocarbons, and the olefin/paraffin ratio is enhanced on the catalyst due to the indirect enhancement of surface basicity by the copper promotion effect.

Effect of vulcanization technique on the physical properties of silica‐filled EPDM rubber

AbstractThe present study aims to enhance EPDM rubber–silica interaction by employing a special technique called Two‐Stage Vulcanization, with the help of a multifunctional rubber additive, bis diisopropyl thiophosphoryl disulfide (DIPDIS). In this process EPDM rubber was heated along with rubber additives up to the time just before the commencement of cure and then filler was incorporated to the preheated rubber to get the final mix. The efficiency of this novel technique is evaluated by the enhancement of physical properties of the silica‐filled vulcanizates. This novel technique is also employed to investigate the effect of a silane‐coupling agent, viz., bis (3‐triethoxy silyl propyl) tetrasulphide (TESPT), in addition to DIPDIS, on the rubber–filler interaction. The positive role of this technique in enhancing the rubber–filler interaction is evidenced by the dynamic mechanical analysis and scanning electron microscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1132–1139, 2006

Study of the effect of hydration on the tensile strength of a silica nanotube

Molecular dynamics simulations using quantum mechanical potentials have been performed to study the interaction of water with silica. The water–water and water–silica interactions were determined at the ab initio coupled cluster and MBPT(2) levels of theory and the resulting forces and reaction mechanisms encoded into a transfer Hamiltonian. Using forces from the rapid evaluation of the transfer Hamiltonian, two prototypical systems, consisting of an (SiO2)10 ring and a 108 atom silica nanorod, were fractured in the presence of water. It was found that inclusion of water caused a reduction in tensile strength of both systems and that the water dimer, as suggested by ab initio calculations, is the most effective mediator of Si–O bond rupture.

Interaction of Eu(III) ion and non-porous silica: Irreversible sorption of Eu(III) on silica and hydrolysis of silica promoted by Eu(III)

The interaction of Eu(III) with non-porous silica was studied using sorption–desorption experiments and Eu(III) was characterized by laser-induced fluorescence spectroscopy. The independence of Eu(III) sorption on ionic strength, the increased affinity of silica for heavier rare earth ions, and the removal of water in the inner-sphere of Eu(III) due to sorption demonstrate that the formation of an inner-sphere complex of Eu(III) with silanol groups at the silica surface is an important process in the initial reaction. A further decrease in the number of hydrated water molecules with time and the irreversibility of the Eu(III) sorption suggest that a new phase incorporating Eu(III) and Si(IV) forms at the silica surface. The presence of Eu(III) greatly promoted the dissolution of silica, which indicated that Eu(III) was effectively hydrolyzing the Si O Si bonds. The formation process of the Eu(III)–silicate phase is discussed.

Effect of pH on ceria–silica interactions during chemical mechanical polishing

To understand the ceria–silica chemical mechanical polishing (CMP) mechanisms, we studied the effect of ceria slurry pH on silica removal and surface morphology. Also, in situ friction force measurements were conducted. After polishing; atomic force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy were used to quantify the extent of the particle–substrate interaction during CMP. Our results indicate the silica removal by ceria slurries is strongly pH dependent, with the maximum occurring near the isoelectric point of the ceria slurry.

A 13C CP/MAS and 31P NMR study of the interactions of dipalmitoylphosphatidylcholine with respirable silica and kaolin

The interaction of silica and kaolin with dipalmitoylphosphatidylcholine (DPPC) has been studied using 13C and 31P solid state nuclear magnetic resonance spectroscopy. These studies explore the molecular interactions of these respirable dusts with a model lung surfactant species to characterize silica toxicity in mixed systems. The choline head group of DPPC was found to remain mobile when adsorbed on kaolin, in contrast to an immobile head group on silica. Further, glycerol carbon intensities were greatly diminished relative to that of choline carbons, a result attributed to broadening effects. These preliminary findings suggest that silica toxicity may not be related to choline mobility as previously noted [J. Colloid Interface Sci. 172 (1995) 536–538].

Drying dissipative structures of the deionized aqueous suspensions of colloidal silica spheres ranging from 29 nm to 1 μm in diameter

Macroscopic and microscopic dissipative structural patterns formed in the course of drying a series of the colloidal silica spheres ranging from 29 nm to 1 μm in diameter have been observed in the aqueous deionized suspension on a cover glass. The broad ring patterns of the hill accumulated with the silica spheres are formed around the outside edges in the macroscopic scale for all spheres examined. The spoke-like cracks are also observed in the macroscopic scale and their number decreases sharply as sphere size increases. The pattern area and the time for the dryness have been discussed as a function of sphere size and concentration. The convection flow of water accompanied with that of the silica spheres and interactions among the silica spheres and substrate are important for the macroscopic pattern formation. The microscopic fractal structures of the wave-like patterns and branched strings are formed. Their fractal dimensions are determined. Microscopic patterns form in the narrow range of sphere sizes and concentrations and are determined mainly by the electrostatic and polar interactions between the spheres and/or between the sphere and substrate in the course of solidification.

The experimental silicification of Aquificales and their role in hot spring sinter formation

ABSTRACTArchean microfossils provide some of the earliest physical evidence for life on Earth, yet there remains a great deal of uncertainty regarding which micro‐organisms were actually preserved. Because of the limited cellular detail remaining, interpretation of those microfossils has been based solely on size and morphology. This has led to significant controversy surrounding the presence or absence of cyanobacteria as early as 3.5 billion years. Accordingly, there has been an experimental bias towards studying their silicification. Here we report the very first findings on thermophilic bacteria–silica interactions, and investigate how Sulfurihydrogenibium azorense, a representative of the Aquificales often found as prominent members of modern hot spring vent communities, interacts with highly siliceous hydrothermal fluids. We show that adsorption of silica is limited to silica polymers and colloids, and that the magnitude of silica adsorption is dependent on its chemolithoautotrophic pathway. Intriguingly, when S. azorense is grown as a H2‐oxidizer, it responds to increasing silica concentrations by producing a protein‐rich biofilm that may afford the cells protection against cell wall silicification. Although the biofilms of Aquificales could potentially contribute to or accelerate siliceous sinter formation under certain growth conditions, the cells themselves show a low preservation potential and are unlikely to have been preserved in the ancient rock record, despite phylogenetic analyses suggesting that they represent one of the most primordial life forms.

Plasma-surface interactions of nanoporous silica during plasma-based pattern transfer using C4F8 and C4F8∕Ar gas mixtures

We have investigated plasma surface interactions of nanoporous silica (NPS) films with porosities up to 50%, and SiO2 with C4F8∕Ar discharges used for plasma etching. The pore size was about 2–3nm for all films. In highly polymerizing plasmas (e.g., pure C4F8 discharges), the porous structure of NPS material favors surface polymerization over etching and porosity-corrected etching rates (CER) were suppressed and lower than SiO2 etching rate for the same conditions. The etching rates of NPS were dramatically enhanced in ion rich discharges (e.g., C4F8∕90%Ar) and the CER in this case is greater than the SiO2 etching rate. Both x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (static SIMS) show that fairly thick (∼2–3nm) fluorocarbon layers exist on the NPS surface during C4F8 etching. This layer blocks the direct interaction of ions with the NPS surface and results in a low etching rate. For C4F8∕90%Ar discharges, little fluorocarbon coverage is observed for NPS surfaces and the direct ion surface interaction is significantly enhanced, explaining the enhancement of CER. We can deduce from analysis of angular resolved XPS data that the surface of NPS materials and SiO2 remain smooth during C4F8 etching. For C4F8∕90%Ar etching, the NPS surfaces became rough. The surface roughening is due to angle-dependent ion etching effects. These surface models were directly verified by the transmission electron microscopy. Depth profiling study of NPS partially etched using C4F8 or C4F8∕90%Ar discharges using dynamic SIMS indicates that the plasma induced modification of NPS was enhanced significantly compared with SiO2 due to the porous structure, which allows the plasma attack of the subsurface region. The modified layer thickness is related to the overall porosity and dramatically increases for NPS with an overall porosity of 50%. The distinct etching behavior of high porosity NPS (∼50%) in fluorocarbon-based discharges relative to NPS material with lower overall porosity is possibly due to interconnected pores, which allow plasma species to more easily penetrate into the subsurface region.

A grand canonical Monte Carlo simulation study of water adsorption on Vycor-likehydrophilic mesoporous silica at different temperatures

The grand canonical Monte Carlo (GCMC) simulation technique is used to studywater adsorption and condensation in a realistic Vycor-like silica mesoporoussystem at various temperatures. Water–water interactions are described usingthe SPC model while water–silica interactions are calculated in the frameworkof the PN-TrAZ model. Thermodynamic quantities (namely water adsorptionisotherms and isosteric heat of adsorption curves) have been calculated along withwater–water and water–Vycor pair distribution functions. The simulated adsorptionisotherm at room temperature compares well with published experimental data.Isosteric heat curves are characteristic of adsorption in a heterogeneous environment.We also show that the BET method for specific surface determination is notvalid in the case of water confined in silica mesoporous materials. By analysingwater–water and water–Vycor pair correlation functions, we demonstrate theexistence of strong distortion compared to bulk water due to the influence of thesilica surface. The hydrogen bond is significantly elongated and angle distorted.

Silica-supported zirconocene/(perfluorophenyl)borate catalyst for propylene polymerization

Abstract IR spectroscopy has been used to study the interaction of silica with PhNEt 2 (N) and B(C 6 F 5 ) 3 (B) and subsequent interaction of the support SiO 2 /[N + B] with dimethylzirconocene Me 2 Si(2-Me-Ind) 2 ZrMe 2 (“Zr”). The data were obtained on the composition of the surface compounds appeared at both stages of catalyst synthesis. It has been shown that (B) and (N) interact with OH groups of silica to form ionic pair [H NR 3 ] + [(C 6 F 5 ) 3 B O Si ] − (IP-1). Cation fragment of this pair contains highly reactive N H bond with a.b. at 3230 cm −1 . It has been found that N H groups in a part of IP-1 complexes react with neighboring OH groups of silica by hydrogen bonding that gives complexes IP-2. It has been shown that “Zr” complexes interact both with complexes IP-1 and IP-2. As “Zr” reacts with IP-1, zirconium ionic complexes IP-3 containing Zr Me bond are formed on silica. These complexes are suggested to be the precursor of the polymerization active sites. The reaction of “Zr” with IP-2, most likely, produces surface zirconium compound containing no Zr Me bonds and inactive for propylene polymerization.

The interaction of water with silica thin films grown on Mo(112)

The adsorption of water on ultrathin SiO2 films at low temperatures has been studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS (HeI)). High-resolution electron energy-loss spectroscopy (HREELS), work function measurements (Δϕ), and temperature programmed desorption (TPD) were also utilized to study the interaction of water with silica. Evidence for molecular absorption of water on low- and high-defect silica surfaces is presented. The data are consistent with the growth of 3-D water clusters even at low coverage, i.e., the water–water hydrogen bonding is stronger than the water–silica interaction. No evidence for dissociation of water was found in contrast to previous UPS results.

Interface between End-Functionalized PEO Oligomers and a Silica Nanoparticle Studied by Molecular Dynamics Simulations

Fully atomistic molecular dynamics (MD) simulations have been carried out on a series of bulk melt pure PEO oligomers and PEO oligomer−silica systems, which differed by their macromolecular end groups. A realistic hybrid model of a silica nanoparticle combining an ionic core as well as the fine-tuning of the surface thickness and number of OH groups per unit surface area was used. The PEO oligomers were decorrelated in all systems under study in order to prevent any artifacts related to the preparation procedure. Significant changes were found to occur in the immediate vicinity of the interface with flattened PEO backbones arranged in densily packed shells and stabilized by the added PEO−silica interactions. Their conformations were also more coiled in order to better adapt to the surface structure. While methyl end groups did not show special characteristics other than their steric effect, hydroxyl end groups had a much higher affinity for the surface and tended to position themselves perpendicular to the surface, thus forming dynamic hydrogen-bonding complexes between the hydroxyl oxygens and the silanol hydrogens. The range of influence of the nanoparticle was evident for structural properties only up to two or three molecular layers, 10−15 Å, but was approximately twice that for dynamical ones.

Formation of silver nanoparticles in an acid-catalyzed silica colloidal solution

In a weak basic, weak acidic or neutral water–alcohol solution, silver nanoparticles were generated by the reduction of Ag + ions in the present of colloidal silica. Silica as a substrate played an important role in the formation of Silver particles. The plasmon band of silver particles supported on the surface of silica was considerably shifted to longer wavelength compared with the pure silver sol. The shift in absorption spectra was explained in terms of surface effects induced by the interaction of silver and silica, as well as size effects and irregular shape.

A comparison of water adsorption on ordered and disordered silica substrates

The adsorption properties of water adsorbed on various silica substrates are investigated by way of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN-TrAZ potential are used to describe water–water and water–silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6 nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor. The chemistry of the surface is made realistic by hydroxylation. The simulated adsorption isotherm and isosteric differential enthalpy of adsorption compare well to experimental data for Vycor, showing the ability of the PN-TrAZ potential to describe the hydrophilic properties of silica surfaces. This study was extended to several crystallographic faces of cristobalite. Their adsorption properties differ widely from each other. It is shown that the hydrophilic properties are not simply related to surface hydroxyl density but are also related to the local structure of the silica surface. In spite of these large variations, it is possible to reproduce the adsorption isotherm of the mesoporous disordered sample by applying a natural averaging procedure over the different crystallographic faces of cristobalite.